Air Changes Per Hour To m3/h Converter
Convert a target ACH and room volume into ventilation flow in m3/h, litres per second and CFM, with a reverse check for existing fan flow.
Room And ACH Inputs
Converted Flow Rate
Required airflow including allowance.
ACH is a volume conversion. It does not prove that air reaches every occupied zone, removes pollutants, or satisfies a legal duty.
What ACH To m3/h Means
Air changes per hour, or ACH, describes how many times the volume of a room is supplied, extracted or cleaned in one hour. To turn ACH into cubic metres per hour, you need the room volume. A small room and a hall can share the same ACH target but need very different airflow. A 20 m3 storeroom at 6 ACH needs 120 m3/h before allowance. A 200 m3 hall at the same 6 ACH needs 1,200 m3/h before allowance.
This converter keeps the relationship clear: room volume multiplied by ACH gives m3/h. It then shows litres per second, CFM and litres per second per person because those units appear in fan data sheets, ventilation notes, commissioning records and UK workplace guidance. The reverse fan-flow field lets you enter an existing extract or supply rate and see the ACH it would provide for the measured room.
Important Boundary For Ventilation Use
The output is a mathematical conversion, not a full ventilation design. Good ventilation depends on fresh-air supply, extract location, mixing, door positions, heat gains, pollutant sources, recirculation, filter condition, maintenance and how people use the room. HSE guidance notes that ventilation rate is only one part of assessing poor ventilation, and that CO2 monitoring can help in some occupied spaces. For complex systems, specialist ventilation advice may be needed.
Do not add supply and extract flows together unless a ventilation engineer has told you to do so for that system. For many practical checks, the limiting fresh-air flow is the useful value. A fan recirculating the same room air is not the same as fresh outdoor air unless filtration and fresh-air make-up are part of the design.
Conversion Formula
room volume in m3 = length x width x height
required m3/h = room volume x ACH
litres per second = m3/h x 1000 / 3600
CFM = m3/h x 0.588577
If the room is measured in feet, the calculator converts cubic feet to cubic metres using 1 cubic foot = 0.0283168466 m3. The allowance percentage is applied after the base m3/h conversion. Use the allowance for early planning, not as a substitute for pressure loss, duct sizing or commissioning data.
Worked Example: Meeting Room
A meeting room is 5 m long, 4 m wide and 2.6 m high. The volume is 52 m3. If the design target is 6 ACH, the base airflow is 52 x 6, which is 312 m3/h. With a 10% planning allowance, the calculator shows about 343 m3/h. That is also about 95 L/s or 202 CFM. If six people use the room, the flow is about 15.9 L/s per person after allowance.
If an existing fan is rated at 250 m3/h, the reverse check is 250 / 52 = 4.8 ACH. That does not automatically fail the room, but it gives a clear prompt to compare the fan, occupancy, CO2 readings, fresh-air path and room use with the actual requirement.
Quick Conversion Table
| Room Volume | 3 ACH | 6 ACH | 10 ACH | 15 ACH |
|---|---|---|---|---|
| 25 m3 | 75 m3/h | 150 m3/h | 250 m3/h | 375 m3/h |
| 50 m3 | 150 m3/h | 300 m3/h | 500 m3/h | 750 m3/h |
| 100 m3 | 300 m3/h | 600 m3/h | 1,000 m3/h | 1,500 m3/h |
| 250 m3 | 750 m3/h | 1,500 m3/h | 2,500 m3/h | 3,750 m3/h |
| 500 m3 | 1,500 m3/h | 3,000 m3/h | 5,000 m3/h | 7,500 m3/h |
Measuring The Room Before Conversion
Simple Rectangular Rooms
Measure internal length, width and height in metres. Ignore skirting boards, shelves and small recesses unless they significantly change the air volume. Use the clear height that people occupy, not the external building height.
Irregular Rooms
Split the area into simple rectangles, calculate each volume and add them. For sloped ceilings, use an average height. For mezzanines, open voids or connected spaces, ask whether the air path treats them as one volume.
Common Mistakes
- Using floor area instead of room volume. ACH needs cubic metres, not square metres.
- Adding supply and extract fan ratings together without checking the system balance.
- Using a fan’s free-air rating when ducts, filters or grilles reduce the real installed flow.
- Treating recirculated air as fresh air without checking filtration and outdoor air make-up.
- Forgetting that a high ACH number can still leave dead zones if air enters and leaves from poor locations.
When To Ask A Specialist
Ask a competent ventilation engineer or building-services professional where the space has hazardous substances, welding fumes, dust, high heat loads, combustion appliances, complex ductwork, multiple rooms, recirculation, pressure requirements or a legal compliance issue. Local exhaust ventilation, infection-control rooms, laboratories, kitchens and industrial processes need more than an ACH-to-m3/h conversion.
FAQ
How do I convert ACH to m3/h?
Multiply the room volume in cubic metres by the ACH target. A 50 m3 room at 6 ACH needs 300 m3/h before any allowance.
What is the difference between ACH and m3/h?
ACH is a rate relative to the room volume. m3/h is the actual airflow volume per hour. The same ACH can mean a small or large m3/h value depending on room size.
Can I use feet instead of metres?
Yes. Select feet and enter length, width and height. The calculator converts cubic feet to cubic metres before applying the ACH formula.
Why show litres per second per person?
UK ventilation guidance often discusses fresh-air flow per occupant. The value helps compare the calculated airflow with occupied-room guidance, but it is not a full risk assessment.
Does a higher ACH always mean better ventilation?
Not always. Air distribution, fresh-air source, extract points, comfort, noise, energy use and pollutant sources all matter.
Is CFM the same as m3/h?
No. CFM is cubic feet per minute. The calculator uses 1 m3/h = 0.588577 CFM for the displayed equivalent.
Sources
- Health and Safety Executive. (n.d.). Ventilation in the workplace. HSE. https://www.hse.gov.uk/ventilation/
- Health and Safety Executive. (n.d.). Assessing the risk of poor ventilation. HSE. https://www.hse.gov.uk/ventilation/assessing-the-risk-of-poor-ventilation.htm
- Chartered Institution of Building Services Engineers. (2015). Top Tips 1: Ventilation in Buildings. CIBSE. https://www.cibse.org/knowledge-research/knowledge-resources/engineering-guidance/top-tips/cibse-top-tips-1-ventilation-in-buildings-2015
- Bureau International des Poids et Mesures. (2019). The International System of Units (SI Brochure) (9th ed.). BIPM. https://www.bipm.org/en/publications/si-brochure