# Air Changes Per Hour Calculator

Calculate air changes per hour (ACH) from room size and airflow CFM. Find the required ventilation rate for any room. Free ACH calculator.

## What this calculates

Air changes per hour (ACH) tells you how many times the entire volume of air in a room gets replaced in one hour. This calculator works two ways: enter your airflow in CFM to find the resulting ACH, or enter a target ACH to find how much CFM you need. Essential for HVAC design, exhaust fan sizing, and indoor air quality.

## Inputs

- **Room Length** (ft) — min 1
- **Room Width** (ft) — min 1
- **Ceiling Height** (ft) — min 6, max 30
- **Airflow Rate** (CFM) — min 0 — Cubic feet per minute of supply air. Leave at 0 to calculate from target ACH.
- **Target ACH** — min 0, max 100 — Desired air changes per hour. Used to calculate required CFM if airflow is 0.

## Outputs

- **Room Volume** (ft³)
- **Air Changes Per Hour** (ACH) — How many times the room air is replaced per hour
- **Required CFM** (CFM) — Airflow needed to achieve target ACH
- **Airflow Rate** (L/s) — Equivalent airflow in liters per second

## Details

The formula is ACH = (CFM x 60) / Room Volume. CFM is cubic feet per minute, the 60 converts minutes to hours, and room volume is length x width x ceiling height in cubic feet. Flip it around and you get Required CFM = (ACH x Volume) / 60.

Different rooms need different ACH rates. Bathrooms typically need 8 ACH (that is why exhaust fans are sized for the room). Kitchens need 15-20 ACH for cooking fumes. Offices need 4-6 ACH. Hospitals require 6-12 ACH depending on the area. Warehouses may only need 1-2 ACH since they are large, open spaces.

For residential ventilation, ASHRAE 62.2 recommends a minimum continuous rate of 0.35 ACH or 15 CFM per person, whichever is greater. This is much lower than the rates needed for kitchens and bathrooms because it is a whole-house average, not a spot ventilation rate.

## Frequently Asked Questions

**Q: What is a good ACH rate for a house?**

A: ASHRAE recommends a minimum of 0.35 ACH for whole-house ventilation. Modern tight homes may naturally achieve only 0.1-0.2 ACH, which requires mechanical ventilation (HRV or ERV). Older, leaky homes may have 1-2 ACH from infiltration alone. Individual rooms like bathrooms need 8 ACH and kitchens need 15-20 ACH during use.

**Q: How do I calculate the CFM for a bathroom exhaust fan?**

A: For bathrooms under 100 sq ft, the simple rule is 1 CFM per square foot. An 80 sq ft bathroom needs an 80 CFM fan. For larger bathrooms, calculate based on 8 ACH: multiply the room volume by 8 and divide by 60. A 10x12 bathroom with 8-foot ceilings needs (960 x 8) / 60 = 128 CFM.

**Q: What is the difference between ACH and CFM?**

A: CFM (cubic feet per minute) measures airflow volume. ACH (air changes per hour) measures how often that airflow replaces the room's air. A 200 CFM fan in a small room has a higher ACH than the same fan in a large room. ACH depends on both the airflow rate and the room size.

**Q: Does a higher ACH mean better air quality?**

A: Generally yes, up to a point. Higher ACH means pollutants, odors, and humidity are diluted faster. But excessively high ACH wastes energy because you are conditioning outdoor air that immediately leaves. The goal is to match ACH to the room's use: more for kitchens and labs, less for storage and hallways.

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Source: https://vastcalc.com/calculators/construction/air-changes-per-hour
Category: Construction
Last updated: 2026-04-08