Cleanroom Air Change Rate, Supply Airflow and Recovery Time Calculator
Enter the room size and either the supply airflow or the air change rate you want. The calculator gives the other one, the number of HEPA terminals, and an estimate of the 100:1 recovery time measured under ISO 14644-3.
Engineering estimate only. Confirm by testing and validation for your equipment and load.
How the calculation works
Air change rate and supply airflow
N = air changes per hour (h−1), Q = filtered supply airflow (m³/h), V = room volume (m³)
The number of HEPA terminals is the supply airflow divided by the airflow of one terminal, rounded up. A 1220 × 610 mm terminal at a face velocity of 0.45 m/s supplies about 1,200 m³/h; check the filter and diffuser data for your design.
Recovery time
In a non-unidirectional (turbulent, mixed) cleanroom, particles released into the room are removed by dilution. With perfect mixing the concentration falls as C(t) = C0 × e−N·t. Real rooms do not mix perfectly, so the calculator applies a mixing efficiency ε:
t in hours (× 60 for minutes). For a 10:1 reduction use ln(10) ≈ 2.3.
ISO 14644-3 describes the recovery test: the room is loaded with test particles, the source is stopped and the time for the concentration to fall by a factor of 100 is measured at representative locations.
What the numbers mean
- Air change rate is a design figure, not a class. ISO 14644-1 classifies a room by its measured particle concentration. The air change rate needed depends on how many particles people and processes generate, the heat load, room shape and how well the air mixes.
- Recovery time. EU GMP Annex 1 expects the time to return from "in operation" to "at rest" to be established during qualification. A clean-up period of 15–20 minutes has long been used as a guidance value.
- FDA. The FDA guidance on sterile drug products produced by aseptic processing (2004) describes an air change rate of at least 20 per hour as typically acceptable for Class 100,000 (ISO 8) rooms.
- Unidirectional airflow (Grade A, ISO 5 zones) is designed by air velocity over the protected area, not by air changes. This calculator is for mixed-airflow rooms.
Limits of this calculator
- The dilution model assumes particles are spread through the room. Near a source, behind equipment or in dead corners, local recovery is slower.
- The mixing efficiency is an assumption. Measure the recovery time in the real room; use smoke studies to see where air does not mix.
- It does not size the air handling unit, return air or pressure cascade.
Related: ISO 14644-1 classification calculator, ISO 14644-1 cleanroom classification and cleanroom airlock types.
Common questions
How do I calculate air changes per hour in a cleanroom?
Divide the filtered supply airflow in m³/h by the room volume in m³. For example 2,700 m³/h into a 90 m³ room is 30 air changes per hour.
How is cleanroom recovery time estimated?
With mixed airflow the particle concentration falls exponentially. The time for a 100:1 reduction is about 4.6 divided by the air change rate (in hours), longer if the room does not mix well. ISO 14644-3 describes how to measure it.
What is a good cleanroom recovery time?
A clean-up period of 15–20 minutes from in operation to at rest has long been used as a guidance value under EU GMP Annex 1. The actual time is established during qualification.
How many air changes does an ISO 8 cleanroom need?
There is no fixed number in ISO 14644. The FDA aseptic processing guidance describes at least 20 air changes per hour as typically acceptable for Class 100,000 (ISO 8) rooms; the design depends on particle generation and heat load.
Send your numbers to an engineer
Use the button under the result to fill this form, or describe your space and equipment. An engineer reviews it and replies.
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