CFM Calculator
Airflow in cubic feet per minute from room size and air changes.
Reviewed by the Calculation Machine team · Updated 24 October 2026 · How we check our maths
Your values
ⓘ Live results
- Required airflow
- 224 CFM
- Room volume
- 2,240 cu ft
- In m³/h
- 381 m³/h
- In L/s
- 106 L/s
- Minutes per full air change
- 10
Working
- 1volume 2,240 cu ft x 6 ACH / 60 = 224 CFM
Quick answer
CFM (cubic feet per minute) for a room is its volume multiplied by the air changes per hour you want, divided by 60: CFM = L × W × H × ACH ÷ 60. A 20 × 14 ft room with an 8 ft ceiling holds 2,240 cubic feet; at 6 air changes per hour it needs 2,240 × 6 ÷ 60 = 224 CFM.
What the CFM calculator does
This calculator sizes airflow for a room from its length, width, ceiling height and a target number of air changes per hour (ACH). It returns the required CFM, the room volume, the same airflow in m³/h and L/s, and how many minutes one full air change takes. Use it to choose an exhaust or supply fan for a bedroom, living room, basement, laundry room, workshop, RV, van or boat cabin.
ACH-based sizing is a general method. Where a building code or standard sets a specific number – bathroom fans, kitchen range hoods, whole-house ventilation – that figure takes priority. Two dedicated tools cover the most common cases: the bathroom fan CFM calculator and the range hood CFM calculator.
A fan's rated CFM is measured at a low static pressure. Long duct runs, bends and grilles reduce real airflow, so choose a fan rated at the static pressure of your duct system, usually 0.1–0.25 in. w.c. for residential fans.
The formula
CFM = (L × W × H × ACH) ÷ 60 CFM per sq ft = H × ACH ÷ 60 Minutes per air change = 60 ÷ ACH
| Symbol | Meaning | Unit |
|---|---|---|
| L, W | Room length and width | ft |
| H | Average ceiling height | ft |
| ACH | Air changes per hour – how many times the room's air volume is replaced each hour | 1/h |
| 60 | Minutes per hour, converting cubic feet per hour to per minute | min/h |
This is the standard air-change method used in ASHRAE handbooks and HVAC design texts. It assumes air mixes evenly through the room; in practice, an exhaust fan placed far from the pollutant source or next to the supply air path will perform worse than the number suggests. For CFM per square foot, multiply ceiling height by ACH and divide by 60 – an 8 ft ceiling at 6 ACH is 0.8 CFM per sq ft. Metric conversions: 1 CFM = 1.699 m³/h = 0.472 L/s.
Worked examples
Bedroom ventilation
A 12 × 12 ft bedroom with an 8 ft ceiling, targeting 4 air changes per hour for fresh air.
Inputs
- Room length12 ft
- Room width12 ft
- Ceiling height8 ft
- Air changes per hour4 ACH
Result
- Required airflow77 CFM
- Room volume1,152 cu ft
- In m³/h130 m³/h
- In L/s36 L/s
- Minutes per full air change15
About 77 CFM, one air change every 15 minutes. A small inline or window fan rated around 80–100 CFM covers this with margin for duct losses.
Basement exhaust fan
A 30 × 24 ft unfinished basement with a 7.5 ft ceiling that smells damp, targeting 1 air change per hour continuously.
Inputs
- Room length30 ft
- Room width24 ft
- Ceiling height7.5 ft
- Air changes per hour1 ACH
Result
- Required airflow90 CFM
- Room volume5,400 cu ft
- In m³/h153 m³/h
- In L/s42 L/s
- Minutes per full air change60
90 CFM running continuously replaces the air hourly. Exhausting a basement pulls in replacement air from elsewhere; in humid summer weather that air can raise moisture, so pair it with a dehumidifier if humidity stays above about 60%.
Woodworking workshop
A 24 × 20 ft garage workshop with a 10 ft ceiling, aiming for 10 air changes per hour while finishing and sanding.
