Free Ventilation Sizing Tool

CFM Calculator — How Much Airflow Does Your Room Need?

Enter the room dimensions, pick the type of space, and get the exhaust or supply airflow you need in CFM, m³/h and L/s. Uses the air-changes-per-hour (ACH) method with recommended rates for more than 60 residential, commercial, industrial and agricultural spaces, plus a duct-size suggestion.

No signup · Works on mobile · Formulas shown below · Updated 2026-08-23

Ventilation Airflow Calculator (ACH Method)

Airflow = Room volume × Air changes per hour. Choose a space type to load a typical ACH, or enter your own.

Quick preset

Units

Room length

ft / m

Room width

ft / m

Ceiling height

ft / m

Type of space (sets ACH)

Custom air changes per hour

ACH

Safety margin (filters, ducts, grilles)

Airflow of one fan you are considering (optional)

CFM / m³/h

Leave blank to skip. Tells you how many fans you need.

Results

Engineering estimate for preliminary sizing. Verify against the fan curve, local codes and your system measurements before purchase. Ask a Longwell engineer to check your duty point free of charge.

How the CFM calculator works

The air-changes method is the standard first-pass way to size a ventilation fan. It asks: how many times per hour should the entire volume of air in the room be replaced? Multiply the room volume by that rate and you have the airflow.

CFM = Room volume (ft³) × ACH ÷ 60
m³/h = Room volume (m³) × ACH
// Room volume = length × width × height
// 1 CFM = 1.699 m³/h = 0.4719 L/s

Then add a safety margin. Fan catalog airflow is measured in free air (zero static pressure). Every metre of duct, every elbow, grille and filter adds resistance, and the delivered airflow falls along the fan curve. A 20 % margin covers a short, clean duct run; 30–50 % is realistic for carbon filters or long flexible duct. For precise work, calculate the static pressure with our Duct Static Pressure Calculator and read the fan curve at that point.

The suggested duct diameter keeps air velocity near 5 m/s (≈1,000 fpm), a good compromise between duct cost and noise for small exhaust systems. Go larger if the fan is near bedrooms or offices.

Recommended air changes per hour by room type

Typical design values used by HVAC engineers. Codes and standards (ASHRAE 62.1/62.2, ASHRAE 170, local building codes, NFPA for booths) override these where they apply. Where a per-person or per-area rule exists, it is noted.

Sources: ASHRAE 62.1/62.2/170 design practice, industrial ventilation (ACGIH) guidance, agricultural extension data and Longwell application records. Use as preliminary values.
Space	Typical ACH	Range	Notes
Bathroom / toilet (residential)	8	6–10	Or 1 CFM per ft² / min 50 CFM intermittent
Kitchen (residential)	15	10–15	Range hood sized separately → see Kitchen Hood tool
Commercial kitchen	25	20–30	Hood capture drives the real number
Laundry room	10	8–12	Dryer exhaust is separate
Living room / bedroom	4	3–6	Whole-house: 0.35 ACH min (ASHRAE 62.2)
Basement (damp)	4	3–6	Pair with dehumidification
Crawl space	2	1–3	Or 1 CFM per 50 ft² floor
Attic (summer venting)	10	8–12	Or 0.7 CFM per ft² attic floor
Garage (residential, 2-car)	6	4–8	Exhaust to outside, low-level
Home workshop	8	6–10	Dust collection separate
Wine cellar	3	2–4	Usually cooling-driven, not ACH
Office (open plan)	6	4–8	Or 17–20 CFM per person
Classroom	6	5–8	Or 15 CFM per student
Conference room	8	6–10
Hotel corridor	4	3–5
Restaurant dining room	8	6–10
Food truck / mobile kitchen	30	20–40	Hood capture is critical
Gym / fitness studio	8	6–12	Higher for group classes
Indoor pool	6	4–8	Humidity-driven; 0.5 CFM per ft² pool area
Sauna / steam room	4	3–6
Retail store	6	4–8
Studio / rehearsal room	6	5–8
Warehouse (general)	4	2–6	Heat removal may dominate in summer
Distribution center	4	3–6
Machine shop	8	6–12
Welding bay	12	10–20	Source capture preferred
Woodworking shop	10	8–15	Plus dust collector
Paint booth / spray area	60	40–100	Code: 100 fpm face velocity
Plating / chemical line	15	10–20	Local exhaust on tanks
Textile mill	6	5–10
Printing plant	10	8–15	Solvent removal
Bakery (oven area)	20	15–30	Heat removal
Coffee roasting	20	15–30	Afterburner exhaust separate
Pizza oven area	20	15–30
Smokehouse	15	10–20
Forge / kiln room	20	15–30	Radiant heat — spot cooling helps
Laboratory (general)	8	6–12	Fume hoods separate
Pharma suite	20	15–30	Cascade pressure design
Cleanroom ISO 8	20	10–30	Plus FFU coverage
Cleanroom ISO 7	60	30–90	FFU 5–15 % ceiling coverage
Isolation room (negative)	12	12	ASHRAE 170: 12 ACH
Server closet / small IT room	20	15–30	Usually heat-load driven → Enclosure tool
Data center hall	25	20–40	Always size by kW heat load
Battery room (VRLA)	6	4–12	Hydrogen dilution: 1 CFM/ft² common
Generator enclosure	12	10–30	Engine heat rejection dominates
Transformer room	10	8–15	Heat-load method preferred
Electrical / switch room	6	4–8
Elevator machine room	10	8–12
Parking garage (enclosed)	6	4–6	CO-demand control common
Boat engine room	20	15–30
RV / van interior	6	4–10	Roof fan
Horse trailer	6	5–10
Greenhouse (summer)	45	30–60	1 air change per minute = 60 ACH
Grow tent	60	60	Full exchange every minute
Vertical farm	20	15–30	Per-rack air movement separate
Mushroom farm	6	4–8	CO₂-driven
Poultry house (summer)	8	6–10	Tunnel: 2–3 m/s air speed
Chicken coop (backyard)	8	6–10
Dairy barn	8	6–10	Summer: 40–60 ACH for heat stress
Swine barn	8	6–10	Winter minimum ~2 ACH
Reptile / animal enclosure	4	3–6
Wastewater / odor control	12	10–15
Portable sanitation / restroom trailer	10	8–12
Container workshop (20/40 ft)	8	6–12

