SMACNA Duct Weight Calculator
An online tool to estimate the weight of sheet metal ductwork based on SMACNA standards.
Duct Weight Estimation
Enter the total length of the duct run in feet.
Enter the width (or smaller dimension for rectangular ducts) in inches.
Enter the height (or larger dimension for rectangular ducts) in inches.
Galvanized Steel
Stainless Steel
Aluminum
Select the metal used for the ductwork.
26 BWG
24 BWG
22 BWG
20 BWG
18 BWG
16 BWG
14 BWG
Enter the gauge of the sheet metal (Birmingham Wire Gauge).
Surface Area
—
Material Thickness (in)
—
Weight per SqFt (lbs)
—
Formula Used: Total Weight = Surface Area (sq ft) * Weight per SqFt (lbs/sq ft)
Duct Weight Data Table
| Material | Gauge (BWG) | Thickness (in) | Weight per SqFt (lbs) |
|---|
Data based on standard SMACNA specifications.
Weight Distribution Chart
Comparison of total weight across different materials for the specified duct dimensions and gauge.
{primary_keyword}
The SMACNA duct weight calculator is a specialized tool designed for HVAC professionals, sheet metal fabricators, engineers, and estimators. It allows users to quickly and accurately determine the estimated weight of fabricated sheet metal ductwork. This calculation is crucial for various aspects of HVAC projects, including material procurement, transportation logistics, structural support design, and overall project cost estimation. By inputting key parameters such as duct dimensions, length, material type, and gauge, the calculator leverages SMACNA (Sheet Metal and Air Conditioning Contractors' National Association) standards and data to provide a reliable weight estimate. This tool is invaluable for ensuring that projects remain within budget and that all logistical and structural considerations are adequately addressed, making it an essential part of efficient {primary_keyword} processes.
Who Should Use It: This calculator is primarily for HVAC contractors, sheet metal fabricators, mechanical engineers, estimators, project managers, and anyone involved in the design, fabrication, or installation of sheet metal duct systems. It helps in quoting jobs, ordering materials, and planning installation phases.
Common Misconceptions: A common misconception is that duct weight is negligible or can be estimated loosely. In reality, especially for large commercial or industrial projects, the cumulative weight of ductwork can be substantial, impacting structural loads and installation complexity. Another misconception is that all sheet metal of the same gauge weighs the same; however, density differences between materials like galvanized steel, stainless steel, and aluminum mean that gauge alone isn't sufficient for accurate weight calculation. Our tool addresses these by considering material type.
{primary_keyword} Formula and Mathematical Explanation
The core of the {primary_keyword} calculation relies on determining the total surface area of the ductwork and then multiplying it by the material's weight per unit area. SMACNA provides standard thickness and density values for various gauges and materials, which are used to derive the weight per square foot.
The calculation follows these steps:
- Calculate Surface Area: For a rectangular duct, the surface area is the sum of the areas of all its sides. Assuming a duct with length L, width W, and height H, the surface area (SA) is calculated as:
SA = 2 * (L * W) + 2 * (L * H) + 2 * (W * H)
However, for HVAC ducting calculations, we often simplify this by considering the perimeter of the cross-section and multiplying by the length.
Perimeter (P) = 2 * (Width + Height)
Surface Area (SA) in square feet = (P in inches / 12 inches/ft) * L in feetSA = ((2 * (Width + Height)) / 12) * Length - Determine Material Thickness: The gauge of the sheet metal (e.g., 26, 24, 22 BWG) corresponds to a specific thickness. These values are standardized by SMACNA.
- Calculate Weight per Square Foot: This is derived from the material's density and the thickness. The formula is:
Weight per SqFt = Thickness (ft) * Density (lbs/cu ft)
SMACNA data tables often provide this value directly for common materials and gauges. - Calculate Total Weight: The final step is to multiply the total surface area by the weight per square foot.
