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Duct Velocity Calculator – HVAC Airflow Design Tool .duct-calc-container { font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, Helvetica, Arial, sans-serif; max-width: 800px; margin: 20px auto; padding: 25px; border: 1px solid #e1e1e1; border-radius: 8px; background-color: #f9f9f9; box-shadow: 0 4px 6px rgba(0,0,0,0.05); } .duct-calc-header { text-align: center; margin-bottom: 25px; } .duct-calc-header h2 { color: #2c3e50; margin-bottom: 10px; } .duct-calc-row { margin-bottom: 20px; } .duct-calc-row label { display: block; font-weight: 600; margin-bottom: 8px; color: #34495e; } .duct-calc-row input, .duct-calc-row select { width: 100%; padding: 12px; border: 1px solid #ccc; border-radius: 4px; box-sizing: border-box; font-size: 16px; } .duct-calc-btn { width: 100%; padding: 15px; background-color: #27ae60; color: white; border: none; border-radius: 4px; font-size: 18px; font-weight: bold; cursor: pointer; transition: background-color 0.3s; } .duct-calc-btn:hover { background-color: #219150; } .duct-calc-result { margin-top: 25px; padding: 20px; background-color: #ffffff; border-left: 5px solid #27ae60; display: none; } .duct-calc-result h3 { margin: 0 0 10px 0; color: #2c3e50; } .velocity-value { font-size: 28px; font-weight: 800; color: #27ae60; } .duct-article { margin-top: 40px; line-height: 1.6; color: #333; } .duct-article h2 { color: #2c3e50; border-bottom: 2px solid #eee; padding-bottom: 10px; } .duct-article h3 { color: #2980b9; } .duct-article table { width: 100%; border-collapse: collapse; margin: 20px 0; } .duct-article table th, .duct-article table td { border: 1px solid #ddd; padding: 12px; text-align: left; } .duct-article table th { background-color: #f4f4f4; } .hidden { display: none; }

Duct Velocity Calculator

Calculate airflow speed (FPM) based on CFM and duct dimensions.

Rectangular / Square Round

Calculated Duct Velocity:

0 FPM

Understanding Duct Velocity in HVAC Systems

Duct velocity is the speed at which air travels through the ductwork of a heating, ventilation, and air conditioning (HVAC) system. It is measured in Feet Per Minute (FPM). Calculating and maintaining the correct velocity is critical for ensuring system efficiency, minimizing noise, and managing static pressure.

The Duct Velocity Formula

The relationship between airflow volume, duct area, and velocity is expressed by the fundamental equation:

Velocity (V) = Flow Rate (Q) / Area (A)

Where:

  • V (Velocity): Airspeed in FPM.
  • Q (Flow Rate): Airflow volume in CFM.
  • A (Area): Cross-sectional area of the duct in square feet.

Recommended Velocity Limits

Standard design practices suggest specific velocity ranges depending on the application to prevent excessive noise (rushing air sounds) and high energy costs. Below are common industry standards:

Application Recommended FPM (Main Ducts) Recommended FPM (Branch Ducts)
Residential 700 – 900 600
Commercial (Schools/Offices) 1,000 – 1,500 800 – 1,000
Industrial 1,500 – 2,500 1,200 – 1,800

Why Does Velocity Matter?

If the velocity is too high, you will experience significant air noise and higher friction losses, which forces the blower motor to work harder, increasing energy consumption. If the velocity is too low, the air may not reach the intended rooms effectively, and in heating applications, it may cool down too much before reaching the register.

Practical Example

Imagine you have a residential furnace moving 1,200 CFM through a main trunk line that is 20 inches wide and 10 inches high.

  1. Calculate Area in Sq. Inches: 20″ x 10″ = 200 sq. inches.
  2. Convert to Sq. Feet: 200 / 144 = 1.389 sq. feet.
  3. Calculate Velocity: 1,200 CFM / 1.389 sq. ft = 864 FPM.

Since 864 FPM falls within the 700-900 FPM range for residential main ducts, this design is considered efficient and quiet.

function toggleDuctInputs() { var shape = document.getElementById('ductShape').value; var rectDiv = document.getElementById('rectangularInputs'); var roundDiv = document.getElementById('roundInputs'); if (shape === 'rectangular') { rectDiv.style.display = 'block'; roundDiv.style.display = 'none'; } else { rectDiv.style.display = 'none'; roundDiv.style.display = 'block'; } } function calculateDuctVelocity() { var cfm = parseFloat(document.getElementById('airVolume').value); var shape = document.getElementById('ductShape').value; var areaSqFt = 0; if (isNaN(cfm) || cfm <= 0) { alert("Please enter a valid Airflow Volume (CFM)."); return; } if (shape === 'rectangular') { var width = parseFloat(document.getElementById('ductWidth').value); var height = parseFloat(document.getElementById('ductHeight').value); if (isNaN(width) || isNaN(height) || width <= 0 || height <= 0) { alert("Please enter valid width and height dimensions."); return; } // Area in sq ft = (Width * Height) / 144 areaSqFt = (width * height) / 144; } else { var diameter = parseFloat(document.getElementById('ductDiameter').value); if (isNaN(diameter) || diameter 1500) { recText.innerHTML = "Note: This velocity is quite high for standard commercial/residential applications. Consider increasing duct size to reduce noise and static pressure."; } else if (velocity < 400) { recText.innerHTML = "Note: This velocity is very low. Ensure your system has enough pressure to distribute air to the furthest registers."; } else { recText.innerHTML = "This velocity is within standard operating ranges for most HVAC systems."; } resultDiv.scrollIntoView({behavior: 'smooth', block: 'nearest'}); }

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