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Air Changes Per Hour Calculator

Air changes per hour calculator for HVAC design, ventilation sizing, and indoor air quality. Enter room length, width, height, and CFM airflow to get ACH (air changes per hour) and a target zone for offices, kitchens, bathrooms, hospitals, and other applications.

Quick Answer

To calculate air changes per hour, multiply CFM by 60 and divide by the room volume in cubic feet. A 12 x 14 x 8 ft office (1,344 cu ft) supplied with 90 CFM has an ACH of 4.0, which sits at the low end of the typical office target of 4 to 10 ACH.

Enter Values

Room Length

Room Width

Ceiling Height

Airflow (CFM)

Room Type

Result

Enter values above and click Calculate to see your result.

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Formula

Core Formula

\text{ACH} = \frac{\text{CFM} \times 60}{V_{\text{room (ft}^3\text{)}}}

How it works: Air changes per hour measures how many times the entire volume of a room is replaced by fresh or filtered air each hour. Multiply CFM (cubic feet per minute) by 60 to get cubic feet per hour, then divide by the room volume in cubic feet. Higher ACH means faster pollutant removal but also higher fan energy and noise.

Review and Methodology

Updated Apr 29, 2026

This calculator runs locally in your browser. Inputs are converted into the units required by the formula, and the result is paired with supporting references so you can verify the method before using it for planning or estimates.

Worked Example

Restaurant kitchen 20 ft x 15 ft x 10 ft with 600 CFM exhaust:

1Step 1: Room volume = 20 x 15 x 10 = 3,000 cu ft

2Step 2: Airflow per hour = 600 x 60 = 36,000 cu ft per hour

3Step 3: ACH = 36,000 / 3,000 = 12 air changes per hour

4Step 4: Target zone for commercial kitchens is 15-30 ACH

5Step 5: Diagnosis = Slightly under target. Increase exhaust to 750-1,500 CFM.

How to Calculate Air Changes Per Hour

Air changes per hour (ACH) is the standard measure of how quickly a room or building is ventilated. One ACH means the full volume of air in the space is replaced once per hour.

The formula is simple: multiply CFM by 60 to get cubic feet of air per hour, then divide by the room volume in cubic feet. CFM is the design airflow at the supply or exhaust register; you can read it on a balance report or measure it with a flow hood.

Different rooms have different targets. Bedrooms and offices need 4 to 10 ACH for comfort and CO2 control. Restaurant kitchens push 15 to 30 ACH to remove smoke and grease vapors. Operating rooms and biosafety labs need 15 to 25 ACH and use HEPA-filtered makeup air to control infection.

ACH alone is not enough for compliance. ASHRAE 62.1 also requires a minimum outdoor-air CFM per person and CFM per square foot. Use ACH as a sanity check and a comparison metric across spaces.

ACH = (CFM x 60) / room volume in cubic feet
Bedrooms: 5-6 ACH; offices: 4-10; bathrooms: 6-8
Restaurant kitchens: 15-30 ACH for smoke and grease removal
Hospital patient rooms: 6-12 ACH; operating rooms: 15-25 ACH
Laboratories and cleanrooms: 10-20 ACH with HEPA filtration

Pair ACH with ASHRAE 62.1 outdoor-air CFM per person targets. For sizing the equipment that delivers the airflow, also check duct static pressure and fan curves.

You can also calculate changes using our HVAC Room Volume Calculator, Superheat and Subcooling Calculator, Rectangle Volume Calculator or BTU Calculator.

ACH Targets by Room Type

Industry-standard air change rates from ASHRAE 62.1, ASHRAE 170, and CDC guidance.

Room Type	Target ACH	Common Source
Bedroom	5-6	ASHRAE 62.2
Office	4-10	ASHRAE 62.1
Conference room	8-12	ASHRAE 62.1
Bathroom	6-8	IRC M1507
Restaurant kitchen	15-30	IMC 507
Hospital patient room	6-12	ASHRAE 170
Operating room	15-25	ASHRAE 170
Cleanroom (ISO 7)	30-60	ISO 14644

Note: Targets vary by jurisdiction and project type. Always confirm with the governing code (ASHRAE, ASHRAE 170, IMC, ISO 14644).

Frequently Asked Questions

How to calculate air changes per hour?

Multiply CFM by 60 to get cubic feet of air per hour, then divide by the room volume in cubic feet. A 1,000 cu ft room with 100 CFM supply has an ACH of 6.0. ACH = (CFM x 60) / volume.

What is a good ACH for a bedroom?

Most building codes and ASHRAE 62.2 target 5 to 6 ACH for residential bedrooms. That balances fresh air, energy use, and noise. Higher ACH is needed when occupants exceed two or in zones with high outdoor pollution.

How many ACH should a restaurant kitchen have?

Commercial kitchens typically run 15 to 30 ACH to remove smoke, grease vapors, and heat from cooking equipment. Heavy-duty hoods over fryers and char-broilers can push 60 ACH momentarily during peak cooking.

What is the ACH for an operating room?

ASHRAE 170 requires a minimum of 20 air changes per hour for hospital operating rooms, with at least 4 of those being outdoor air. Some facilities run 25 to 30 ACH for orthopedic or implant procedures.

Is higher ACH always better?

Not always. Higher ACH improves pollutant and pathogen removal but raises fan energy, noise, and conditioning load. Balance ACH against ASHRAE 62.1 outdoor-air requirements and equipment capacity. For most offices, 6 to 10 ACH is the comfort sweet spot.

Does ACH include outdoor air?

ACH measures total airflow through the space. The portion that is outdoor (rather than recirculated) air is set by ASHRAE 62.1 as CFM per person plus CFM per square foot. You need both metrics for code compliance.

Is it possible to embed the Air Changes Per Hour Calculator on another website?

Yes, embedding the Air Changes Per Hour Calculator is free. Hit the "Embed" button on this page, adjust the width, height, and theme, then grab the iframe code. It works on WordPress, Wix, Squarespace, Shopify, and plain HTML pages. No registration needed. Full instructions at calculory.com/services/embed-calculators.

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