Plastic Part Weight Calculator
Accurately estimate material weight, volume, and cost for injection molding and manufacturing.
Rectangular Block / Plate
Solid Cylinder / Rod
Hollow Tube / Pipe
Solid Sphere
Custom Volume (Known cm³)
Select the basic geometric shape that best fits your part.
Polypropylene (PP) – 0.90 g/cm³
LDPE – 0.92 g/cm³
HDPE – 0.95 g/cm³
ABS – 1.04 g/cm³
Polystyrene (PS) – 1.05 g/cm³
Nylon 6 (PA6) – 1.13 g/cm³
Acrylic (PMMA) – 1.19 g/cm³
Polycarbonate (PC) – 1.20 g/cm³
PET – 1.38 g/cm³
PVC (Rigid) – 1.40 g/cm³
Custom Density
Please enter a valid positive density.
Quantity must be at least 1.
Average market price for the raw resin.
Total Batch Weight
0.00 kg
Formula Used: Weight = Volume × Density × Quantity.
Calculated based on geometric volume and specific gravity of the selected resin.
Calculated based on geometric volume and specific gravity of the selected resin.
Single Part Weight
0.00 g
Single Part Volume
0.00 cm³
Total Material Cost
$0.00
| Metric | Value | Unit |
|---|
What is How to Calculate Plastic Part Weight?
Understanding how to calculate plastic part weight is a fundamental skill in manufacturing, engineering, and supply chain management. It refers to the mathematical process of estimating the mass of a molded or machined plastic component before it is physically produced. This calculation is derived from the part's geometric volume and the specific density (specific gravity) of the chosen polymer resin.
Engineers, mold designers, and procurement managers use this calculation to estimate raw material requirements, calculate shipping costs, and determine the cycle time for injection molding machines. A precise calculation helps prevent material wastage and ensures accurate financial forecasting for production runs.
Common misconceptions include assuming all plastics have the same weight or ignoring the impact of shrinkage and additives like glass fiber. In reality, the density difference between Polypropylene (0.90 g/cm³) and PVC (1.40 g/cm³) can result in a 55% weight difference for the exact same shape.
Plastic Part Weight Formula and Mathematical Explanation
The core formula for how to calculate plastic part weight is based on the relationship between mass, density, and volume. The calculation is performed in three steps:
Step 1: Calculate Volume (V)
First, determine the volume of the part in cubic centimeters (cm³). For complex injection molded parts, this is often done using CAD software, but for basic shapes, geometric formulas apply:
- Rectangular Block: Length × Width × Height
- Cylinder: π × Radius² × Height
- Tube: π × (Outer Radius² – Inner Radius²) × Height
Step 2: Apply Density (ρ)
Multiply the volume by the material's density. The formula is:
Weight (W) = Volume (V) × Density (ρ)
Variable Reference Table
| Variable | Meaning | Common Unit | Typical Range |
|---|---|---|---|
| W | Weight / Mass | Grams (g) or Kilograms (kg) | 0.1g – 50kg+ |
| V | Volume | Cubic Centimeters (cm³) | Varies by part |
| ρ (Rho) | Density | g/cm³ | 0.90 – 1.50 g/cm³ |
Practical Examples (Real-World Use Cases)
Example 1: Injection Molded ABS Enclosure
An electronics manufacturer needs to know how to calculate plastic part weight for a rectangular housing made of ABS plastic.
- Dimensions: 10cm (L) × 5cm (W) × 0.2cm (Thickness/Height effectively)
- Volume: 10 × 5 × 0.2 = 10 cm³
- Material: ABS (Density ≈ 1.04 g/cm³)
- Calculation: 10 cm³ × 1.04 g/cm³ = 10.4 grams
- Financial Impact: If resin costs $2.00/kg, the material cost per part is $0.0208.
Example 2: Nylon Gear Production
A factory plans to produce 1,000 cylindrical gears using Nylon 6.
- Dimensions: Radius 3cm, Height 1cm.
- Volume: π × 3² × 1 ≈ 28.27 cm³
- Material: Nylon 6 (Density ≈ 1.13 g/cm³)
- Single Weight: 28.27 × 1.13 = 31.95 grams
- Total Batch Weight: 31.95g × 1,000 = 31,950g (31.95 kg)
- Interpretation: The procurement team must order at least 32kg of raw Nylon pellets, plus a buffer for sprues and runners.
How to Use This Plastic Part Weight Calculator
- Select Shape: Choose the geometry that matches your part (Box, Cylinder, Tube, etc.).
- Enter Dimensions: Input the length, width, height, or radius in centimeters (cm).
- Choose Material: Select a common plastic from the dropdown (e.g., PP, ABS, PC) or enter a custom density if you are using a filled compound.
- Set Quantity: Enter the number of parts you intend to manufacture.
- Input Cost: Enter the current market price of the resin per kilogram to see financial estimates.
- Review Results: The calculator instantly updates the single part weight, total batch weight, and estimated material cost.
Key Factors That Affect Plastic Part Weight Results
When learning how to calculate plastic part weight, consider these six critical factors that influence the final mass and cost:
- Specific Gravity (Density): This is the most direct multiplier. A switch from PP (0.90) to PVC (1.40) increases weight by over 50% without changing dimensions.
