Air Velocity Conversion

CFM to FPM Calculator — Air Velocity Converter

Convert CFM to FPM (or FPM back to CFM) for any round or rectangular duct, with live results in ft/s, m/s, MPH, km/h and LFM. Enter the duct size and the tool computes the area for you — no formula lookup required.

CFM ↔ FPM converter

Airflow — CFM

Duct shape

Duct diameter — inches

Result

Normal

718 FPM

141 CFM through 0.196 sq ft of duct

FPM

718

ft/s (FPS)

12.0

m/s

3.65

MPH

8.2

km/h

13.1

LFM

718

Typical residential duct design range — good balance of airflow and noise.

Measured Scenarios

Six real ducts, converted both ways

Every card below is an exact conversion — the airflow a fan actually delivers through the duct listed, at the velocity the geometry forces. Use the filter to isolate the quiet cards (≤ 900 FPM, the residential design band) or the noisy ones, and check your own project against the closest card.

Highlight cards:

Hall bathroom exhaust

6" round duct

Quiet Zone ✓

545 FPM

8 ACH on a 10 × 10 × 8 ft bath

Duct area0.196 sq ft
Velocity546 FPM
Speed6.2 mph · 2.78 m/s

Kitchen range hood

8" round duct

Quiet Zone ✓

716 FPM

capture rule for a 30-inch cooktop

Duct area0.349 sq ft
Velocity716 FPM
Speed8.1 mph · 3.64 m/s

4 × 4 grow tent, filtered

10" round duct

Quiet Zone ✓

733 FPM

30 ACH + carbon filter margin

Duct area0.545 sq ft
Velocity734 FPM
Speed8.3 mph · 3.73 m/s

Central return-air trunk

14" round trunk

Quiet Zone ✓

842 FPM

3-ton system, 300 CFM per ton return

Duct area1.069 sq ft
Velocity842 FPM
Speed9.6 mph · 4.28 m/s

One-car garage exhaust

8" round duct

Noisy

1,003 FPM

6 ACH on a 12 × 20 × 8 ft garage

Duct area0.349 sq ft
Velocity1,003 FPM
Speed11.4 mph · 5.10 m/s

Clothes dryer vent (max spec)

4" round duct

Noisy

1,833 FPM

typical dryer rating, before lint load

Duct area0.087 sq ft
Velocity1,833 FPM
Speed20.8 mph · 9.31 m/s

The FPM to CFM formula (and the reverse)

Airflow and velocity are the same fact viewed two ways, joined by the duct's free area. CFM counts the cubic feet of air that pass a point each minute; FPM counts the feet that air travels each minute. Multiply the travel speed by the opening it flows through and you get the volume — that is the entire relationship:

Velocity from flow

FPM = CFM ÷ Area (sq ft)

Divide the airflow by the duct's cross-section. This is the direction you use to check noise before buying a fan.

Flow from velocity

CFM = FPM × Area (sq ft)

The anemometer direction: measure speed, multiply by the opening, get the delivery. This is how field airflow is verified.

Duct area

A = π·D² ÷ 4 ÷ 144

For round ducts, in square feet. For rectangular ducts it is width × height ÷ 144 — the calculator does both automatically.

Worked conversions you can check by hand

100 FPM to CFM on a 6-inch duct

Area = π × 6² ÷ 4 ÷ 144 = 0.196 sq ft. Flow = 100 × 0.196 = 20 CFM. Slow-moving air like this suits filter media and ERV cores, not exhaust runs.

304 FPM to CFM on an 8-inch duct

Area = π × 8² ÷ 4 ÷ 144 = 0.349 sq ft. Flow = 304 × 0.349 = 106 CFM — a quiet, low-friction delivery for a bathroom fan running below its rating.

Calculate CFM from 600 FPM on a 10-inch duct

Area = 0.545 sq ft. Flow = 600 × 0.545 = 327 CFM. The 600 FPM start of the quiet band is where economical duct sizing begins.

141 CFM to FPM through a 3.25 × 10 wall stack

Area = 3.25 × 10 ÷ 144 = 0.226 sq ft. Velocity = 141 ÷ 0.226 = 625 FPM — the classic wall stack just clears the quiet band. Its ASHRAE equivalent round diameter is 6.1 inches.

