Free Conversion Tool

CFM to m³/h Converter — Airflow, Pa to Inches of Water & Velocity Units

The three unit systems fan datasheets use — CFM (US), m³/h (Europe/China) and L/s (UK/AU, ASHRAE SI) — converted exactly, together with static pressure and air velocity. Type any one value; everything else updates.

No signup · Works on mobile · Formulas shown below · Updated 2026-08-23

Convert airflow, pressure and velocity

Enter a value in any box of a group; the others recalculate. The last group computes airflow from velocity and duct size.

Results

Engineering estimate for preliminary sizing. Verify against the fan curve, local codes and your system measurements before purchase. Ask a Longwell engineer to check your duty point free of charge.

Exact conversion factors

From	To	Multiply by	Quick mental check
CFM	m³/h	1.699 01	CFM × 1.7
m³/h	CFM	0.588 58	m³/h × 0.6
CFM	L/s	0.471 947	CFM ÷ 2
L/s	CFM	2.118 88	L/s × 2
m³/h	L/s	0.277 78	m³/h ÷ 3.6
m³/s	CFM	2 118.88
Pa	in. w.g.	0.004 014 63	250 Pa ≈ 1 in. w.g.
in. w.g.	Pa	249.089	0.1 in ≈ 25 Pa
mm H₂O	Pa	9.806 65	≈ ×10
psi	Pa	6 894.76
fpm	m/s	0.005 08	1,000 fpm ≈ 5 m/s
m/s	fpm	196.85	m/s × 200

Airflow = Velocity × Area // m³/s = m/s × m²; CFM = fpm × ft²
Velocity pressure p_v = ½ ρ v² // Pa, with ρ = 1.2 kg/m³; in. w.g. = (fpm ÷ 4005)²

Why fan datasheets disagree (and what to check)

CMH vs CMM. Asian datasheets sometimes quote m³/min (CMM). 60× difference — if a 120 mm fan claims "3 CMM" that is 180 m³/h, ~106 CFM.
Free-air vs working point. Max airflow is at 0 Pa. Max pressure is at 0 airflow. Neither is your operating point; read the curve.
Standard air. CFM/m³/h is volumetric. At altitude or high temperature, the same volume carries less mass → less cooling. Use the altitude correction tool.
Static vs total pressure. European catalogs usually plot static pressure (p_sf); some plot total. Total = static + velocity pressure at the outlet; the difference matters for high-velocity plug fans.
SCFM and ACFM appear in compressed-air and combustion work: SCFM is corrected to standard conditions, ACFM is actual. Fan catalogs use ACFM unless stated.

Frequently Asked Questions

How do I convert CFM to m³/h?

Multiply CFM by 1.699. 1,000 CFM = 1,699 m³/h. To go the other way, multiply m³/h by 0.5886 (or divide by 1.7 for a quick estimate).

What is 1 inch of water in Pascals?

1 inch of water gauge (in. w.g. / in. H₂O at 4 °C) = 249.089 Pa. A handy approximation is 250 Pa. 0.5 in. w.g. ≈ 125 Pa, which is a typical external static pressure for residential ducted systems.

How do I calculate airflow from velocity and duct size?

Airflow = velocity × cross-sectional area. A 250 mm round duct has an area of 0.0491 m²; at 5 m/s it carries 0.245 m³/s = 884 m³/h = 520 CFM. In imperial units, CFM = fpm × ft².

What is the difference between L/s and CFM?

1 L/s = 2.119 CFM. L/s is used in the UK, Australia and ASHRAE SI standards; CFM in North America. A 50 CFM bathroom fan is about 24 L/s.

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