You can feel a stuffy room before you can measure it. The air gets heavy, people get sleepy, and smells stay around. Ventilation fixes this by swapping the air inside a room with fresh air from outside. Air changes per hour, or ACH, tells you how often that swap happens.
Enter the airflow and the size of the room below, and the calculator will give you the ACH.
What is air changes per hour?
Air changes per hour (ACH) is the number of times the full volume of air in a room is replaced in one hour. If a room has 6 ACH, then in theory all the air in it is replaced six times every hour, or once every ten minutes.
It is one of the most common ways to describe ventilation. Offices, workshops, laboratories, kitchens, hospitals and spray booths all have an ACH target that depends on what happens inside.
The ACH formula
ACH = Airflow (m3 per hour) / Room volume (m3)
Room volume is length x width x height. If your airflow is in another unit, convert it to cubic metres per hour first. The calculator does this for you:
- Litres per second x 3.6 = cubic metres per hour
- Cubic feet per minute (CFM) x 1.699 = cubic metres per hour
Worked example
A workshop is 8 m long, 5 m wide and 3 m high. An extraction fan moves 600 cubic metres of air per hour.
- Room volume = 8 x 5 x 3 = 120 m3
- ACH = 600 / 120 = 5
The air in the workshop is replaced about five times an hour.
Using CFM. A different room is 10 m x 6 m x 3 m, which is 180 m3. The fan is rated at 400 CFM.
- Airflow = 400 x 1.699 = about 680 m3 per hour
- ACH = 680 / 180 = about 3.8
Why ventilation matters at work
- Fumes, dust and vapours: fresh air dilutes and removes them before they build up
- Carbon dioxide and oxygen: in a crowded or closed room, CO2 rises and people feel tired and lose focus
- Heat and humidity: moving air helps people stay comfortable and cuts heat stress
- Germs in the air: good ventilation lowers the chance of infections spreading in shared rooms
- Fire and explosion risk: in some places, ventilation stops flammable gas or vapour from reaching dangerous levels
How many air changes do you need?
There is no single number. The right ACH depends on what the room is used for, how many people are in it, what is released into the air and what the law or your standard says. A storeroom needs far fewer air changes than a paint shop, a laboratory or an isolation room in a hospital.
Look at the guidance that applies to your place, such as ASHRAE guidance, your local building code, or the rules for your industry. If a hazardous substance is involved, the amount of airflow is decided by the risk assessment, not by a general rule.
What ACH does not tell you
ACH is an average for the whole room. It does not show whether fresh air reaches every corner. In some rooms, air travels straight from the inlet to the fan and leaves dead spots untouched. A room can show a good ACH on paper and still have a stuffy corner where a worker stands all day.
That is why it helps to check in other ways too. Watch where smoke from a smoke stick moves. Measure CO2 at the places where people sit. Ask workers where the air feels bad.
How to get your airflow figure
- Use the fan or air handler data sheet, but remember that the rated flow is often for free air. With ducts, filters and bends, the real flow is lower.
- Measure the air speed at the grille or duct with an anemometer, and multiply by the open area to get the flow.
- Ask the ventilation contractor for the measured flow from the last test.
Step by step
- Measure the length, width and height of the room in metres.
- Find the airflow and note its unit.
- Enter the numbers and choose the right unit.
- Read the ACH.
- Compare it with the figure your standard or risk assessment asks for.
- If it is too low, improve the airflow and test again.
Ways to improve ventilation
- Fit a bigger fan, or add a second one
- Clean or replace blocked filters and grilles
- Place air inlets and outlets so that air crosses the work area, not just the middle of the room
- Use local extraction at the source of fumes, which is often better than moving the whole room’s air
- Open doors and windows when it is safe and the work allows it
- Service the system on a schedule, because fans and ducts lose power with dirt and wear
Common mistakes
- Using the fan label figure as the real airflow
- Forgetting the room height, and getting the volume wrong
- Mixing units, for example putting CFM in as cubic metres per hour
- Counting a ceiling fan that only stirs the air as ventilation
- Assuming a high ACH means the air is clean in every part of the room
- Not retesting after the room layout or the work changes
Linking it to your wider plan
Poor ventilation is a hazard that should go through your usual risk assessment. Score it with the risk matrix calculator, and show the effect of a new fan or extraction system with the residual risk calculator. In hot rooms, check the heat stress calculator as well.
Frequently asked questions
What does ACH mean?
ACH means air changes per hour. It is how many times the air in a room is replaced in one hour.
Is a higher ACH always better?
Not always. More airflow can cost more, create draughts and make noise. The aim is to reach the level your use needs.
How do I change litres per second to ACH?
Multiply litres per second by 3.6 to get cubic metres per hour, then divide by the room volume.
Does opening a window count?
It helps, but it is hard to measure and depends on wind and weather. For a rated target, use a measured mechanical airflow.
Can I use this for a large hall?
Yes, but ACH is an average, so check that air reaches the places where people work.
Learn this properly
Ventilation and indoor air quality are covered in NEBOSH, IOSH and OSHA courses. ISH Institutes teaches these in Peshawar. Chat with us on WhatsApp and we will help you pick the right course.