Technician measuring airflow inside a ventilation duct with a handheld anemometer

Measure the speed, multiply by the openingPhoto: Unsplash

Round duct CFM ↔ FPM table

Each cell is CFM at that velocity for the diameter in the left column — read it both directions. The 600 and 900 columns bound the quiet design band; the 1,200 column is the residential noise ceiling.

Duct (round)	Area (sq ft)	100 FPM	300 FPM	600 FPM	900 FPM	1,200 FPM
4 inch	0.087	9	26	52	79	105
5 inch	0.136	14	41	82	123	164
6 inch	0.196	20	59	118	177	236
7 inch	0.267	27	80	160	241	321
8 inch	0.349	35	105	209	314	419
10 inch	0.545	55	164	327	491	654
12 inch	0.785	79	236	471	707	942
14 inch	1.069	107	321	641	962	1,283
16 inch	1.396	140	419	838	1,257	1,676
18 inch	1.767	177	530	1,060	1,590	2,121

How to read it: need 250 CFM quietly? Find the smallest duct whose 900 FPM column still covers 250 — that is the 8-inch row (314 CFM at 900 FPM). Running the same 250 CFM in a 6-inch duct forces 1,274 FPM, past the noise ceiling.

Air velocity units: FPM, m/s, MPH, km/h, ft/s & LFM

The converter outputs all six speed units at once, but the factors below are worth knowing on sight — they appear on datasheets, anemometer screens and filter specs from different industries. Velocity alone never tells you CFM; every one of these still needs the duct or grille area to become a flow.

FPM → m/s

× 0.00508

1 FPM = 0.00508 m/s exactly (0.3048 m ÷ 60 s)

m/s → FPM

× 196.85

1 m/s = 196.85 FPM — the factor the homepage uses

FPM → ft/s (FPS)

÷ 60

60 feet per minute in one foot per second

MPH → FPM

× 88

5,280 ft ÷ 60 min = 88 FPM per 1 mph, exactly

km/h → FPM

× 54.68

1 km/h = 0.27778 m/s = 54.68 ft/min

LFM ↔ FPM

× 1

linear feet per minute IS feet per minute — same unit, filter-industry name

MPH to CFM and m/s to CFM, by example

340 CFM to MPH (6-inch duct)

340 ÷ 0.196 sq ft = 1,735 FPM. 1,735 ÷ 88 = 19.7 mph. That is clothes-dryer speed in a residential duct — a clear signal to step up a size.

20 m/s to CFM (8-inch duct)

20 m/s × 196.85 = 3,937 FPM. 3,937 × 0.349 sq ft = 1,374 CFM. Industrial dust-collection territory, far past residential design speeds.

Calculate MPH from CFM on a 10-inch duct

500 CFM ÷ 0.545 sq ft = 918 FPM. 918 ÷ 88 = 10.4 mph — just over the quiet band, audible in a bedroom but fine for a workshop.

Field Method

Velocity pressure to CFM — the pitot tube & manometer method

When you cannot trust the fan's nameplate — after duct upgrades, filter loads, or on a system nobody documented — a pitot tube and manometer measure the velocity pressure of the moving air directly. Velocity pressure is the small difference between total pressure and static pressure, and it converts to speed with one square-root formula built on standard air (0.075 lb/ft³ at 70 °F):

Speed from pressure

FPM = 4,005 × √VP

VP in inches of water column. Example: VP = 0.10 in. w.c. → 4,005 × 0.316 = 1,266 FPM. The 4,005 constant encodes standard-air density; at altitude or high temperature the true constant grows, so readings read slightly fast.

Flow from pressure

CFM = 4,005 × A × √VP

Multiply through by duct area. Example: 0.06 in. w.c. measured in a 6-inch duct → FPM = 4,005 × 0.245 = 981 → CFM = 981 × 0.196 = 192 CFM. One measurement, one multiplication, and the delivery is known.

Taking a trustworthy traverse

1
Drill or find access holes 7–10 duct diameters downstream of any fitting — a reading taken two diameters after an elbow can be off by 20–40% because the flow is still swirl-biased.
2
Mark a traverse: two perpendicular lines across the duct, readings at the centers of 10 equal areas per line (the log-linear rule). Averages beat single-point readings by a wide margin.
3
Zero the manometer with both ports open to room air, then connect total pressure to the pitot's impact port and static to the static port — the difference IS the velocity pressure.
4
Average the readings, square-root the mean VP (not the mean of the square roots), then convert: FPM = 4,005 × √(mean VP), CFM = FPM × area.

