HVACCFM Calculator

CFM Calculator

Find the CFM airflow a room needs — by air changes, HVAC tonnage, or bathroom fan size.

Room and airflow

Length (ft)

Width (ft)

Height (ft)

Sets a typical air-changes-per-hour target — adjust below if needed.

Ready to calculate

Enter the room dimensions and type to find the airflow it needs.

Last updated June 18, 2026 by our expert review team

How to Calculate CFM

CFM = room volume × air changes per hour ÷ 60. Find the volume (length × width × ceiling height), multiply by the ACH your room needs, and divide by 60 to get cubic feet per minute. A 12 × 10 ft room with a 9 ft ceiling at 8 ACH needs about 144 CFM.

Required CFM by floor area (8 ft ceiling):

Room size6 ACH8 ACH
100 sq ft80 CFM107 CFM
150 sq ft120 CFM160 CFM
200 sq ft160 CFM213 CFM
300 sq ft240 CFM320 CFM
400 sq ft320 CFM427 CFM

At an 8 ft ceiling. Taller ceilings need proportionally more CFM — the calculator uses your exact height.

Expert Contributors

EG
Creator
Ehsan Ghazanfari
Licensed Structural Engineer
H
Expert Review
Hawkin
Certified Cost & Estimating Professional

Updated June 2026

How we verify our calculators
CFM formula: room volume (length times width times ceiling height) times air changes per hour, divided by 60, equals the airflow in cubic feet per minute.
Airflow scales with the whole room volume, not just floor area — and with how many times per hour you want the air replaced.

Methodology

How the CFM Calculator Works

Ventilation is sized by how fast you replace a room's air. Air changes per hour (ACH) is that rate; multiplying it by the room volume and dividing by 60 converts it into CFM, the airflow rating fans and blowers are sold by.

Bigger rooms and higher ACH targets need more CFM. See DOE Energy Saver for whole-house ventilation guidance.

Formulas

Room volume = length × width × ceiling height

CFM = volume (ft³) × ACH ÷ 60

Metric: m³/h = volume (m³) × ACH

Quick Reference

Bedroom
5–6 ACH
Living room
6–8 ACH
Kitchen
7–8 ACH
Bathroom
8 ACH
1 CFM → m³/h
× 1.699

Air Changes per Hour by Room

Each space has a typical ACH target based on moisture, odors, and how it's used:

RoomRecommended ACH
Bedroom5–6
Living room6–8
Kitchen7–8
Bathroom8
Office6
Basement3–4
Garage / workshop6–8

Typical residential ranges. Codes and equipment manuals are the final word — high-moisture rooms favor the top of the range.

CFM for HVAC: Airflow per Ton

For ducted heating and cooling, airflow is sized off the system's capacity: about 400 CFM per ton of cooling (1 ton = 12,000 BTU). That total is then split across rooms by their individual loads.

System sizeTotal airflow
1.5 ton600 CFM
2 ton800 CFM
2.5 ton1,000 CFM
3 ton1,200 CFM
4 ton1,600 CFM
5 ton2,000 CFM

At 400 CFM/ton. Humid climates run ~350 for more dehumidification; dry climates up to ~450.

HVAC airflow per ton: a residential air handler and ducts, with the rule that one ton of cooling needs about 400 CFM, scaling from 600 CFM at 1.5 tons to 2,000 CFM at 5 tons.

Bathroom Exhaust Fan Sizing

Sizing a bathroom exhaust fan: a ceiling fan venting a bathroom, sized at 1 CFM per square foot up to 100 sq ft, or by fixture — 50 CFM each for toilet, shower, and tub, 100 for a jetted tub.

For bathrooms up to 100 sq ft, use 1 CFM per square foot (50 CFM minimum). Larger or multi-fixture baths size by fixture and add them up:

Bath sizeFan
40 sq ft50 CFM
50 sq ft50 CFM
70 sq ft70 CFM
90 sq ft90 CFM
100 sq ft100 CFM
FixtureAdd
Toilet50 CFM
Shower50 CFM
Bathtub50 CFM
Jetted tub100 CFM

Fixture method (HVI) for baths over 100 sq ft — sum each fixture's CFM.

