Air Change Per Hour Calculator

Calculation mode *

Pick what you want to solve for.

Units *

ACH is always per hour.

Result decimals

Controls rounding in the result box.

Volume input *

Use direct volume for irregular rooms.

Room shape

Custom uses the volume field.

Notes (optional)

Included in exports for documentation.

Length

Used when volume is from dimensions.

Width

Use consistent units across fields.

Height

Ceiling height or effective zone height.

Room volume

ft³

Required for direct volume or custom shape.

Airflow

CFM

Used when computing ACH.

Target ACH

1/h

Used when computing required airflow.

Reset

Formula used

Room volume (rectangular): V = L × W × H
ACH (Imperial): ACH = (CFM × 60) ÷ V(ft³)
ACH (Metric): ACH = Q(m³/h) ÷ V(m³)
Required airflow: rearrange the same equation to solve for Q.

This tool estimates air changes from supply or exhaust airflow. Outdoor air fraction and filtration goals depend on the design basis.

How to use this calculator

Choose the calculation mode: ACH or required airflow.
Select units, then enter room dimensions or direct volume.
Enter airflow or target ACH, depending on your mode.
Press Submit to see results above the form.
Download CSV or PDF for reports and submittals.

Example data table

Space	Volume	Airflow	ACH	Notes
Bedroom	2,250 ft³	60 CFM	1.60 1/h	Balanced ventilation, quiet mode.
Office	4,050 ft³	450 CFM	6.67 1/h	Higher target during occupied hours.
Lab (metric)	120 m³	1,200 m³/h	10.00 1/h	Check pressure cascade requirements.

Example values are illustrative only; project criteria may differ.

FAQs

1) What does ACH measure?

ACH estimates how many times the room air volume is replaced each hour, based on measured or design airflow and the space volume.

2) Is ACH the same as outdoor air changes?

Not always. If supply air is recirculated, ACH reflects total air delivered. Outdoor air changes depend on the outdoor air fraction and economizer operation.

3) Which airflow should I use, supply or exhaust?

Use the airflow that represents the air exchange driver for the zone. For balanced systems, supply is common. For negative-pressure rooms, exhaust is often the driver.

4) How accurate is the volume from dimensions?

It is accurate for simple shapes and full-height mixing. For plenum spaces, partial-height mixing, or irregular geometry, use an effective volume estimate.

5) What ACH should I design for?

Targets depend on standards, occupancy, contaminants, and risk. Homes are often lower, many commercial spaces are moderate, and critical rooms can be higher.

6) How do I convert between CFM and m³/h?

A common conversion is 1 CFM ≈ 1.699 m³/h. Convert airflow first, then compute ACH using the matching volume units.

7) Why does ACH matter for comfort and safety?

Higher ACH can reduce contaminants and odors, but increases energy use and noise. Pair ventilation with filtration, source control, and commissioning checks.