Don't: read velocity pressure at a fitting

VP taken inside an elbow, a transition, or within two duct diameters of either routinely reads 15–40% off the true mean, and the error almost always flatters the result. If access is only available near a fitting, take readings at multiple angles and accept a wider band — or verify with a second method.

How to calculate CFM with an anemometer

An anemometer gives you FPM directly — the work is in where and how many times you measure. Air in a duct does not move at one speed: it moves fastest at the center, slowest at the walls, and a single center reading overstates the average by 10–25%. The grid-average method fixes that:

In-duct (vane anemometer)

Measure at 6–10 points on a grid across the duct face.
Average the readings — that mean is your design FPM.
Multiply by duct area: mean FPM × sq ft = CFM.
Example: 740 FPM average in a 6-inch duct = 145 CFM.

At a grille (hood or capture plate)

Hold the anemometer in a capture hood flush to the grille.
Use the grille's free area: face area × free-area %.
Stamped grilles run roughly 50–70% free area.
Example: 500 FPM × 0.29 sq ft free area = 145 CFM.

Warning: using the full face area of a grille instead of its free area overstates CFM by 30–50% on stamped-face grilles. The error always flatters the system — which is exactly why it goes unnoticed for years.

The 600–900 FPM quiet zone, speed bands & pitfalls

Residential duct design lives between 600 and 900 FPM. Below 600 the duct is bigger than the job needs; above 900 friction and noise compound; past 1,200 the run is objectionable in any occupied space. The bands the calculator uses:

Velocity band	Speed	What it means
Quiet	≤ 600 FPM · ≤ 3.0 m/s · ≤ 6.8 mph	Bedroom-silent. Ideal for baths, bedrooms, living spaces.
Normal	600–900 FPM · 3.0–4.6 m/s · 6.8–10.2 mph	The residential design band — economical duct size, acceptable sound.
Loud	900–1,200 FPM · 4.6–6.1 m/s · 10.2–13.6 mph	Audible airflow. Fine for utility areas; step up a size for living spaces.
Warning	> 1,200 FPM · > 6.1 m/s · > 13.6 mph	Whistling and rumbling. Increase duct size — noise is only the first symptom, static pressure is the second.

💡 Measured insight

Doubling duct diameter cuts velocity fourfold for the same CFM: 100 CFM is 1,149 FPM in a 4-inch duct, 287 FPM in an 8-inch. One size up is usually the cheapest noise fix on the whole install.

⚠️ Pitfall

Flexible corrugated duct measures its full diameter, but the sagging inner liner can block 10–25% of the area. Stretch flex tight and support every 4 feet, or convert the run to smooth metal where you can.

❌ Don't

Don't multiply FPM by duct diameter in feet — that is not area. A "6 × 600 = 3,600 CFM" answer is off by 18×; the real area of a 6-inch duct is 0.196 sq ft, not 6 sq ft.

CFM vs FPM — which number do you actually need?

Size the fan with CFM; check the duct with FPM. The room tells you the airflow it needs — volume × ACH ÷ 60, which is the job the main CFM calculator does. Then velocity becomes the quality check: the same 141 CFM that is perfect for a bathroom is a scream in a 4-inch duct and a murmur in a 6-inch one. Every duct decision on this page starts with a CFM target, which is why the two tools sit one link apart.

This page owns the conversion job. The CFM conversion calculator handles volume units (m³/h, L/s, CMM), the fan performance calculator predicts what an existing fan delivers at a new speed, and the whole-house fan guide sizes equipment that pushes these numbers around.

Wall-mounted ventilation grille with airflow moving through a residential room

Flow sizes the fan — velocity judges the ductPhoto: Unsplash

CFM ↔ FPM conversion FAQ

How do you calculate CFM from FPM?▼

Multiply the feet-per-minute reading by the duct's free area in square feet: CFM = FPM × Area. For a 6-inch round duct, area = π × 6² ÷ 4 = 28.27 sq in = 0.196 sq ft, so 600 FPM × 0.196 = 118 CFM. The calculator at the top runs this in both directions and auto-computes the area from your duct dimensions.

