Fan CFM Calculator
Size the fan from the room — volume × the air-changes standard your space actually has, in CFM.
Fan CFM Calculator
Results recalculate instantly on every keystroke. Nothing you type is transmitted.
What this result does not account for
- Ideal mixing — no short-circuit or distribution losses
- Standards quoted for screening; the AHJ's edition governs
In short: A 12 × 14 × 10 ft kitchen holds 1,680 ft³, and the continuous-kitchen line is 5 air changes an hour: 140 CFM. That is the ASHRAE 62.2 arithmetic to the CFM — the same standard also allows 100 CFM of intermittent range hood instead. A bedroom needs a tenth of that rate; a restaurant kitchen needs six times it. The room sets the volume; the standard sets the clock.
Formula
CFM = length × width × height × ACH ÷ 60
One air change replaces the room's whole volume once. Air changes per hour divided by 60 is the clock in minutes; volume times that rate is the fan, in cubic feet a minute. The standards do the arguing: ASHRAE 62.2 wants a whole house at 0.35 ACH, kitchens at 5 ACH continuous (or 100 CFM of intermittent hood), bathrooms at 20 CFM continuous — and commercial spaces climb from offices at 4–10 ACH to restaurant kitchens at 30–60 under NFPA 96.
Worked Example
- Enter the room's inside dimensions and ceiling height.
- Pick the air-changes standard the space is held to.
- Read the CFM — that is the fan's duty at the register.
Defaults: 140 CFM for the 1,680 ft³ kitchen. Drive the same room to 30 ACH and the fan reads 840 CFM — the commercial-kitchen rate costs six residential fans.
Strengths & Limits Of This Model
Where this engine is strong
- Standards named per space — the number has a source
- The 62.2 kitchen example is the default, verifiable
Where it stops
- Real distribution can halve effective ACH
Practical Use Cases
Bath fans
size the exhaust before buying
Kitchen hoods
the 5-ACH continuous line
Fresh-air plans
whole-house CFM from the 0.35 rule
Methodology & Editorial Standards
Computation runs in IEEE-754 double precision at full internal precision; rounding to two decimal places occurs strictly at the display layer, so no cumulative drift enters the result. All monetary outputs use accounting presentation — grouped thousands, two decimals, negatives in parentheses — so figures can be transcribed directly into a model or working paper. Division-by-zero and out-of-domain inputs return an em-dash rather than a misleading number.
This engine was reconciled against an independent reference implementation and hand-verified for the worked example above before release. Our full five-stage review process is published on the About Us page.
Disclaimer. This calculator is provided for informational and modelling purposes only and does not constitute financial, tax, legal, medical, or engineering advice. Verify all figures with a qualified professional before acting on them.
Fan CFM Calculator — 8 Expert FAQs
8 analyst-written answers to the questions practitioners actually ask — optimised for voice and answer-engine retrieval.
How many CFM does my kitchen hood need?
ASHRAE 62.2 draws the line two ways: 100 CFM of intermittent vented hood, or 5 air changes an hour of continuous exhaust. On a 12 × 14 × 10 kitchen the continuous line reads 140 CFM — bigger than the 100 CFM intermittent, which is exactly the point of running it all day.
What about the bathroom?
The code numbers are fixed: 50 CFM intermittent or 20 CFM continuous. The ACH route is the sanity check — a 6 × 8 × 8 bath at 20 CFM continuous turns over about 3.1 times an hour.
How much fresh air does a whole house need?
ASHRAE 62.2 sets 0.35 air changes an hour, never less than 15 CFM a person. On a 1,800 ft³-ish small house that reads in the tens of CFM — a whisper next to the kitchen's 140.
Why does the same room need 30 ACH in a restaurant?
Because grease-laden vapor and heat are the load, not odor: NFPA 96 wants commercial kitchens at 30–60 ACH. The formula does not care which standard you feed it — only the number changes.
What is a good ACH for a bedroom?
About 0.35 to 1 under ASHRAE 62.2 — sleeping occupants exhale little, so the dilution bar is low. A 12 × 12 × 8 bedroom at 1 ACH is 19.2 CFM, a quiet fan by design.
Is ACH the same as air changes per minute?
No — and the division by 60 is where typos go to hide. Five air changes an hour is one change every twelve minutes; the CFM formula carries that conversion inside it.
How does this connect to cooling?
Airflow in CFM also prices cooling air: the rule is 400 CFM per ton, and the sensible-heat lane reads CFM = BTU/h ÷ (1.08 × ΔT). Ventilation sizes the fan for health; the cooling load page sizes it for temperature.
Does fan type change the CFM?
The duty is the duty — 140 CFM is 140 CFM at the register. What changes by fan and duct is static pressure: long or flex ducts make a fan deliver less than its label, which is why the duct page sizes the run before the fan is chosen.