FPM (feet per minute) to CFM (cubic feet per minute) Calculator

Convert FPM to CFM by multiplying air velocity by duct area, or enter a round or rectangular duct size. Also works backward from CFM to FPM.

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Calculator Functions

Function	What it does	What it requires
Flow from area and velocity	Converts FPM to CFM for an opening of known area	Requires the average air velocity and the clear cross-sectional area
Velocity from flow and area	Converts CFM to FPM for an opening of known area	Requires the volume flow rate and the clear cross-sectional area
Area from flow and velocity	Finds the clear area that passes a set flow at a set velocity	Requires the volume flow rate and the average air velocity
Flow through a round duct	Converts FPM to CFM in a round duct, working out the area from the diameter	Requires the inside duct diameter and the average air velocity
Velocity through a round duct	Converts CFM to FPM in a round duct	Requires the inside duct diameter and the volume flow rate
Flow through a rectangular duct	Converts FPM to CFM in a rectangular duct, working out the area from width and height	Requires the inside duct width, the inside duct height and the average air velocity
Velocity through a rectangular duct	Converts CFM to FPM in a rectangular duct	Requires the inside duct width, the inside duct height and the volume flow rate

Velocity can be entered in ft/min, ft/s, m/s or m/min, area in ft², in², m² or yd², flow in CFM or m³/min, and duct dimensions in inches or millimeters. The round duct functions offer 4, 6, 8, 10 and 12 inch diameters as quick picks. Every result also lists the cross-sectional area in ft² and m², the velocity in m/min and m/s, and the flow in m³/min.

FPM to CFM Formula

FPM is a speed and CFM is a volume per minute, so FPM cannot be turned into CFM without an area. 100 FPM is 100 CFM through 1 ft², 200 CFM through 2 ft² and about 35 CFM through an 8 inch round duct. CFM and ft³/min are the same unit, so no conversion is needed between them.

Flow from area and velocity

The formula above applies directly: 500 FPM through 2 ft² is 500 × 2 = 1,000 CFM. A velocity in feet per second is multiplied by 60 first, so 10 ft/s is 600 FPM. Metric entries are converted with 1 ft = 0.3048 m exactly [2].

Velocity from flow and area

This is the CFM to FPM conversion. 1,000 CFM through 2 ft² moves at 1,000 ÷ 2 = 500 FPM. The same flow through a smaller opening moves faster.

Area from flow and velocity

1,200 CFM at 800 FPM needs 1,200 ÷ 800 = 1.5 ft² of clear area. A velocity of zero is rejected, because no area can pass a flow at zero speed.

Flow through a round duct

D is the inside diameter in feet, so a diameter in inches is divided by 12 first. An 8 inch duct has an area of π × 0.6667² ÷ 4 = 0.3491 ft², and 1,000 FPM through it is 349.07 CFM.

FPM	CFM through 1 ft²	CFM in a 6 in round duct	CFM in an 8 in round duct	CFM in a 12 in round duct
100	100	20	35	79
300	300	59	105	236
400	400	79	140	314
500	500	98	175	393
600	600	118	209	471
700	700	137	244	550
800	800	157	279	628
900	900	177	314	707
1,000	1,000	196	349	785
1,200	1,200	236	419	942
1,500	1,500	295	524	1,178
2,000	2,000	393	698	1,571
5,000	5,000	982	1,745	3,927

Round duct values are rounded to the nearest whole CFM. For another size, multiply the 1 ft² column by the duct area in square feet.

Velocity through a round duct

The area comes from the diameter as above, then FPM = CFM ÷ A. 400 CFM in an 8 inch duct moves at 400 ÷ 0.3491 = 1,145.92 FPM.

Flow through a rectangular duct

W and H are the inside width and height in feet. A 12 by 8 inch duct has an area of 1 × 0.6667 = 0.6667 ft², and 900 FPM through it is 600 CFM.

Velocity through a rectangular duct

The area comes from width times height, then FPM = CFM ÷ A. 600 CFM in a 12 by 8 inch duct moves at 600 ÷ 0.6667 = 900 FPM.

Limits of This Estimate

• The velocity must be the average across the opening. Air moves fastest at the center of a duct, and in a straight round duct the average is about 90% of a centerline reading [1].
• The result is actual CFM at the temperature and pressure of the air being measured. It is not standard CFM (SCFM), which is corrected to dry air at 68 °F and 29.92 in Hg [1], and no density correction is made.
• Area means the clear inside opening. Grilles, louvers, liners and insulation reduce it, and nominal duct sizes can differ from the inside dimension.

Sources