CFM Calculator (Air Flow)

To calculate airflow in CFM, multiply the air velocity in feet per minute (FPM) by the duct's cross-sectional area in square feet: CFM = FPM × ft². A 12-inch round duct has an area of 0.785 ft², so at 800 FPM it carries 628 CFM. Use the inside diameter and the average velocity across the duct, as a single centreline reading runs high. The calculator also gives m³/h and L/s.

Already have a figure to convert? Use the CFM to m³/h and m³/h to CFM converters.

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Air flow results

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How do I convert air velocity (m/s) to L/s or CFM?

Multiply the velocity by the duct's cross-sectional area. L/s = m/s × area in m² × 1,000. A 300 mm round duct has an area of 0.0707 m², so at 5 m/s it carries 353.4 L/s, which is 1,272 m³/h or 749 CFM. With imperial units, CFM = FPM × area in ft², and 1 m/s is 196.85 FPM.

How It Works

Use the calculator to turn an anemometer reading in a duct into CFM, m³/h or L/s, to check a design velocity against the ranges below, or read a duct size off the tables for a target airflow. It multiplies the average velocity by the duct's cross-sectional area and converts the units.

Air Flow Equation

Q = V × A

Where:

  • • Q = Volumetric flow rate (m³/s, CFM, or L/s)
  • • V = Average air velocity (m/s or ft/min)
  • • A = Duct cross-sectional area (m² or ft²)

Circular Duct Area

A = π × D² / 4

Where D is the internal duct diameter. For example, a 300 mm diameter circular duct has a cross-sectional area of 0.0707 m².

Rectangular Duct Area

A = W × H

Where W is duct width and H is duct height. A 400 mm × 200 mm rectangular duct has an area of 0.08 m².

Equivalent Circular Diameter

When comparing rectangular and circular ducts, the equivalent diameter for equal friction loss is:

D_eq = 1.30 × (W × H)^0.625 / (W + H)^0.250

Typical HVAC Velocity Ranges

Application m/s FPM
Residential supply ducts2 to 4400 to 800
Commercial supply ducts5 to 81,000 to 1,600
Main supply trunks6 to 101,200 to 2,000
Return air ducts3 to 5600 to 1,000
Exhaust ducts5 to 101,000 to 2,000
Industrial ventilation10 to 202,000 to 4,000

Higher velocities reduce duct size but increase noise and pressure drop. Branches and outlets serving occupied rooms are usually kept below about 7.5 m/s (1,500 FPM) to limit noise; ASHRAE sets the limit by duct location and the room's noise criterion.

Airflow in CFM by Round Duct Size

CFM = FPM × area in ft², for the inside diameter of a round duct. Read across at the velocity your design uses.

Duct (in) Area (ft²) 600 FPM 800 FPM 1,000 FPM
4 0.087 52 CFM 70 CFM 87 CFM
5 0.136 82 CFM 109 CFM 136 CFM
6 0.196 118 CFM 157 CFM 196 CFM
7 0.267 160 CFM 214 CFM 267 CFM
8 0.349 209 CFM 279 CFM 349 CFM
9 0.442 265 CFM 353 CFM 442 CFM
10 0.545 327 CFM 436 CFM 545 CFM
12 0.785 471 CFM 628 CFM 785 CFM
14 1.069 641 CFM 855 CFM 1,069 CFM
16 1.396 838 CFM 1,117 CFM 1,396 CFM
18 1.767 1,060 CFM 1,414 CFM 1,767 CFM
20 2.182 1,309 CFM 1,745 CFM 2,182 CFM
24 3.142 1,885 CFM 2,513 CFM 3,142 CFM

Airflow in L/s and m³/h by Round Duct Size

L/s = m/s × area in m² × 1,000, and m³/h = L/s × 3.6, for the inside diameter of a round duct.

