How to Calculate Volume Flow Rate of Air

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Air Volume Flow Rate Calculator

Calculate airflow (CFM) based on duct size and air velocity.

Rectangular / Square Round / Circular
Typical residential HVAC velocity is 400-900 FPM.
Volume Flow Rate (Imperial)
0 CFM
Volume Flow Rate (Metric)
0 m³/hr
0 L/s
Duct Cross-Sectional Area
0 sq. ft.

How to Calculate Volume Flow Rate of Air

Understanding how to calculate the volume flow rate of air is essential for HVAC technicians, engineers, and building maintenance professionals. The volume flow rate determines how much air is moving through a duct system over a specific period, directly impacting heating and cooling efficiency, ventilation adequacy, and energy consumption.

The Core Formula

The calculation of air volume flow rate is based on the relationship between the area of the duct and the speed at which the air is traveling. The fundamental equation used in fluid dynamics for this purpose is:

Q = A × v

Where:

  • Q = Volume Flow Rate (typically measured in Cubic Feet per Minute, or CFM).
  • A = Cross-sectional Area of the duct (measured in Square Feet, or ft²).
  • v = Velocity of the air (measured in Feet Per Minute, or FPM).

Step-by-Step Calculation Guide

1. Determine the Duct Area

Most ductwork is measured in inches, but the velocity is in feet per minute. Therefore, you must first convert the duct dimensions into square feet.

For Rectangular Ducts:

Measure the width and height in inches.
Area (ft²) = (Width inches × Height inches) ÷ 144

For Round Ducts:

Measure the diameter in inches.
Area (ft²) = π × (Diameter inches ÷ 12 ÷ 2)²
Or more simply: Area (ft²) = (Diameter inches)² × 0.005454

2. Measure Air Velocity

Use an anemometer or pitot tube to measure the velocity of the air moving through the duct. Velocity is usually expressed in Feet Per Minute (FPM). For accurate results, take multiple readings across the cross-section of the duct and calculate the average velocity.

3. Calculate the Flow Rate (Q)

Multiply the calculated Area (in square feet) by the Average Velocity (in FPM). The result is your airflow volume in CFM.

Example Calculation

Suppose you have a rectangular duct measuring 12 inches by 10 inches, and your anemometer reads an air velocity of 600 FPM.

  1. Calculate Area: (12 × 10) ÷ 144 = 120 ÷ 144 = 0.833 sq. ft.
  2. Multiply by Velocity: 0.833 sq. ft. × 600 FPM = 500 CFM.

The volume flow rate of air in this system is 500 Cubic Feet per Minute.

Why Is This Calculation Important?

  • HVAC Balancing: Ensures each room receives the correct amount of conditioned air.
  • Equipment Sizing: Verifies that fans and blowers are not undersized or oversized for the ductwork.
  • Air Quality: Necessary for calculating Air Changes Per Hour (ACH) to maintain healthy indoor air quality.
function toggleDuctInputs() { var shape = document.getElementById('ductShape').value; var rectInputs = document.getElementById('rectangularInputs'); var roundInputs = document.getElementById('roundInputs'); if (shape === 'rectangular') { rectInputs.classList.remove('avfr-hidden'); roundInputs.classList.add('avfr-hidden'); } else { rectInputs.classList.add('avfr-hidden'); roundInputs.classList.remove('avfr-hidden'); } } function calculateAirFlow() { // Inputs var shape = document.getElementById('ductShape').value; var velocity = parseFloat(document.getElementById('airVelocity').value); var areaSqFt = 0; // Error flag var isValid = true; if (isNaN(velocity) || velocity < 0) { isValid = false; } // Calculate Area based on shape 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) { isValid = false; } else { // Width * Height (sq inches) / 144 = sq feet areaSqFt = (width * height) / 144; } } else { var diameter = parseFloat(document.getElementById('ductDiameter').value); if (isNaN(diameter) || diameter <= 0) { isValid = false; } else { // Formula: Pi * r^2. r = (diameter/12)/2 feet. var radiusFeet = (diameter / 12) / 2; areaSqFt = Math.PI * (radiusFeet * radiusFeet); } } // Display logic var resultsDiv = document.getElementById('resultsArea'); if (isValid) { // Calculation: Q = A * V var cfm = areaSqFt * velocity; // Metric Conversions // 1 CFM = 1.69901 m3/hr // 1 CFM = 0.471947 L/s var cmh = cfm * 1.69901; var lps = cfm * 0.471947; // Update DOM document.getElementById('resCFM').innerHTML = cfm.toFixed(1) + " CFM"; document.getElementById('resCMH').innerHTML = cmh.toFixed(1) + " m³/hr"; document.getElementById('resLPS').innerHTML = lps.toFixed(1) + " L/s"; document.getElementById('resArea').innerHTML = areaSqFt.toFixed(3) + " sq. ft."; resultsDiv.style.display = "block"; } else { alert("Please enter valid positive numbers for all dimensions and velocity."); resultsDiv.style.display = "none"; } }

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