Worked example: 8 × 10 ft bathroom

Given: 10 ft × 8 ft bathroom, 8 ft ceiling, typical 8 ACH, 20 % duct margin.

Volume = 10 × 8 × 8 = 640 ft³
Base airflow = 640 × 8 ÷ 60 = 85 CFM
With 20 % margin = 85 × 1.2 = 102 CFM → choose a 100–110 CFM rated fan, or a fan that still delivers 85 CFM at 0.25 in. w.g.
Duct: 102 CFM at ~1,000 fpm → 4.3 in → use a 5 in (125 mm) duct for lower noise.

Cross-check with the per-area rule (1 CFM/ft² → 80 CFM) and the code minimum (50 CFM intermittent). All three agree within range — a 100 CFM fan is the right call.

When air changes are the wrong method

Heat removal (server rooms, battery cabinets, transformer rooms, kitchens in summer): size by kW of heat and allowable temperature rise instead — use the Enclosure Cooling Airflow Calculator.
Contaminant capture (fume hoods, welding, paint): size by capture velocity at the source, then check the room ACH as a minimum.
Occupancy (offices, classrooms, restaurants): ASHRAE 62.1 per-person rates often exceed the ACH figure for crowded rooms. Use the larger.
Humidity (pools, laundries, basements): moisture load governs; ACH is only a sanity check.
High altitude or hot climates: air is thinner, so mass flow per CFM drops. Correct with the Air Density & Altitude Calculator.

Frequently Asked Questions

How do I calculate CFM for a room?

Multiply the room volume in cubic feet (length × width × height) by the recommended air changes per hour for that type of space, then divide by 60. Example: a 640 ft³ bathroom at 8 ACH needs 640 × 8 ÷ 60 ≈ 85 CFM. Add 10–30 % for duct and filter losses.

What is a good ACH for a bathroom, kitchen or garage?

Bathrooms are typically designed at 8 ACH (code minimum 50 CFM intermittent or 20 CFM continuous), residential kitchens at 15 ACH (plus a range hood), and garages at 4–6 ACH. See the full table above for 60+ space types.

How do I convert CFM to m³/h?

Multiply CFM by 1.699 to get m³/h, or divide m³/h by 1.699 to get CFM. 1 CFM is also 0.472 L/s. Our Airflow Unit Converter handles CFM, m³/h, L/s and m³/s in both directions.

Why is my fan delivering less than its rated CFM?

Catalog ratings are at zero static pressure (free air). Ducts, elbows, grilles and filters add resistance; the fan then operates further up its curve and delivers less air. Axial fans are especially sensitive. Choose a fan whose curve still meets your CFM at the expected static pressure, or use a centrifugal / mixed-flow duct fan for ducted systems.

Should I size by air changes or by heat load?

If the room's problem is heat (electronics, motors, ovens, people in a gym), size by heat load: airflow = heat (W) ÷ (1.08 × ΔT°F) in CFM. If the problem is odor, moisture or general freshness, use air changes. When in doubt, calculate both and take the larger.

Can I use this calculator for supply fans as well as exhaust fans?

Yes. The required airflow is the same whether you push air in or pull it out. For positive-pressure rooms (cleanrooms, labs with outward leakage) supply about 10–15 % more than exhaust; for negative-pressure rooms (kitchens, isolation rooms, workshops) exhaust more than you supply.

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