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CFM Calculator

Airflow in CFM from room volume and target air changes.

Your measurementsEvery assumption editable01

How many times the room's full volume is replaced each hour.

Airflow requiredCalculated live02

Result

240CFM

240 CFM at 6 air changes per hour

Room volume300 sq ft × 8 ft
2400 cu ft
Air changes per hour
6
Airflow requiredvolume × ACH ÷ 60 minutes
240 CFM

CFM is cubic feet per minute while ACH is per hour, which is where the 60 comes from. Getting that conversion backwards is the most common error in ventilation arithmetic and produces an answer sixty times wrong.

Air changes per hour is a target, not a measurement. What a room actually gets depends on duct sizing, register placement and whether the return path is adequate — a room with a supply and no return path does not achieve its nominal air change rate however big the duct is.

For a whole system rather than a room, the rule of thumb is roughly 400 CFM per ton of cooling, and that is the figure a technician will measure against.

Enter the room dimensions and how many times an hour you want its full volume replaced. The conversion between air changes per hour and cubic feet per minute is where most ventilation arithmetic goes wrong, and it goes wrong by a factor of sixty.

Why use this tool?What it does differently03

Why use this tool?

ACH targets built in

Presets for bedrooms, living spaces, bathrooms, workshops and commercial kitchens.

The conversion, correctly

CFM is per minute and ACH is per hour. The factor of sixty between them is the commonest error in ventilation maths.

Volume shown

The room volume is itemised, which is the figure you need for almost any other ventilation calculation.

Any ceiling height

Volume rather than floor area, so vaulted and tall rooms come out correctly.

How this worksThe method04

How this cfm calculator works

Room volume is floor area times ceiling height. Multiplying by air changes per hour gives cubic feet per hour, and dividing by sixty converts to cubic feet per minute.

That divide-by-sixty is where the arithmetic usually fails. Getting it backwards produces an answer sixty times wrong in either direction, which is large enough to be obviously absurd and yet common enough to appear in published tables.

Air changes per hour is a design target rather than a measurement. What a room actually achieves depends on duct sizing, register placement and whether there is an adequate return path — a room with a supply and no way for air to leave does not achieve its nominal rate however large the duct.

How to use itStep by step05

How to use it

  1. Step 1: Measure the room

    Floor area and ceiling height. Volume is what matters, not area.

  2. Step 2: Choose an ACH target

    4 for bedrooms, 6 for living spaces, 8 for bathrooms and kitchens, 15 for workshops, 30 for commercial kitchens.

  3. Step 3: Check the return path

    Air can only enter as fast as it can leave. Door undercuts or transfer grilles matter as much as the supply.

  4. Step 4: Size the duct

    Take the CFM figure to the duct size calculator.

Example usageWorked figures06

Example usage

A living room
300 sq ft with 8 ft ceilings is 2,400 cubic feet. At 6 air changes per hour that is 240 CFM.
A bathroom
The same room at 8 ACH needs 320 CFM. Wet rooms need more because moisture removal, not temperature, is the job.
A vaulted living room
300 sq ft at 16 ft is 4,800 cubic feet — 480 CFM at 6 ACH, double the standard-ceiling figure for the same floor plan.
Frequently asked questionsCommon questions07

Frequently asked questions

How do I convert air changes per hour to CFM?

Multiply room volume in cubic feet by the air changes per hour, then divide by 60. The divide by 60 converts hours to minutes and is the step most often missed.

How many air changes per hour do I need?

About 4 for bedrooms, 6 for general living space, 8 for bathrooms and kitchens, 15 for workshops with fumes, and 30 or more for commercial kitchens.

How much CFM per square foot?

It depends on ceiling height, which is why this works from volume. As a rough check, a standard 8 ft room at 6 ACH needs about 0.8 CFM per square foot.

Why does my room not get its rated airflow?

Usually because air cannot leave. A supply register with no return path, closed doors and no undercut will throttle actual airflow well below the nominal figure.

How much CFM per ton of air conditioning?

Roughly 400 CFM per ton is the standard design figure, and it is what a technician measures against when commissioning a system.

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