Duct Airflow Calculator (CFM)
Calculate duct airflow in CFM, m³/h and L/s for round or rectangular ducts from air velocity, pitot velocity pressure or a design friction rate.
Airflow is area-averaged velocity multiplied by a duct’s clear internal cross-sectional area. This calculator uses that identity for round or rectangular ducts, or estimates flow from an effective velocity pressure or an empirical straight-metal-duct friction rate. Friction mode estimates one section under stated assumptions; duct size alone does not determine what a connected fan will deliver.
How to use this tool
- Pick round or rectangular and enter the inside dimensions in inches.
- Choose what you know: air velocity, velocity pressure or friction rate.
- Enter the value from your meter or design.
- Read the airflow in CFM, m³/h and L/s, and compare duct sizes in the table.
Formula
Q = V × A; V = 4005 × √VP; at friction rate f: Q = (f × d^5.02 ÷ 0.109136)^(1/1.9)
Q is CFM, V is ft/min, and area is πd²/576 or wh/144 in ft² for inch dimensions. VP is inches of water; 4005 assumes density 0.075 lb/ft³. Rectangular friction uses Huebscher equivalent diameter. 1 CFM = 1.699 m³/h = 0.472 L/s. Zero velocity or pressure gives zero flow; the friction fit assumes turbulent flow.
Example
A 12-inch duct has area π/4 = 0.7854 ft². At 800 ft/min, flow is 628.32 CFM, approximately 1,068 m³/h. For an 8-inch duct at effective velocity pressure 0.04 in. w.c., velocity is 4005 × 0.2 = 801 ft/min and flow is 279.60 CFM.
Frequently asked questions
How do I measure duct velocity?
Use a suitable velocity or Pitot traverse across the duct at specified locations, away from flow disturbances. Convert individual pressure readings to velocities before area-weighted averaging; the square root of an arbitrary mean pressure is not generally the mean velocity. One center reading is insufficient.
How many CFM does a 6-inch duct carry?
The empirical friction option gives roughly 95–100 CFM for a 6-inch metal duct at 0.08 in. w.c. per 100 ft. Actual flow also depends on fan pressure, fittings, terminals, leakage and the whole system. Flex duct requires its manufacturer’s installed pressure-loss data.
Can I use a reading taken at the grille?
Not with the duct size. Grilles have small, uneven openings, so the velocity at the face is not the duct velocity. You need the grille's free area from the manufacturer, or a flow hood that measures CFM directly.
Does air temperature matter?
Yes. The velocity-pressure calculation assumes standard density 0.075 lb/ft³. At another density, velocity is multiplied by sqrt(standard density / actual density). This tool does not apply that correction, so avoid treating its pressure result as a corrected high-altitude measurement.
Sources
Last updated: 2026-10-30