Airflow, Taken Seriously
CFM Calculator exists for one question — how much fan does this room actually need? — answered with published formulas, honest duct losses, and sizing advice that treats every CFM as air a real fan will have to move, quietly, for the next ten years.
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Scenario presets with auto-filled ACH
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Room types in the ACH reference table
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Duct diameters in the capacity chart
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Accounts, uploads, or paywalls
Why this site exists
Air is the material everyone treats as free until a room proves otherwise. A bathroom fan too small by forty CFM does not fail loudly — it fails slowly, as a mirror that never quite clears, a ceiling that grows a shadow above the shower, and a smell the next guest notices before you do. A grow tent undersized by one duct size shows up as heat stress in week three. A range hood that whispers on setting one and roars on setting two was almost never undersized in CFM — it was choked by a duct run nobody calculated.
Yet most sizing answers online are either a fan store hiding behind a chart, or forum arithmetic that stops at “volume times air changes divided by sixty” and never mentions that the duct is eating twenty percent of the answer. The results of guessing wrong are not abstract: mold remediation in a bathroom that needed 110 CFM and got 50, a kitchen hood that recirculates grease because the wall stack was sized by habit, a carbon filter that killed a fan rating before the first harvest. CFM is the first number every ventilation decision consumes — fans, ducts, filters, noise, and energy bills all size themselves from it — and it was the number the internet was worst at producing honestly.
We built cfmCalculator.net to be the boring, correct alternative: one free tool, one published formula — CFM = (room volume × ACH) ÷ 60 — and every assumption in plain sight. The calculator is the product; this page is the contract: how the engine works, what it does and does not promise, and the standards the guides around it are held to.
Every duct in that ceiling was sized by someone — correctly or notPhoto: UnsplashHow the calculation actually works
Every input takes the same six stops. Because the chain is fixed and public, two different answers from this tool are mathematically impossible.
Your Room
Length, width, and ceiling height — or floor area plus height, or a volume you already know. Measured on the space, not guessed from a floor plan.
Volume
The room's cubic footage: area × height in imperial, or length × width × height in metres converted behind the scenes. This is the air the fan has to move.
ACH
Air changes per hour — how many times the room's full air volume should be replaced every hour. Chosen from your room type or set by hand.
Base CFM
Volume × ACH ÷ 60. The clean textbook answer before reality touches it — the number every CFM chart on the internet stops at.
System Losses
Duct run and elbows add static pressure that chokes a fan below its rating: +10% per 50 ft of duct, +5% per 90° bend, added transparently.
Safety Factor
Your multiplier (×1.0 to ×2.0, ×1.2 by default) covers dirty filters, crushed flex duct, and the gap between rated and installed fan performance.
Scenario Presets That Fill Everything In
- Bathroom, Kitchen Hood, Garage, and Grow Tent chips on the input panel
- Each preset auto-fills ACH, safety factor, starter duct size, and typical room dimensions
- One tap takes you from a blank form to a defensible fan size
Two Ways to Describe a Room
- Dimensions mode: length × width × ceiling height, in feet or metres
- Area mode: floor area plus ceiling height, for rooms you have only measured on paper
- Smart input parsing accepts 100, 100.5, 10' 6, and 120 inches alike
Duct Reality, Not Free-Air Fantasy
- Friction correction: +10% per 50 ft of duct run, shown as its own line item
- Elbow loss: +5% per 90° bend, because every kink in the run costs airflow
- Rectangular duct converter: the ASHRAE equivalent-diameter formula for wall stacks and joist bays
Velocity & Noise You Can Hear Coming
- Live FPM velocity for round and rectangular ducts in imperial or metric
- Color-coded verdicts: quiet, normal, noisy, and warning bands
- A fan-size recommender that rounds your final CFM up to the next stocked blower
Five audiences, one number
The same arithmetic serves a first-time DIYer and a commercial kitchen planner — they just read different parts of the result. These are the people we design for.
HVAC Contractors & Installers
You quote jobs by the register and get blamed by the customer when the bathroom fogs. The calculator turns room takeoffs into defensible CFM figures with the loss math attached — numbers you can hand to a supplier, write on an invoice, and stand behind when the inspector asks how you sized the fan.
DIY Renovators & Homeowners
A bathroom fan, a range hood, a stuffy bedroom over the garage — every one of these projects starts with 'will the fan I found on sale actually do the job?' Enter the room, pick the preset, read the answer. The site explains every number, so you know why 50 CFM was never going to clear that shower's steam.
Indoor Growers
Tents are the hardest ventilation problem in the house: heat, humidity, and CO₂ all move with the air. The Grow Tent preset starts at 30 ACH, and the duct-loss fields exist because a 4-inch carbon filter on a 25-foot run is not the fan the box label promised. Size the exhaust before the smell sizes it for you.
Kitchen & Restaurant Planners
Grease, steam, and heat do not negotiate. From a residential cooktop hood to a commercial kitchen at 14–30 ACH, the calculator separates the dining-room refresh rate from the hood capture velocity — the two numbers most kitchen ventilation arguments are actually about.
Students & Educators
CFM = volume × ACH ÷ 60 is one of the most teachable formulas in building science: one multiplication connects geometry, time, and comfort. Change one input at a time, watch which outputs move, and build real intuition for how air actually behaves in a room you can picture.
