Air Changes per Hour Calculator (ACH, CFM & m³/h)

How to use the air changes per hour calculator

Enter the length, width and ceiling height of the room and the airflow supplied (or extracted) by the fan, air handler or air purifier. The calculator gives the room volume, the air changes per hour, the time for one air change and the time needed to remove 99% and 99.9% of an airborne contaminant. Enter a target ACH to see the airflow you need in m³/h or CFM. Switch to imperial units to work in feet and CFM.

Air changes per hour formula

ACH = Q ÷ V  (Q in m³/h, V in m³)  ·  ACH = CFM × 60 ÷ V (V in ft³)

Q_required = ACH × V  ·  time to remove a fraction f = ln[1 ÷ (1 − f)] ÷ ACH

The removal time assumes the supply air is clean and mixes well with the room air, with no new contaminant released during the period.

Worked example

A 5 m × 4 m office with a 2.7 m ceiling is supplied with 300 m³/h of air:

• Room volume V = 5 × 4 × 2.7 = 54 m³ (1,907 ft³)
• ACH = 300 ÷ 54 = 5.56 air changes per hour
• One air change takes 60 ÷ 5.56 = 10.8 min
• 99% removal takes ln(100) ÷ 5.56 × 60 = 49.7 min, and 99.9% takes 74.6 min
• For a target of 6 ACH the airflow must be 6 × 54 = 324 m³/h (190.7 CFM)

Airflow needed for a given ACH

Time to remove airborne contaminants

Well-mixed room with clean supply air:

Practical notes

• Mixing matters: short-circuiting between a supply diffuser and a nearby extract grille can leave parts of the room poorly ventilated, so the effective ACH is lower than the calculated value.
• Air purifiers: use the clean air delivery rate (CADR) as the airflow — a purifier with a CADR of 250 m³/h in a 50 m³ room adds 5 ACH of clean air.
• Recirculated air only counts towards contaminant removal if it passes through an effective filter; outdoor-air requirements in ventilation codes are usually stated separately.
• To change a fan’s airflow by changing its speed, use the fan affinity laws calculator; to convert between m³/h, L/s and CFM, use the flow rate converter.

Related calculators

• ⚙️EngineeringFan Power & Fan Affinity Laws CalculatorCalculate fan air and shaft power from airflow, pressure rise and efficiency, then use the fan affinity laws to predict flow, pressure and power at a new speed.Open calculator
• 🔁ConversionFlow Rate ConverterConvert volumetric flow between m³/h, L/min, L/s, US gpm, CFM and m³/s, for pumps, pipes and flow meters.Open calculator
• ⚙️EngineeringChiller Tonnage CalculatorCalculate chiller capacity in tons of refrigeration (TR) and kW from chilled water flow and temperature difference, Q = m·cp·ΔT, with a sizing table.Open calculator
• ➗MathBox Volume CalculatorCalculate the volume, surface area and space diagonal of a box or rectangular prism in litres, cubic metres, cubic feet or gallons, plus volumetric weight.Open calculator
• ⚙️EngineeringCompressed Air Leak CalculatorCalculate the air flow lost through a compressed air leak from hole size and pressure, the compressor power it wastes and the yearly energy and cost.Open calculator
• ⚙️EngineeringAir Receiver Size CalculatorCalculate compressed air receiver tank size for a peak demand event and for compressor load/unload cycling, from flow, pressure band and time.Open calculator

<iframe src="https://zentacalculator.com/embed/engineering/air-changes-per-hour/" title="Air Changes per Hour Calculator (ACH, CFM & m³/h)" width="100%" height="640" style="border:1px solid #e2e5ee;border-radius:12px;max-width:720px" loading="lazy"></iframe>
<p style="font-size:13px"><a href="https://zentacalculator.com/engineering/air-changes-per-hour/">Air Changes per Hour Calculator (ACH, CFM & m³/h)</a> by <a href="https://zentacalculator.com/">Zenta Calculator</a></p>
<script>addEventListener('message',function(e){if(e.origin==='https://zentacalculator.com'&&e.data&&e.data.zentaHeight){document.querySelectorAll('iframe[src="https://zentacalculator.com/embed/engineering/air-changes-per-hour/"]').forEach(function(f){if(f.contentWindow===e.source)f.style.height=e.data.zentaHeight+'px'})}})</script>