Total Weight = SA (sq ft) * Weight per SqFt (lbs/sq ft)
Variables Used:
| Variable | Meaning | Unit | Typical Range |
|---|---|---|---|
| Duct Length (L) | Total length of the duct run | feet (ft) | 1 – 1000+ |
| Duct Width (W) | Width of the rectangular duct or diameter for round ducts | inches (in) | 1 – 120+ |
| Duct Height (H) | Height of the rectangular duct (use same as Width for round ducts) | inches (in) | 1 – 120+ |
| Material Type | The metal used for fabrication | N/A | Galvanized Steel, Stainless Steel, Aluminum |
| Gauge (G) | Sheet metal thickness standard (Birmingham Wire Gauge) | BWG | 14 – 30 |
| Surface Area (SA) | Total exterior surface area of the ductwork | square feet (sq ft) | Calculated |
| Thickness (T) | Actual thickness of the metal sheet | inches (in) | 0.0179 – 0.0747 (for BWG 26-14) |
| Density (D) | Mass per unit volume of the material | lbs/cu ft | Galvanized Steel: ~490, Stainless Steel: ~495, Aluminum: ~165 |
| Weight per SqFt (WPSF) | Weight of one square foot of the material at the specified gauge | lbs/sq ft | Calculated / Looked up |
| Total Weight (TW) | Estimated total weight of the duct section | lbs | Calculated |
Practical Examples (Real-World Use Cases)
Understanding the {primary_keyword} in practice is key. Here are a couple of examples:
Example 1: Standard Return Air Duct
A mechanical contractor is installing a 20 ft long section of a return air duct for a commercial building. The duct dimensions are 24 inches wide by 18 inches high. It's fabricated from 22 gauge galvanized steel.
Inputs:
- Duct Length: 20 ft
- Duct Width: 24 in
- Duct Height: 18 in
- Material: Galvanized Steel
- Gauge: 22 BWG
Calculation Steps (Illustrative):
- Perimeter = 2 * (24 + 18) = 2 * 42 = 84 inches
- Surface Area = (84 in / 12 in/ft) * 20 ft = 7 ft * 20 ft = 140 sq ft
- From SMACNA data or calculator lookup: 22 gauge galvanized steel has a thickness of approx. 0.0359 inches and a weight of approx. 2.24 lbs/sq ft.
- Total Weight = 140 sq ft * 2.24 lbs/sq ft = 313.6 lbs
Result: The estimated weight for this duct section is approximately 313.6 lbs. This information is vital for the estimator to include in the material quote and for the installation team to plan for handling and potentially required lifting aids.
Example 2: Main Supply Duct Run
An HVAC project requires a 50 ft run of main supply duct with dimensions of 36 inches wide by 30 inches high. The material specified is 20 gauge stainless steel.
Inputs:
- Duct Length: 50 ft
- Duct Width: 36 in
- Duct Height: 30 in
- Material: Stainless Steel
- Gauge: 20 BWG
Calculation Steps (Illustrative):
- Perimeter = 2 * (36 + 30) = 2 * 66 = 132 inches
- Surface Area = (132 in / 12 in/ft) * 50 ft = 11 ft * 50 ft = 550 sq ft
- From SMACNA data or calculator lookup: 20 gauge stainless steel has a thickness of approx. 0.0397 inches and a weight of approx. 3.28 lbs/sq ft.
- Total Weight = 550 sq ft * 3.28 lbs/sq ft = 1804 lbs
Result: The estimated weight for this section of ductwork is approximately 1804 lbs. This significant weight indicates the need for robust structural supports and careful planning for transportation and installation, especially considering the higher density of stainless steel compared to galvanized steel.
How to Use This {primary_keyword} Calculator
Using our SMACNA Duct Weight Calculator is straightforward. Follow these steps:
- Enter Duct Dimensions: Input the total Duct Length in feet. Then, enter the Duct Width and Duct Height in inches. For round ducts, use the diameter for both Width and Height.
- Select Material and Gauge: Choose the Material Type (Galvanized Steel, Stainless Steel, or Aluminum) from the dropdown menu. Then, select the corresponding Material Gauge (e.g., 26, 24, 22 BWG).
- Calculate: Click the "Calculate Weight" button. The calculator will process your inputs.
- Review Results: The primary result, "Estimated Total Weight," will be displayed prominently. You will also see key intermediate values: Surface Area, Material Thickness, and Weight per Square Foot.
- Understand the Formula: A brief explanation of the calculation formula is provided below the results.
- Use the Data Table: Refer to the table for standard thickness and weight per square foot values for various materials and gauges.
- Visualize with the Chart: The bar chart provides a visual comparison of the estimated total weight across different material types for your specified dimensions and gauge.
- Copy Results: Use the "Copy Results" button to easily transfer the main result, intermediate values, and key assumptions to your clipboard for reports or quotes.