How do you convert FPM to CFM for a round duct?▼

Compute the round duct's area in square feet — (π × diameter²) ÷ 4 ÷ 144 — then multiply by FPM. A 10-inch duct has 0.545 sq ft of area, so 800 FPM converts to 436 CFM. If the duct is rectangular, multiply width × height in inches ÷ 144 instead, but expect roughly 5–10% less actual flow than the area math suggests because rectangular corners add friction.

How many CFM is 100 FPM?▼

It depends entirely on the duct area — 100 FPM is a speed, not a flow. On a 6-inch round duct (0.196 sq ft), 100 FPM is 20 CFM. On a 10-inch duct (0.545 sq ft) it is 54 CFM, and on a 14-inch trunk (1.069 sq ft) it is 107 CFM. That is why every FPM-to-CFM conversion needs the duct size as a second input.

What is 304 FPM in CFM?▼

On a 6-inch round duct, 304 FPM × 0.196 sq ft = 60 CFM. On an 8-inch duct (0.349 sq ft) it is 106 CFM, and on a 12-inch duct (0.785 sq ft) it is 239 CFM. 304 FPM sits below the quiet zone — useful for filter media and ERV cores, but low for an exhaust run, where 600–900 FPM keeps the duct sized economically.

How do you convert CFM to m/s?▼

Convert CFM to FPM first by dividing by duct area in sq ft, then multiply FPM by 0.00508 to get meters per second. Example: 200 CFM in a 6-inch duct is 200 ÷ 0.196 = 1,020 FPM, which is 1,020 × 0.00508 = 5.18 m/s. There is no direct CFM-to-m/s factor without the duct area, because CFM measures volume while m/s measures speed.

How many CFM is 20 MPH of air velocity?▼

20 mph equals 1,760 FPM (20 × 88). Multiply by the duct area to get flow: in a 6-inch round duct that is 1,760 × 0.196 = 345 CFM; in an 8-inch duct it is 614 CFM. Air at 20 mph in a residential duct is far past the 1,200 FPM noise line — expect a loud, whistling run and high static pressure.

How do you calculate CFM from velocity pressure?▼

Measure velocity pressure (VP) with a pitot tube and manometer, then apply FPM = 4,005 × √VP. Example: a VP reading of 0.10 in. w.c. gives 4,005 × 0.316 = 1,266 FPM. Multiply by the duct area for CFM — 1,266 × 0.196 (6-inch duct) = 248 CFM. The 4,005 constant comes from √(1,096 × 5.2), where 4,005² ≈ standard-air pressure conversion.

How do you calculate CFM with an anemometer?▼

Take velocity readings at 5–10 points across the duct face on a grid, average them to get mean FPM, then multiply by the free area in square feet. Example: an average of 740 FPM across a 6-inch duct face is 740 × 0.196 = 145 CFM. For grilles, multiply by the grille's free area (typically 50–70% of face area), not the face size — using full face area overstates CFM by 30–50%.

What is the difference between CFM and FPM?▼

CFM measures volume of air moved per minute — what sizes the fan. FPM measures how fast that air travels — what sets the noise and friction. The two are linked only through duct area (CFM = FPM × area). A fixed 100 CFM can be a whisper at 512 FPM in an 8-inch duct or a roar at 2,049 FPM in a 4-inch duct; the fan moves the same air either way.

What is the 600–900 FPM rule?▼

Residential duct design targets 600–900 FPM: below 600 the duct is larger than it needs to be, and above 900 noise and friction climb fast. At 900 FPM a 6-inch duct carries 176 CFM, an 8-inch duct 314 CFM, and a 10-inch duct 490 CFM. Above 1,200 FPM airflow noise becomes objectionable in living spaces — that is the calculator's high-noise warning line.

How do you calculate vent CFM from FPM at a grille?▼

Use the grille's free area, not its face size. A 10 × 6 in return grille has a 60 sq in face, but with 70% free area the airflow area is 42 sq in (0.29 sq ft). At a measured 500 FPM, the vent delivers 500 × 0.29 = 145 CFM. Manufacturers publish free-area percentages; if unknown, 60–70% is a safe planning figure for stamped grilles.

Is LFM different from FPM?▼

No — LFM (linear feet per minute) is the same measurement as FPM, just named differently in the filter and cleanroom industries. A HEAF filter rated at 500 LFM sees 500 FPM of face velocity. Converting LFM to CFM is identical to FPM: multiply by the filter's face area in square feet, and derate for the filter's open-area ratio if the datasheet lists one.