Two questions, two formulas
"How many CFM do I need" has two honest answers depending on what you are sizing.
- Ventilation airflow — how much outside air a room needs to stay fresh.
CFM = room volume (cu ft) × air changes per hour ÷ 60 - Equipment airflow — how much air must move to deliver a given amount of heating or cooling.
CFM = sensible BTU/h ÷ (1.08 × ΔT °F)
The calculator above runs the first one. The second is below with worked examples, because the two are commonly confused and the second usually gives the bigger number.
The constant 1.08 in the second formula is not arbitrary: it is 60 minutes × 0.075 lb/cu ft (air density) × 0.24 BTU/(lb·°F) (specific heat of air). Air is light, which is why moving heat with air takes so much volume.
How many air changes per hour does a room need?
Air changes per hour (ACH) is how many times the full volume of the room is replaced in an hour. It rises with moisture, odours and occupancy.
| Room | Typical ACH | Why |
|---|---|---|
| Bedroom, living room | 4–6 | General occupancy and odour control |
| Kitchen | 10–15 | Cooking moisture, grease and combustion by-products |
| Bathroom | 6–10 | Shower humidity, which drives mould risk |
| Utility / laundry | 8–10 | Dryer moisture |
| Open-plan living space | 4–6 | Larger volume, lower relative occupancy |
Ventilation rates are set by code and standard in real projects — ASHRAE 62.2 covers whole-house and local exhaust rates for residences. This calculator is for planning and for checking whether a fan you already own is in the right range.
Three worked examples
- Bedroom, 200 sq ft with a 9 ft ceiling, 5 ACH. Volume = 1,800 cu ft. CFM = 1,800 × 5 ÷ 60 = 150 CFM.
- Bathroom, 150 sq ft with an 8 ft ceiling, 8 ACH. Volume = 1,200 cu ft. CFM = 1,200 × 8 ÷ 60 = 160 CFM — which is why bathroom fans commonly land at 110–150 CFM for a smaller room and 150–200 for a larger one.
- Open living space, 400 sq ft with a 9 ft ceiling, 6 ACH. Volume = 3,600 cu ft. CFM = 3,600 × 6 ÷ 60 = 360 CFM.
Now the equipment version, which often governs:
- 18,000 BTU/h with a 20°F temperature rise. CFM = 18,000 ÷ (1.08 × 20) = 833 CFM.
- 36,000 BTU/h with a 20°F rise. CFM = 36,000 ÷ (1.08 × 20) = 1,667 CFM.
- 48,000 BTU/h with a 25°F rise. CFM = 48,000 ÷ (1.08 × 25) = 1,778 CFM.
The pattern is worth internalising: the same equipment at a smaller temperature rise needs proportionally more air. When airflow is restricted — a dirty filter, a crushed duct, a closed register — the temperature rise climbs and the heat exchanger runs hotter.
Why the two numbers disagree, and which one wins
Ventilation CFM answers a health and comfort question. Equipment CFM answers a physics question: the air has to carry the heat that the equipment produces or removes. On a room with modest occupancy and a large heating load, equipment airflow is usually the larger requirement and the duct has to be sized for it. On a kitchen or bathroom, ventilation and local exhaust dominate.
They are also not additive in the way people assume. A balanced ventilation strategy and a separately ducted heating system serve different purposes and are usually designed independently.
Common mistakes
- Using floor area instead of volume. CFM comes from cubic feet. A 200 sq ft room with a 12 ft ceiling needs half again more air than the same floor at 8 ft.
- Assuming one ACH figure for every room. A kitchen at 10–15 and a bedroom at 4–6 are very different jobs.
- Comparing a fan's free-air rating to a ducted requirement. Ratings are measured with no static pressure. Real ductwork costs you airflow, and a restrictive run can cut delivered CFM substantially.
- Confusing CFM with velocity. CFM is volume per minute; velocity is how fast it moves through a section. Both matter, and they are related by the duct area.
- Ignoring the filter. A loaded filter is one of the most common causes of low airflow and high temperature rise in a furnace.
- Sizing exhaust without make-up air. A powerful kitchen or whole-house exhaust needs a path for replacement air, or it will depressurise the house and backdraft combustion appliances.
Measuring and moving the air you just calculated
A CFM figure is only useful if you can check it against a real room. These are the instruments and parts that turn the number into something you can verify.
- Anemometer / airflow meter — The direct way to check delivered CFM at a register: measure velocity, multiply by the grille area. Inexpensive hot-wire models are accurate enough for residential work.
- Bathroom exhaust fan — Rated by CFM, which is exactly what the calculator outputs. Buy to the number for your room volume and duct length, not to the cheapest on the shelf.
- HVAC air filter — A loaded filter is the most common reason measured airflow falls below the rating. Replace before you conclude the fan is undersized.
- Duct sealing mastic — Sealing leaks recovers airflow you are already paying to move. Foil tape and mastic beat cloth duct tape, which dries out and falls off.
As an Amazon Associate I earn from qualifying purchases. The links above are affiliate links — if you buy through one, the price you pay is unchanged and a small commission helps keep these calculators free. Tools are listed because the job calls for them, not because of commission rates.
Sources and standards
Where these figures come from
Every formula on this page is implemented from the published reference material below. Where a value depends on the equipment manufacturer, the page says so instead of guessing one.
- ASHRAE — Standard 62.2 residential ventilation rates and airflow engineering practice.
- U.S. Department of Energy — home ventilation, duct sealing and HVAC airflow guidance.
Figures, formulas and reference tables last verified: 21 September 2026.