- Reset: If you need to start over or try different values, click the "Reset" button to revert to default settings.
Decision-Making Guidance: The results help in making informed decisions regarding material ordering, transportation costs, structural load calculations for support systems, and overall project budgeting. Higher weights may necessitate stronger supports and specialized handling equipment.
Key Factors That Affect {primary_keyword} Results
Several factors influence the accuracy and magnitude of the calculated duct weight. Understanding these is crucial for precise estimation:
- Duct Dimensions (Width, Height, Length): This is the most direct factor. Larger dimensions mean greater surface area, leading to higher weight. Even small increases in width or height compound significantly over long duct runs.
- Material Type: Different metals have vastly different densities. Stainless steel is significantly denser than galvanized steel, which is denser than aluminum. This directly impacts the weight per square foot, even at the same gauge. For instance, stainless steel ductwork will always weigh more than equivalent galvanized steel ductwork.
- Material Gauge (Thickness): Lower gauge numbers indicate thicker metal. Thicker metal (e.g., 16 gauge) weighs considerably more per square foot than thinner metal (e.g., 26 gauge). The choice of gauge often depends on duct size, pressure class, and support spacing requirements.
- Duct Shape and Seams: While this calculator primarily focuses on rectangular ducts (or round treated as rectangular for simplicity), complex shapes or extensive use of reinforcing seams can add marginal weight not fully captured by basic surface area calculations. SMACNA standards account for typical seam allowances.
- Fittings and Transitions: Elbows, take-offs, reducers, and transitions add material and complexity. While this calculator focuses on straight duct sections, the weight of fittings is an essential part of the total project weight and must be estimated separately or included in advanced calculations.
- Cladding or Insulation: For many applications, ductwork is insulated or clad. The weight of insulation materials (e.g., fiberglass, mineral wool) and outer jackets (e.g., foil, vinyl) must be added to the sheet metal weight for a complete picture of the installed weight.
- Fasteners and Accessories: The weight of associated components like flanges, hangers, fasteners, access doors, and internal liners can add a small but cumulative percentage to the overall weight.
Frequently Asked Questions (FAQ)
Q1: What is SMACNA, and why is it important for duct weight calculations?
SMACNA (Sheet Metal and Air Conditioning Contractors' National Association) publishes industry standards and data manuals that provide specifications for duct construction, including material thicknesses, weights, and fabrication practices. Using SMACNA data ensures calculations are based on industry best practices and reliable figures.
Q2: Does this calculator account for round ducts?
Yes, the calculator can be used for round ducts. For round ducts, enter the diameter for both the "Duct Width" and "Duct Height" fields. The perimeter calculation will correctly approximate the circumference.
Q3: What is the difference between galvanized steel, stainless steel, and aluminum in terms of weight?
Stainless steel is the heaviest, followed by galvanized steel, and then aluminum, which is the lightest. This is due to their differing densities. Stainless steel has a density around 495 lbs/cu ft, galvanized steel around 490 lbs/cu ft, and aluminum around 165 lbs/cu ft. At the same gauge, stainless steel ducting will be significantly heavier.
Q4: How does material gauge affect the weight?
Lower gauge numbers mean thicker metal. Thicker metal has more mass per unit area, so it weighs more. For example, 18 gauge steel is significantly heavier than 26 gauge steel.
Q5: Does this calculator include the weight of insulation?
No, this calculator focuses solely on the weight of the sheet metal itself. Insulation weight must be calculated separately based on the insulation type, thickness, and surface area.
Q6: How accurate are these estimates?
The estimates are highly accurate for straight duct sections based on standard SMACNA data. However, actual weight can vary slightly due to manufacturing tolerances, specific joint designs, and the addition of fittings, reinforcements, or external materials.
Q7: Can I use this for calculating shipping costs?
Yes, the estimated weight is a crucial input for calculating shipping costs. Knowing the weight allows for accurate quoting from freight carriers and ensures proper vehicle capacity planning.
Q8: What is BWG?
BWG stands for Birmingham Wire Gauge, a standard system used for specifying the thickness of sheet metal. Lower numbers indicate thicker metal.
Q9: Should I round up the dimensions or weight?
It is often advisable to round up material quantities and potentially the final weight estimate slightly to account for waste, standard material lengths, and the cumulative weight of fittings and accessories not explicitly calculated here.