CFM Examples

Master Bedroom

14 × 12 × 8 ft · 5 ACH
112 CFM1,344 ft³5 ACH

Bedrooms need modest airflow — about 5 air changes per hour keeps sleeping air fresh without drafts.

Kitchen

12 × 10 × 9 ft · 8 ACH
144 CFM1,080 ft³8 ACH

Cooking moisture and odors push kitchens to 7–8 ACH. A range hood handles the spike at the stove.

Finished Basement

30 × 25 × 8 ft · 4 ACH
400 CFM6,000 ft³4 ACH

Big, low-traffic spaces use a lower ACH, but the large volume still adds up to real airflow.

Avoid these

CFM Sizing Mistakes

Sizing by floor area alone

Airflow depends on volume. A high ceiling needs more CFM for the same floor area.

Using one ACH for every room

Kitchens and baths need far more air changes than bedrooms or basements.

Buying a fan at exactly the calculated CFM

Rated CFM drops under duct resistance. Size up, and keep ducts short and smooth.

Ignoring make-up air

Powerful exhaust needs a path for replacement air, or the fan starves and back-drafts.

CFM Calculator FAQs

How do I calculate CFM for a room?
Multiply the room's volume (length × width × ceiling height) by the air changes per hour (ACH) you want, then divide by 60. For example, a 12 × 10 ft room with a 9 ft ceiling is 1,080 ft³; at 8 ACH that's 1,080 × 8 ÷ 60 = 144 CFM.
What is ACH (air changes per hour)?
ACH is how many times the entire volume of air in a room is replaced in one hour. A bedroom at 6 ACH has all its air swapped six times an hour. Higher-moisture or higher-odor rooms (kitchens, bathrooms) need more changes; low-traffic spaces need fewer.
How many CFM do I need per square foot?
At a standard 8 ft ceiling and 8 ACH, you need roughly 1.07 CFM per square foot. So a 200 sq ft room needs about 213 CFM. The exact figure depends on your ceiling height and the room's ACH target, which is why volume — not just floor area — drives the math.
What ACH should each room have?
Common targets are 5–6 for bedrooms, 6–8 for living rooms, 7–8 for kitchens, 8 for bathrooms, 6 for offices, and 3–4 for basements. Garages and workshops run 6–8 because of fumes and dust. Always check local code and the equipment manual.
How do I size a bathroom exhaust fan?
A common rule is 1 CFM per square foot of bathroom floor (8 ft ceiling), with a 50 CFM minimum. For bathrooms over 100 sq ft, size by fixtures: 50 CFM each for the toilet, shower, and tub, plus 100 CFM for a jetted tub, per HVI guidance.
How many CFM per ton does an HVAC system need?
About 400 CFM per ton of cooling is the standard rule of thumb — so a 3-ton (36,000 BTU) system moves roughly 1,200 CFM. It ranges from about 350 CFM/ton in humid climates (slower air for more dehumidification) to 450 in dry climates. Divide that total across rooms by their individual loads.
What's the difference between CFM and ACH?
ACH is a rate of air changes relative to room size; CFM is the actual volume of air moved per minute. CFM is what you shop for in a fan or blower. You convert from one to the other with the room volume: CFM = volume × ACH ÷ 60.
How do I convert CFM to m³/h?
Multiply CFM by 1.699 to get cubic meters per hour. So 150 CFM ≈ 255 m³/h. In metric this calculator reports airflow directly as m³/h (room volume in m³ × ACH).

Sizing the whole system? Pair this with the BTU calculator for heating and cooling load, or the mini split calculator for ductless zones.

Important Disclaimer

These estimates are for planning purposes only. Actual costs vary by location, material availability, and project complexity. Always get at least 3 local quotes. This calculator does not replace professional advice.

Sources

ASHRAE Standard 62.2: Residential Ventilation
ashrae.org
Home Ventilating Institute (HVI): Fan Sizing
hvi.org
U.S. DOE Energy Saver: Whole-House Ventilation
energy.gov

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