- Wall Thickness: In injection molding, wall thickness drives volume. Even a 0.5mm increase across a large part can significantly increase total weight and cooling time.
- Fillers and Additives: Glass fibers, talc, or mineral fillers increase density. Glass-filled Nylon is significantly heavier than unfilled Nylon.
- Shrinkage Rates: Plastics shrink as they cool. While the mold is larger, the final part volume is smaller. Calculations should be based on the final part dimensions.
- Runners and Sprues: The calculated weight is usually the "net" weight. The "gross" weight required for manufacturing includes the feed system (runners), which can add 20-50% to material usage unless hot runners are used.
- Scrap Rate: Real-world manufacturing has a scrap rate (typically 2-5%). Financial calculations should account for this material loss.
Frequently Asked Questions (FAQ)
Does this calculator account for shrinkage?
No, this calculator determines the weight based on the final dimensions of the part. To account for mold dimensions, you would need to subtract the shrinkage rate from your mold inputs.
How do I calculate weight for complex shapes?
For complex organic shapes, it is best to use the "Custom Volume" option. You can obtain the volume (cm³) from your CAD software (SolidWorks, AutoCAD, Fusion 360) and input it directly.
Why is density important in cost calculation?
Plastic is sold by weight (kg or lb) but used by volume (to fill a mold). A lower density material yields more parts per kilogram, reducing the effective unit cost.
What is the difference between specific gravity and density?
In the context of plastics, they are often used interchangeably. Specific gravity is a ratio relative to water, while density is mass per unit volume. Since water is ~1 g/cm³, the numerical values are usually identical.
How does glass fiber affect weight?
Glass fiber has a density of ~2.54 g/cm³. Adding 30% glass fiber to Polypropylene (0.90 g/cm³) will significantly increase the composite density, making the part heavier and stiffer.
Can I calculate weight in pounds (lbs)?
This calculator uses metric units (grams/kg) as they are the industry standard for resin density. To convert: 1 kg = 2.20462 lbs.
Does colorant affect weight?
Masterbatch (colorant) is usually added at 1-3%. While some pigments (like Titanium Dioxide) are heavy, at such low ratios, the impact on total part weight is usually negligible for general estimation.
How accurate is this calculation?
The calculation is mathematically precise based on the inputs. However, real-world variations in wall thickness, packing pressure during molding, and material batch density can cause slight deviations.
' +
'' +
" +
'
';
}
// Handle Material Dropdown
function handleMaterialChange() {
var select = document.getElementById('materialSelect');
var customGroup = document.getElementById('customDensityGroup');
if (select.value === 'custom') {
customGroup.style.display = 'block';
density = parseFloat(document.getElementById('customDensity').value) || 0;
} else {
customGroup.style.display = 'none';
density = parseFloat(select.value);
}
calculateWeight();
}
// Main Calculation Logic
function calculateWeight() {
// 1. Get Inputs
var qty = parseFloat(document.getElementById('quantity').value) || 0;
var costPerKg = parseFloat(document.getElementById('materialCost').value) || 0;
// Update density if custom
if (document.getElementById('materialSelect').value === 'custom') {
density = parseFloat(document.getElementById('customDensity').value) || 0;
}
// 2. Calculate Volume
var volume = 0;
if (currentShape === 'box') {
var l = parseFloat(document.getElementById('dimL').value) || 0;
var w = parseFloat(document.getElementById('dimW').value) || 0;
var h = parseFloat(document.getElementById('dimH').value) || 0;
volume = l * w * h;
} else if (currentShape === 'cylinder') {
var r = parseFloat(document.getElementById('dimR').value) || 0;
var h = parseFloat(document.getElementById('dimH').value) || 0;
volume = Math.PI * r * r * h;
} else if (currentShape === 'tube') {
var r1 = parseFloat(document.getElementById('dimR1').value) || 0;
var r2 = parseFloat(document.getElementById('dimR2').value) || 0;
var h = parseFloat(document.getElementById('dimH').value) || 0;
// Ensure r1 > r2
var outer = Math.max(r1, r2);
var inner = Math.min(r1, r2);
volume = Math.PI * (outer * outer – inner * inner) * h;
} else if (currentShape === 'sphere') {
var r = parseFloat(document.getElementById('dimR').value) || 0;
volume = (4/3) * Math.PI * Math.pow(r, 3);
} else if (currentShape === 'custom') {
volume = parseFloat(document.getElementById('dimV').value) || 0;
}
// 3. Calculate Results
var singleWeightGrams = volume * density;
var totalWeightGrams = singleWeightGrams * qty;
var totalWeightKg = totalWeightGrams / 1000;
var totalCost = totalWeightKg * costPerKg;
// 4. Update UI
document.getElementById('volumeResult').innerText = volume.toFixed(2) + ' cm³';
document.getElementById('singleWeightResult').innerText = singleWeightGrams.toFixed(2) + ' g';
document.getElementById('totalWeightResult').innerText = totalWeightKg.toFixed(3) + ' kg';
document.getElementById('costResult').innerText = '$' + totalCost.toFixed(2);
// Update Table
updateTable(volume, density, singleWeightGrams, totalWeightKg, totalCost);
// Update Chart
drawChart(singleWeightGrams);
}
function updateTable(vol, den, singleW, totalW, cost) {
var tbody = document.getElementById('resultsTableBody');
tbody.innerHTML =
'