How to Calculate Air Flow Rate in Cfm

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Air Flow Rate (CFM) Calculator

Rectangular Duct Round Duct
Feet Per Minute (measured with an anemometer)

Calculation Results

Cross-Sectional Area: 0 sq. ft.

Air Flow Rate: 0 CFM

Please enter valid numbers for all fields.

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"; } // Hide results when switching to avoid confusion document.getElementById("resultSection").style.display = "none"; document.getElementById("errorMsg").style.display = "none"; } function calculateCFM() { var velocity = parseFloat(document.getElementById("airVelocity").value); var shape = document.getElementById("ductShape").value; var areaSqFt = 0; var isValid = true; if (isNaN(velocity) || velocity <= 0) { isValid = false; } if (isValid) { 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 { // Convert inches to square feet: (W * H) / 144 areaSqFt = (width * height) / 144; } } else { var diameter = parseFloat(document.getElementById("ductDiameter").value); if (isNaN(diameter) || diameter <= 0) { isValid = false; } else { // Convert diameter to radius var radius = diameter / 2; // Area in sq inches = PI * r^2 var areaSqIn = Math.PI * (radius * radius); // Convert to sq feet areaSqFt = areaSqIn / 144; } } } var errorMsg = document.getElementById("errorMsg"); var resultSection = document.getElementById("resultSection"); if (!isValid) { errorMsg.style.display = "block"; resultSection.style.display = "none"; } else { errorMsg.style.display = "none"; resultSection.style.display = "block"; // Calculate CFM = Area (sq ft) * Velocity (FPM) var cfm = areaSqFt * velocity; document.getElementById("resultArea").innerHTML = areaSqFt.toFixed(3); document.getElementById("resultCFM").innerHTML = Math.round(cfm); } }

How to Calculate Air Flow Rate in CFM

Calculating the air flow rate in CFM (Cubic Feet per Minute) is a fundamental skill in HVAC engineering, balancing ventilation systems, and ensuring proper air exchange in residential or industrial spaces. CFM represents the volume of air moving through a duct or space in one minute. Accurate calculation ensures your heating, cooling, and ventilation systems operate efficiently.

The Basic Formula for Air Flow (CFM)

The standard equation for calculating air flow rate is derived from the continuity equation of fluid dynamics. To find the CFM, you simply multiply the velocity of the air by the cross-sectional area of the duct through which it flows.

Formula:
Q = A × V
  • Q = Air Flow Rate (CFM – Cubic Feet per Minute)
  • A = Cross-Sectional Area (Square Feet)
  • V = Air Velocity (FPM – Feet per Minute)

Step-by-Step Calculation Guide

1. Measure Air Velocity

First, you need to know how fast the air is moving. This is typically measured using an anemometer. The standard unit for this measurement is Feet Per Minute (FPM). For accurate results, take multiple readings across the duct face and average them to account for turbulence.

2. Calculate the Cross-Sectional Area

The trickiest part of the equation is often calculating the area (A), primarily because duct dimensions are usually measured in inches, but the formula requires the area in Square Feet.

For Rectangular Ducts:

  • Measure the Width (W) and Height (H) in inches.
  • Calculate area in square inches: Area_sq_in = Width × Height
  • Convert to square feet: Area_sq_ft = Area_sq_in ÷ 144

For Round Ducts:

  • Measure the Diameter (D) in inches.
  • Calculate the radius (r): r = D ÷ 2
  • Calculate area in square inches: Area_sq_in = π × r² (where π ≈ 3.14159)
  • Convert to square feet: Area_sq_ft = Area_sq_in ÷ 144

3. Compute the Final CFM

Once you have the Area in square feet and the Velocity in FPM, multiply them together to get the CFM.

Real-World Example

Let's say you are measuring a rectangular HVAC duct that is 12 inches wide and 10 inches high. Using an anemometer, you measure the average air velocity to be 600 FPM.

  1. Calculate Area in Sq Inches: 12 × 10 = 120 sq inches.
  2. Convert to Sq Feet: 120 ÷ 144 = 0.833 sq ft.
  3. Calculate CFM: 0.833 sq ft × 600 FPM = 500 CFM.

Why is CFM Important?

Correct airflow is critical for system performance. Low CFM can lead to frozen evaporator coils in AC units or overheating heat exchangers in furnaces. High CFM can result in noise issues and inability to properly dehumidify the air. Using a calculator like the one above helps technicians verify that the installed ductwork matches the equipment specifications.

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