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Airflow CFM Calculator (Velocity + Area)

Convert duct velocity and cross-sectional area into volumetric CFM delivery. Supports anemometer FPM readings or Pitot tube differential pressure with live ASHRAE pathway compliance evaluation.

Airflow CFM Inputs

Metric Capture Method

Velocity Source

Velocity Performance

Duct Geometry Profile

System Designation

Live Results

Volumetric Airflow Delivery

500.0 CFM

Balanced HVAC Airflow — Velocity parameters align perfectly with ASHRAE standards for your system pathway. Distribution profile is quiet and efficient.

Calculated Cross-Sectional Area

0.7 Sq. Ft.

Evaluated Circuit Air Speed

750.0 FPM

Target Operational Boundary: 700 - 900 FPM

How to Use This Airflow CFM Calculator

  1. Choose your velocity capture method. Toggle between Direct Velocity (FPM) for anemometer readings or Pitot Tube Pressure (in. w.c.) for differential pressure measurements. Pitot tube velocity is derived using V = 4005 × √(VP).
  2. Enter velocity performance data. For direct FPM mode, enter the measured airflow speed from your anemometer. For Pitot tube mode, enter the velocity pressure in inches of water column — the calculator converts this to equivalent FPM automatically.
  3. Define duct geometry. Select Rectangular or Round Duct shape, then enter cross-section dimensions. Rectangular area uses (width × height) ÷ 144 sq ft; round area uses (π × D²) ÷ 576 sq ft.
  4. Select the mechanical application pathway. Choose Supply Air Line (700–900 FPM), Return Air Duct (600–800 FPM), or Exhaust Air Vent (500–700 FPM). These ASHRAE-aligned velocity bands drive the compliance badge evaluation.
  5. Review CFM delivery and balancing compliance. The Live Results panel shows volumetric CFM, cross-sectional area, evaluated air speed, target operational boundary, and a RED/YELLOW/GREEN system air balancing compliance badge.

Formulas & Example

Volumetric airflow combines measured or derived velocity with duct cross-sectional area, then evaluates the result against pathway-specific velocity targets for balanced HVAC distribution.

Rectangular Area (sq ft) = (width × height) ÷ 144
Round Area (sq ft) = (π × D²) ÷ 576   [D = diameter, inches]

Pitot Velocity (FPM) = 4005 × √(VP)   [VP = velocity pressure, in. w.c.]
CFM = active velocity (FPM) × cross-sectional area (sq ft)

Pathway velocity targets:
  Supply Air Line:   700 – 900 FPM
  Return Air Duct:   600 – 800 FPM
  Exhaust Air Vent:  500 – 700 FPM

Compliance:
  RED    → velocity > max
  YELLOW → velocity ≥ 90% of max, or velocity < min
  GREEN  → velocity within balanced range

Worked Example

750 FPM direct reading, 12" × 8" rectangular duct, Supply Air Line pathway:

Area = (12 × 8) ÷ 144 = 0.7 sq ft
CFM = 750 × 0.7 = 525.0 CFM

Velocity 750 FPM is within 700–900 FPM supply target → GREEN badge

Switch to Pitot tube mode at 0.035 in. w.c. and the derived velocity is 4005 × √0.035 ≈ 749.0 FPM — producing nearly identical CFM output for cross-checking field instrumentation.

Frequently Asked Questions

How is CFM calculated from velocity and duct area?▾
CFM equals air velocity in FPM multiplied by cross-sectional area in square feet. For rectangular ducts: Area = (width × height) ÷ 144. For round ducts: Area = (π × D²) ÷ 576, where D is diameter in inches. Example: 750 FPM through a 12" × 8" duct (0.7 sq ft) yields 525 CFM.
How does the Pitot tube velocity conversion work?▾
When using Pitot tube differential pressure, velocity is derived as V (FPM) = 4005 × √(VP), where VP is velocity pressure in inches of water column. At 0.035 in. w.c., this yields approximately 749 FPM — matching a typical anemometer reading on a supply branch line.
What velocity ranges apply to each application pathway?▾
Supply Air Line targets 700–900 FPM, Return Air Duct targets 600–800 FPM, and Exhaust Air Vent targets 500–700 FPM. These bands reflect ASHRAE trade practice for quiet distribution, adequate throw, and balanced static pressure across residential and light commercial systems.
What do the RED, YELLOW, and GREEN compliance badges mean?▾
Red indicates velocity exceeds the pathway maximum — risking duct whistling and restricted register delivery. Yellow flags under-velocity (poor throw, stagnant zones) or upper-threshold boundary (≥90% of max). Green confirms velocity sits within the ideal balanced range for the selected pathway.
Can I use this calculator for round and rectangular ductwork?▾
Yes. Select Rectangular and enter width and height in inches, or select Round Duct and enter the physical diameter. The calculator computes the correct cross-sectional area for either geometry and multiplies by your active velocity to produce CFM.

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