Duct (mm) Area (m²) 2 m/s 3 m/s 5 m/s
100 0.0079 15.7 L/s (57 m³/h) 23.6 L/s (85 m³/h) 39.3 L/s (141 m³/h)
125 0.0123 24.5 L/s (88 m³/h) 36.8 L/s (133 m³/h) 61.4 L/s (221 m³/h)
150 0.0177 35.3 L/s (127 m³/h) 53.0 L/s (191 m³/h) 88.4 L/s (318 m³/h)
160 0.0201 40.2 L/s (145 m³/h) 60.3 L/s (217 m³/h) 100.5 L/s (362 m³/h)
200 0.0314 62.8 L/s (226 m³/h) 94.2 L/s (339 m³/h) 157.1 L/s (565 m³/h)
250 0.0491 98.2 L/s (353 m³/h) 147.3 L/s (530 m³/h) 245.4 L/s (884 m³/h)
315 0.0779 155.9 L/s (561 m³/h) 233.8 L/s (842 m³/h) 389.7 L/s (1,403 m³/h)
400 0.1257 251.3 L/s (905 m³/h) 377.0 L/s (1,357 m³/h) 628.3 L/s (2,262 m³/h)
500 0.1963 392.7 L/s (1,414 m³/h) 589.0 L/s (2,121 m³/h) 981.7 L/s (3,534 m³/h)
630 0.3117 623.4 L/s (2,244 m³/h) 935.2 L/s (3,367 m³/h) 1,558.6 L/s (5,611 m³/h)

FAQ

Common questions about calculating air flow in CFM, m³/h and L/s.

Frequently Asked Questions

How do I calculate CFM from duct size?

Multiply the air velocity in feet per minute (FPM) by the duct's cross-sectional area in square feet. A round duct's area is π × D² ÷ 4 with D in feet, so a 12-inch duct is 0.785 ft² and at 800 FPM carries 628 CFM. For a rectangular duct, multiply width by height: a 16 × 8 inch duct is 0.889 ft² and at 800 FPM carries 711 CFM.

How do I convert between CFM, m³/h and L/s?

1 CFM equals 1.699 m³/h, and 1 m³/h equals 0.5886 CFM. For L/s, 1 CFM is 0.4719 L/s and 1 L/s is 2.119 CFM.

What units is air flow measured in?

Volumetric air flow is measured in CFM (cubic feet per minute) in the US, m³/h in most of Europe, and L/s in the UK, Australia and many ISO standards; large plant also uses m³/s. Air velocity is measured in m/s or FPM, and it becomes a flow rate only once it is multiplied by the duct area. Psychrometric and heat load calculations work in mass flow (kg/s or lb/h), which stays the same as the air warms or cools.

How many CFM does a room need?

Multiply the room volume in cubic feet by the air changes per hour (ACH) you need, then divide by 60: CFM = ft³ × ACH ÷ 60. A 10 × 10 ft room with an 8 ft ceiling is 800 ft³, so 6 air changes an hour needs 800 × 6 ÷ 60 = 80 CFM. Take the ACH from the ventilation standard or the duty the room serves.

What velocities are typical in HVAC ducts?

Residential supply ducts typically run at 2 to 4 m/s (400 to 800 FPM), commercial supply ducts at 5 to 8 m/s (1,000 to 1,600 FPM), and industrial ventilation at 10 to 20 m/s (2,000 to 4,000 FPM). Branches and outlets serving occupied rooms are usually kept below about 7.5 m/s (1,500 FPM) to limit noise; ASHRAE sets the limit by duct location and the room's noise criterion.

How does air temperature affect flow calculations?

The calculator works in volumetric flow, taken from the velocity and area you enter, so temperature does not change its result. The mass of air moved does change: air density is about 1.225 kg/m³ at 15 °C and 1.127 kg/m³ at 40 °C at sea level. For heating and cooling loads, account for the density difference between supply and return air.

What is the relationship between air flow and pressure drop in ducts?

Pressure drop in a duct rises roughly with the square of velocity, so doubling the air velocity roughly quadruples the pressure drop. Smaller ducts save material but need more fan power to overcome the higher pressure drop, so duct size is a balance against fan energy.

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