Related Tools and Internal Resources
- SMACNA Duct Weight Calculator: Our primary tool for estimating duct metal weight.
- HVAC Load Calculator: Helps determine heating and cooling needs for a space, influencing duct sizing.
- Sheet Metal Gauge to Thickness Converter: Useful for understanding specific material dimensions.
- Air Velocity Calculator: Essential for ensuring proper airflow within ducts.
- HVAC Pressure Loss Calculator: Helps in determining the energy required to move air through the duct system.
- Duct Sizing Calculator: Assists in determining appropriate duct dimensions based on airflow requirements.
- Material Cost Estimator Tool: Use estimated weights to help calculate the cost of materials for your project.
Results copied to clipboard!
var smacnaData = {
"galvanized": {
"26": {"thickness": 0.0179, "weight_per_sqft": 1.12, "density": 490},
"24": {"thickness": 0.0239, "weight_per_sqft": 1.50, "density": 490},
"22": {"thickness": 0.0299, "weight_per_sqft": 1.87, "density": 490},
"20": {"thickness": 0.0359, "weight_per_sqft": 2.24, "density": 490},
"18": {"thickness": 0.0478, "weight_per_sqft": 3.00, "density": 490},
"16": {"thickness": 0.0598, "weight_per_sqft": 3.75, "density": 490},
"14": {"thickness": 0.0747, "weight_per_sqft": 4.69, "density": 490}
},
"stainless": {
"26": {"thickness": 0.0179, "weight_per_sqft": 1.13, "density": 495},
"24": {"thickness": 0.0239, "weight_per_sqft": 1.51, "density": 495},
"22": {"thickness": 0.0299, "weight_per_sqft": 1.88, "density": 495},
"20": {"thickness": 0.0359, "weight_per_sqft": 2.25, "density": 495},
"18": {"thickness": 0.0478, "weight_per_sqft": 3.01, "density": 495},
"16": {"thickness": 0.0598, "weight_per_sqft": 3.77, "density": 495},
"14": {"thickness": 0.0747, "weight_per_sqft": 4.71, "density": 495}
},
"aluminum": {
"26": {"thickness": 0.0179, "weight_per_sqft": 0.38, "density": 165},
"24": {"thickness": 0.0239, "weight_per_sqft": 0.51, "density": 165},
"22": {"thickness": 0.0299, "weight_per_sqft": 0.64, "density": 165},
"20": {"thickness": 0.0359, "weight_per_sqft": 0.77, "density": 165},
"18": {"thickness": 0.0478, "weight_per_sqft": 1.03, "density": 165},
"16": {"thickness": 0.0598, "weight_per_sqft": 1.29, "density": 165},
"14": {"thickness": 0.0747, "weight_per_sqft": 1.61, "density": 165}
}
};
var defaultValues = {
ductLength: 10,
ductWidth: 12,
ductHeight: 12,
material: "galvanized",
gauge: "26"
};
function updateTableData() {
var tableBody = document.getElementById("weightDataTableBody");
tableBody.innerHTML = ""; // Clear existing rows
var materials = Object.keys(smacnaData);
for (var i = 0; i < materials.length; i++) {
var material = materials[i];
var gauges = Object.keys(smacnaData[material]).sort(function(a, b) {
return parseInt(a) – parseInt(b);
});
for (var j = 0; j < gauges.length; j++) {
var gauge = gauges[j];
var data = smacnaData[material][gauge];
var row = tableBody.insertRow();
row.insertCell(0).textContent = material.charAt(0).toUpperCase() + material.slice(1).replace('_', ' ');
row.insertCell(1).textContent = gauge + " BWG";
row.insertCell(2).textContent = data.thickness.toFixed(4);
row.insertCell(3).textContent = data.weight_per_sqft.toFixed(2);
}
}
}
function updateChart() {
var canvas = document.getElementById('weightChart');
if (!canvas) return;
var ctx = canvas.getContext('2d');
if (window.weightChartInstance) {
window.weightChartInstance.destroy();
}
var length = parseFloat(document.getElementById("ductLength").value);
var width = parseFloat(document.getElementById("ductWidth").value);
var height = parseFloat(document.getElementById("ductHeight").value);
var gauge = document.getElementById("gauge").value;
if (isNaN(length) || isNaN(width) || isNaN(height) || length <= 0 || width <= 0 || height <= 0) {
return; // Don't draw if inputs are invalid
}
var perimeter = 2 * (width + height);
var surfaceArea = (perimeter / 12) * length;
var chartData = {
labels: [],
datasets: [{
label: 'Estimated Weight (lbs)',
data: [],
backgroundColor: [
'rgba(0, 74, 153, 0.6)', // Primary blue
