Educational explanation, not professional design advice. Last reviewed 11 November 2026. Disclaimer
In plain English
Ventilation replaces stale or polluted room air with fresh air. A simple way to describe how much is supplied is air changes per hour (ACH): the airflow in one hour divided by the volume of the room. Six air changes per hour means that every hour, six room-volumes of air are blown in (and the same amount leaves).
ACH is easy to picture and is often used for workshops, toilets, kitchens, plant rooms and spray booths. Once you know the airflow, you can choose a fan and size the ducts that carry the air.
Technical definition
Air changes per hour is the volumetric airflow rate V̇ supplied to (or extracted from) a space, divided by the volume of the space: ACH = V̇ × 3600 ÷ Vroom, with V̇ in m³/s and Vroom in m³. In everyday units, ACH = (m³/h) ÷ m³. In a duct, the same airflow is related to the average air velocity v and the inside cross-section area A by V̇ = A × v.
How it works and where it is used
One air change does not replace all of the room air. When the incoming air mixes well with the room air, the share of the original air that has been replaced after n air changes is 1 − e−n: about 63% after one air change and 95% after three. This is why higher ACH values are used where fumes, smells or moisture must be cleared quickly.
Many standards do not specify ventilation as ACH. ASHRAE 62.1, for example, sets outdoor air per person plus per square metre of floor, and local regulations or process rules may set other values. ACH is still useful for comparing designs and for spaces where the room volume, not the number of people, controls the problem.
Air velocity in a duct is a balance. Fast air allows small, cheap ducts but causes noise and high pressure loss, which means a bigger fan and more electricity. Slow air is quiet but needs larger ducts. Designers therefore pick a target velocity for each part of the system and size the duct from the airflow.
Formula
ACH = V̇ (m³/h) ÷ Vroom (m³) V̇ (m³/h) = ACH × Vroom
V̇ = A × v D = √(4A ÷ π) for a round duct
Vroom = length × width × height. In inch-pound units, ACH = cfm × 60 ÷ ft³. One air change takes 60 ÷ ACH minutes. A is the inside cross-section area of the duct (m²), v the average velocity (m/s) and D the inside diameter of a round duct.
Worked example
Example 1 — an office room. A room 5 m × 4 m × 3 m has a volume of 60 m³. A fan supplies 120 m³/h, so ACH = 120 ÷ 60 = 2. To reach 6 ACH, the airflow must be 6 × 60 = 360 m³/h = 0.1 m³/s. In a round duct at 4 m/s: A = 0.1 ÷ 4 = 0.025 m², D = √(4 × 0.025 ÷ π) = 178 mm — a 200 mm duct would be the next common size.
Example 2 — a small workshop. A workshop 12 m × 8 m with a 4 m ceiling has a volume of 384 m³. For 6 ACH, the extract airflow is 6 × 384 = 2304 m³/h = 0.64 m³/s. With good mixing, after 30 minutes (3 air changes) about 95% of the original air has been replaced.
In the workshop and in practice
Use ACH to size extract fans for toilets, kitchens and battery rooms, to check whether a workshop or store room has enough fresh air, and to set purge rates after painting or cleaning. Always add make-up air: an extract fan cannot remove air faster than air can get back in, so doors, grilles or a supply fan are needed. When the supply air also cools the room, check the airflow against the cooling load as well — the air must carry the heat, and that may need more airflow than the ventilation requirement.
Common mistakes and misconceptions
- Using the floor area instead of the volume. A room with a high ceiling holds more air and needs more airflow for the same ACH.
- Assuming one air change replaces all the air. With mixing, it replaces about 63%.
- Using the duct’s outside size. Velocity depends on the inside area; insulation and lining reduce it.
- Treating ACH as a legal requirement. Check the ventilation standard and local regulations that apply to your building and process.
Safety note
Spaces with hazardous fumes, flammable vapours, dust or low oxygen need ventilation designed to the relevant safety standards, often with monitoring and alarms. Do not rely on a general ACH figure for these.
Related calculators
- Air Changes per Hour (ACH) Calculator — Air changes per hour from room size and airflow — or the airflow needed for a target ACH.
- Duct Size & Air Velocity Calculator — Air velocity in round or rectangular ducts, round duct size for a velocity, and rectangular-to-round equivalent diameter.
- Air Sensible & Latent Heat Calculator — Sensible, latent and total heat of an air stream — or the airflow or temperature change for a given load.
Related topics
- Cooling Load: Sensible and Latent Heat — The heat an air-conditioning system must remove, split into temperature (sensible) and moisture (latent) parts.
References
- ASHRAE Standard 62.1: Ventilation and Acceptable Indoor Air Quality
- ASHRAE Handbook — Fundamentals: Ventilation and Infiltration; Duct Design
- CIBSE Guide B2: Ventilation and Ductwork