From one bathroom fan to a whole-floor system — same math, different stakesPhoto: UnsplashWhat the CFM figure is actually for
A CFM result is not a finish line — it is an input to four decisions that happen after it. Here is each one, in the order the tool surfaces them:
Buying the Fan
The final CFM — base, plus losses, plus safety factor — is the number to shop against. The fan-size recommender rounds it up to the next stocked blower, so a 107 CFM bathroom points you at a 110 CFM unit, not a wish.
Sizing the Duct
Air velocity between 600 and 900 FPM keeps residential ducts quiet. The capacity chart shows what each round duct diameter can carry at each velocity — and the converter tells you what rectangular stack matches.
Predicting the Noise
Whistles and rumble are velocity problems, not brand problems. Push 400 CFM through a 4-inch duct and every neighbor hears it; the velocity badge warns above 1,000 FPM before you buy, not after you install.
Comparing Quotes
When two estimates disagree, the loss breakdown shows whose assumptions are visible: duct length, bend count, and safety factor are line items — so the conversation becomes about the inputs instead of the price.
Add margin before you buy. The ×1.2 default safety factor exists so filters, wall caps, and flex duct sag are sized for the installation plus reality — not the fan at its exact free-air rating. See a full worked example →
The fan you buy and the duct behind it are one systemPhoto: UnsplashWhat we promise — and what we don't
A sizing calculator earns trust by being precise about its own limits. Here is the complete list of both.
Published formulas only
Every result traces back to CFM = (volume × ACH) ÷ 60, the standard loss adders, and the ASHRAE equivalent-diameter equation. You can verify any answer by hand in under a minute — the formulas section shows all of them.
Honest ACH ranges
The room-type table uses widely published ventilation practice — bathrooms 6–8, bedrooms 5–6, commercial kitchens 14–30, grow rooms 20–40 — and the ACH field always accepts your own number when your code official or engineer specifies one.
Rated vs. installed, stated plainly
A fan's box rating is free-air performance. Real installations lose flow to filters, wall caps, and flex duct. We flag this instead of pretending the rating arrives intact — which is exactly why the loss and safety-factor stages exist.
Margins are yours to set
The default ×1.2 safety factor is a sensible planning figure, not a verdict. Slide it from ×1.0 to ×2.0 to match your duct material, filter load, and how much you hate climbing back into the attic.
Estimates, not engineering
Results are for planning and education. For hospital wards, spray booths, labs, or any code-governed installation, your mechanical engineer and the ventilation code win — every time, and the tool says so on the page.
How the site itself is engineered
The calculation engine is half the story. The other half is the choices behind the page you are reading — why there is no account, no upload, and no premium tier.
Privacy by architecture
The entire engine is client-side JavaScript. Your room dimensions, ACH choices, and duct details are processed in your own browser tab and never transmitted to our servers — not as a policy promise, but as a structural fact. There is no account system, no upload step, and no history stored anywhere on our side.
Instant by design
No spinners, no round trips. Results update on every keystroke because the computation is a handful of multiplications running on your device. The site is statically built and served from the edge, so the tool is typically interactive in well under two seconds on a normal connection.
Free, ad-supported, and labeled
Every feature on the site is free with no usage caps. Advertising — clearly separated from calculator output and editorial content — pays the hosting bill. There is no premium tier hiding the useful half of the tool, and no email gate in front of a result.
Independent on purpose
We sell no fans, take no affiliate commissions, and hold no manufacturer relationships. The presets and ACH ranges exist because they answer real questions, not because a fan brand asked for them. When a product datasheet disagrees with our estimate, the datasheet wins and we say so on the page.
Runs in your browser — nothing to install, nothing sentPhoto: UnsplashHow the guides are written
The calculator is surrounded by guides — on the formula, the ACH table, and worked examples. Every one of them answers to the same five rules — and so do we, on this page.
- We show the working — guides walk through the formula, the units, and a worked example, so you can check our math rather than trust it.
- We label uncertainty — where a number depends on duct material, filter loading, or installed-vs-rated fan flow, we say so in the same breath as the number.
- We cite practice, not vibes — ACH ranges mirror published ventilation guidance, and the loss adders follow the rules of thumb mechanical contractors actually use.
- We update, and we date — content is reviewed against the current calculator behavior; if the tool changes, the guides change with it.
- We write for the trade and the first-timer — the same page serves an estimator sizing a restaurant hood and a homeowner replacing one bathroom fan, and neither gets talked down to.
Straight answers about the site
Tool-specific questions live in the homepage FAQ. These are the ones about us.
How accurate is the CFM calculator?
What are the four scenario presets?
Why do duct length and elbows matter so much?
Who is behind the site?
How is the site funded?
What is a quick rule of thumb for CFM?
Can I use the results for code compliance or engineered systems?
Is the site really free?
Explore the site
Every homepage anchor and every page that explains, extends, and governs the tool.
Legal: Privacy · Terms · Disclaimer
Now size something
The calculator is free, instant, and runs entirely in your browser. Pick a scenario preset, describe the room, and get a CFM figure, a duct size, and a noise forecast in the time it took to read this page.
Calculate Airflow in CFM