'rgba(40, 167, 69, 0.6)', // Success green
'rgba(23, 162, 184, 0.6)' // Info cyan
],
borderColor: [
'rgba(0, 74, 153, 1)',
'rgba(40, 167, 69, 1)',
'rgba(23, 162, 184, 1)'
],
borderWidth: 1
}]
};
var materialsToCompare = ["galvanized", "stainless", "aluminum"];
for (var i = 0; i {
var colors = ['rgba(0, 74, 153, 0.6)', 'rgba(40, 167, 69, 0.6)', 'rgba(23, 162, 184, 0.6)'];
return colors[index % colors.length];
});
chartData.datasets[0].borderColor = chartData.labels.map((_, index) => {
var colors = ['rgba(0, 74, 153, 1)', 'rgba(40, 167, 69, 1)', 'rgba(23, 162, 184, 1)'];
return colors[index % colors.length];
});
window.weightChartInstance = new Chart(ctx, {
type: 'bar',
data: chartData,
options: {
responsive: true,
maintainAspectRatio: true,
scales: {
y: {
beginAtZero: true,
title: {
display: true,
text: 'Weight (lbs)'
}
}
},
plugins: {
legend: {
display: false // Hide legend as labels are on the bars
},
tooltip: {
callbacks: {
label: function(context) {
var label = context.dataset.label || ";
if (label) {
label += ': ';
}
if (context.parsed.y !== null) {
label += context.parsed.y + ' lbs';
}
return label;
}
}
}
}
}
});
}
function calculateWeight() {
var lengthInput = document.getElementById("ductLength");
var widthInput = document.getElementById("ductWidth");
var heightInput = document.getElementById("ductHeight");
var materialSelect = document.getElementById("material");
var gaugeSelect = document.getElementById("gauge");
var lengthError = document.getElementById("ductLengthError");
var widthError = document.getElementById("ductWidthError");
var heightError = document.getElementById("ductHeightError");
var length = parseFloat(lengthInput.value);
var width = parseFloat(widthInput.value);
var height = parseFloat(heightInput.value);
var material = materialSelect.value;
var gauge = gaugeSelect.value;
var isValid = true;
// Reset errors
lengthError.textContent = "";
widthError.textContent = "";
heightError.textContent = "";
if (isNaN(length) || length <= 0) {
lengthError.textContent = "Please enter a valid positive length.";
isValid = false;
}
if (isNaN(width) || width <= 0) {
widthError.textContent = "Please enter a valid positive width.";
isValid = false;
}
if (isNaN(height) || height <= 0) {
heightError.textContent = "Please enter a valid positive height.";
isValid = false;
}
if (!isValid) {
document.getElementById("result").classList.remove("visible");
document.getElementById("surfaceAreaResult").textContent = "–";
document.getElementById("thicknessResult").textContent = "–";
document.getElementById("weightPerSqFtResult").textContent = "–";
return;
}
var perimeter = 2 * (width + height);
var surfaceArea = (perimeter / 12) * length;
var materialData = smacnaData[material];
var gaugeData = materialData ? materialData[gauge] : null;
if (!gaugeData) {
// Fallback or error if data is missing
document.getElementById("result").classList.remove("visible");
document.getElementById("surfaceAreaResult").textContent = surfaceArea.toFixed(2) + " sq ft";
document.getElementById("thicknessResult").textContent = "N/A";
document.getElementById("weightPerSqFtResult").textContent = "N/A";
return;
}
var thickness = gaugeData.thickness;
var weightPerSqFt = gaugeData.weight_per_sqft;
var totalWeight = surfaceArea * weightPerSqFt;
document.getElementById("result").classList.add("visible");
document.getElementById("result").textContent = totalWeight.toFixed(2) + " lbs";
document.getElementById("surfaceAreaResult").textContent = surfaceArea.toFixed(2) + " sq ft";
document.getElementById("thicknessResult").textContent = thickness.toFixed(4) + " in";
document.getElementById("weightPerSqFtResult").textContent = weightPerSqFt.toFixed(2) + " lbs/sq ft";
updateChart();
}
function resetCalculator() {
document.getElementById("ductLength").value = defaultValues.ductLength;
document.getElementById("ductWidth").value = defaultValues.ductWidth;
document.getElementById("ductHeight").value = defaultValues.ductHeight;
document.getElementById("material").value = defaultValues.material;
document.getElementById("gauge").value = defaultValues.gauge;