Inputs
- Room length24 ft
- Room width20 ft
- Ceiling height10 ft
- Air changes per hour10 ACH
Result
- Required airflow800 CFM
- Room volume4,800 cu ft
- In m³/h1,359 m³/h
- In L/s378 L/s
- Minutes per full air change6
800 CFM of general exhaust clears the room every 6 minutes. It does not replace dust collection at the tool – size that separately with the dust collection CFM calculator.
Air changes per hour by room type
These are common rule-of-thumb targets for general ventilation, not code minimums. Use the higher end where there are odors, moisture, fumes or many occupants. For homes, ASHRAE Standard 62.2 sets whole-house ventilation at 0.03 CFM per sq ft of floor area plus 7.5 CFM per person (bedrooms + 1), and minimum local exhaust of 50 CFM for bathrooms and 100 CFM for kitchen range hoods when they run intermittently.
| Space | Typical ACH | Notes |
|---|---|---|
| Bedroom, living room | 2–4 | Higher for home offices or many occupants |
| Basement (unfinished) | 1–2 continuous | Control humidity as well as airflow |
| Laundry room | 5–10 | Dryer must vent outdoors separately |
| Bathroom | 8 | The 50 CFM code minimum usually governs |
| Workshop, general | 6–10 | Use local extraction for dust and fumes |
| RV, van or boat cabin (cooking, sleeping) | 15–30 | Small volume, high moisture from breathing and cooking |
RVs, vans and boat cabins
Small living spaces need many air changes because one or two people breathing, cooking and drying gear produce a lot of moisture in a small volume. A 10 × 7.5 ft van interior with 6.5 ft of headroom holds about 488 cubic feet; at 20 ACH it needs about 163 CFM. Most 12 V roof vent fans are rated well above that on their top setting, so the practical limit is noise and battery draw. Always open a second vent or window as a make-up air path – an exhaust fan in a sealed cabin moves very little air. Never rely on ventilation to deal with fuel-burning heaters or generators: fit a carbon monoxide alarm.
Converting CFM to other units
European and UK fan ratings are usually in m³/h or L/s. The calculator shows both. To convert a room volume in cubic meters, multiply by 35.31 for cubic feet. For other conversions, use the CFM to m³/h converter or the ACH to CFM calculator, which also works backwards from a fan's rating to the air changes it achieves.
How to use the CFM Calculator
- 1Measure the room's length and width in feet; for open-plan or L-shaped spaces, add the volumes of each part.
- 2Enter the average ceiling height – for a vaulted ceiling, the midpoint between lowest and highest.
- 3Choose an ACH target from the table, or use a code requirement if one applies.
- 4Read the required CFM, then pick a fan rated for that airflow at your duct system's static pressure.
Frequently asked questions
How many CFM do I need for a room?
Multiply the room volume in cubic feet by your target air changes per hour and divide by 60. For a typical 12 × 12 × 8 ft bedroom at 4 ACH, that is about 77 CFM.
How many air changes per hour do I need?
Living spaces typically need 2–4 ACH, laundry rooms and workshops 5–10, and small RV or van cabins 15 or more. Codes for bathrooms and kitchens set fixed CFM figures instead.
What is CFM per square foot?
It is airflow divided by floor area, which equals ceiling height × ACH ÷ 60. A common HVAC rule of thumb is about 1 CFM of supply air per sq ft, but a Manual J load calculation gives the real figure.
Is a higher CFM fan always better?
No. An oversized fan is louder, uses more energy, and in a tight house can depressurize the building and back-draft fuel-burning appliances. Aim for the calculated airflow with a modest margin.
How do I convert CFM to m³/h?
Multiply CFM by 1.699. 224 CFM is about 381 m³/h, or 106 L/s.
References
- ANSI/ASHRAE Standard 62.2 – Ventilation and Acceptable Indoor Air Quality in Residential Buildings — ASHRAE
- ASHRAE Handbook – Fundamentals — ASHRAE
- Home Ventilating Guide — Home Ventilating Institute (HVI)
About this calculator
- Built and reviewed by the Calculation Machine team · How we build and test calculators
- Last reviewed: October 24, 2026
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