document.getElementById("ductLengthError").textContent = "";
document.getElementById("ductWidthError").textContent = "";
document.getElementById("ductHeightError").textContent = "";
document.getElementById("result").classList.remove("visible");
document.getElementById("surfaceAreaResult").textContent = "–";
document.getElementById("thicknessResult").textContent = "–";
document.getElementById("weightPerSqFtResult").textContent = "–";
if (window.weightChartInstance) {
window.weightChartInstance.destroy();
}
var canvas = document.getElementById('weightChart');
var ctx = canvas.getContext('2d');
ctx.clearRect(0, 0, canvas.width, canvas.height); // Clear canvas
// Optionally re-calculate with defaults or leave as is until user clicks calculate
calculateWeight(); // Recalculate to show default results
}
function copyResults() {
var mainResult = document.getElementById("result").textContent;
var surfaceArea = document.getElementById("surfaceAreaResult").textContent;
var thickness = document.getElementById("thicknessResult").textContent;
var weightPerSqFt = document.getElementById("weightPerSqFtResult").textContent;
var assumptions = "Assumptions:\n";
assumptions += "Material: " + document.getElementById("material").selectedOptions[0].text + "\n";
assumptions += "Gauge: " + document.getElementById("gauge").selectedOptions[0].text + "\n";
var textToCopy = "SMACNA Duct Weight Calculation:\n\n";
textToCopy += "Estimated Total Weight: " + mainResult + "\n\n";
textToCopy += "Key Intermediate Values:\n";
textToCopy += "- Surface Area: " + surfaceArea + "\n";
textToCopy += "- Material Thickness: " + thickness + "\n";
textToCopy += "- Weight per SqFt: " + weightPerSqFt + "\n\n";
textToCopy += assumptions;
if (navigator.clipboard && window.isSecureContext) {
navigator.clipboard.writeText(textToCopy).then(function() {
showClipboardMessage("Results copied to clipboard!");
}).catch(function(err) {
showClipboardMessage("Failed to copy. Please copy manually.");
console.error('Clipboard copy failed: ', err);
});
} else {
// Fallback for older browsers or non-secure contexts
var textArea = document.createElement("textarea");
textArea.value = textToCopy;
textArea.style.position = "fixed";
textArea.style.left = "-9999px";
document.body.appendChild(textArea);
textArea.focus();
textArea.select();
try {
document.execCommand('copy');
showClipboardMessage("Results copied to clipboard!");
} catch (err) {
showClipboardMessage("Failed to copy. Please copy manually.");
console.error('Fallback clipboard copy failed: ', err);
}
document.body.removeChild(textArea);
}
}
function showClipboardMessage(message) {
var msgBox = document.getElementById("clipboard-message");
msgBox.textContent = message;
msgBox.classList.add("show");
setTimeout(function() {
msgBox.classList.remove("show");
}, 3000);
}
// Load Chart.js library dynamically if not present
function loadChartJs() {
if (typeof Chart === 'undefined') {
var script = document.createElement('script');
script.src = 'https://cdn.jsdelivr.net/npm/chart.js@3.0.0/dist/chart.min.js'; // Use a specific version
script.onload = function() {
updateTableData(); // Populate table after script loads
calculateWeight(); // Initial calculation
updateChart(); // Initial chart rendering
};
script.onerror = function() {
console.error("Failed to load Chart.js library.");
};
document.head.appendChild(script);
} else {
updateTableData(); // Populate table immediately
calculateWeight(); // Initial calculation
updateChart(); // Initial chart rendering
}
}
// Initialize on page load
document.addEventListener('DOMContentLoaded', loadChartJs);
// Ensure inputs trigger calculation on change
document.getElementById("ductLength").addEventListener("input", calculateWeight);
document.getElementById("ductWidth").addEventListener("input", calculateWeight);
document.getElementById("ductHeight").addEventListener("input", calculateWeight);
document.getElementById("material").addEventListener("change", calculateWeight);
document.getElementById("gauge").addEventListener("change", calculateWeight);