Air Changes per Hour Calculator
Find the air changes per hour (ACH) a room gets from its ventilation. Enter the room size and the fan or supply airflow in CFM to get the ACH, the room volume and the minutes per air change — plus the airflow needed to hit a target, checked against common room-type goals. Includes by-airflow and by-room-type tables and a downloadable PDF.
| Airflow | ACH | Min / change | vs target |
|---|---|---|---|
| Enter a space to compare | |||
| Room type | Target ACH | Required CFM | vs current |
|---|---|---|---|
| Enter a space to compare | |||
How to use the air changes per hour calculator
Four quick steps turn a room’s size and its fan airflow into air changes per hour, minutes per air change and the CFM needed to hit a target — with by-airflow and by-room-type tables and a downloadable report.
Enter the room size
Give the length, width and ceiling height in feet to get the room’s air volume.
Add the airflow
Enter the fan, exhaust or supply airflow in CFM from its spec label.
Set a target
Pick a room type to fill in a typical target ACH, or type your own.
Read results & save a PDF
Get ACH, a pass/fail against your target and the CFM needed — then download a report.
How air changes per hour are calculated
Airflow per hour divided by the room’s volume gives the air changes; the same relationship run backward gives the airflow you need. Here is the exact math the calculator runs.
Worked example — 15 × 12 ft living room, 8 ft ceiling, 150 CFM fan, 6 ACH target
Volume: 15 × 12 × 8 = 1,440 ft³ · ACH: 150 × 60 ÷ 1,440 = 6.25
Minutes per change: 1,440 ÷ 150 = 9.6 min
Required for 6 ACH: 6 × 1,440 ÷ 60 = 144 CFM · Check: 6.25 ≥ 6 → meets target
≈ 6.3 air changes per hour — one every 9.6 minutes, meeting the 6 ACH target with 6 CFM to spare.
Air change reference charts
Handy lookups for typical room targets, how airflow changes ACH, and the CFM each room type needs. Local codes and specific uses can call for more.
| Room type | Target ACH | Minutes / change | Why |
|---|---|---|---|
| Bedroom | 5 | 12 | Low activity, mostly sleeping |
| Living room | 6 | 10 | General occupancy |
| Office | 6 | 10 | Occupancy, equipment heat |
| Kitchen | 8 | 7.5 | Cooking heat, moisture, odors |
| Workshop | 8 | 7.5 | Dust and fumes |
| Bathroom | 10 | 6 | Shower moisture, odors |
Minutes per change = 60 ÷ ACH. These are planning ranges, not code minimums.
| Airflow | ACH | Minutes / change | vs 6 ACH target |
|---|---|---|---|
| 50 CFM | 2.1 | 28.8 | Low |
| 100 CFM | 4.2 | 14.4 | Low |
| 150 CFM | 6.3 | 9.6 | OK |
| 200 CFM | 8.3 | 7.2 | OK |
| 300 CFM | 12.5 | 4.8 | OK |
| 400 CFM | 16.7 | 3.6 | OK |
| 500 CFM | 20.8 | 2.9 | OK |
A 15 × 12 ft room with an 8 ft ceiling. Doubling the airflow doubles the air changes.
| Room type | Target ACH | Required CFM | With 150 CFM |
|---|---|---|---|
| Bedroom | 5 | 120 | OK |
| Living room | 6 | 144 | OK |
| Office | 6 | 144 | OK |
| Kitchen | 8 | 192 | Low |
| Workshop | 8 | 192 | Low |
| Bathroom | 10 | 240 | Low |
Required CFM = target ACH × volume ÷ 60. Measured fan airflow is often below the rating once ducting is added.
Everything the calculator works out
A room size and an airflow give you the full ventilation picture — air changes, timing, the airflow you need and a clear pass or fail.
Key figures behind the estimate
Built for any room that needs fresh air
From a steamy bathroom to a dusty workshop, the same calculator tells you whether a fan is moving enough air.
Dealing with a foggy mirror, mildew or lingering odors and wants to know if the exhaust fan is big enough.
- Measure the room
- Pick the room type
- Read the extra CFM needed
Sanding, finishing or soldering in a garage or shop and needs enough airflow to clear dust and fumes.
- Use the workshop target
- Compare fan sizes
- Add headroom for ducting
Comparing air purifiers by their CADR and wants to see how many air changes each gives in a room.
- Enter the CADR as airflow
- Check the ACH
- Size up for bigger rooms
7 things to know about air changes
A few details keep a ventilation estimate realistic and the room actually getting the fresh air it needs.
Air changes per hour calculator FAQ
The airflow, ventilation and fan sizing questions people ask most about air changes per hour.
Air changes per hour, or ACH, is how many times the full volume of air in a room is replaced with fresh or filtered air in one hour. A room with 6 ACH has an amount of air equal to its whole volume moved through it six times every hour, or once every 10 minutes.
ACH is a simple way to compare ventilation between rooms of different sizes, because it accounts for the room’s volume rather than just the fan’s airflow. A 100 CFM fan gives a small bathroom far more air changes than it would a large living room.
Higher ACH generally clears moisture, odors and airborne pollutants faster, but more ventilation also means more heated or cooled air leaving the home.
Multiply the airflow in cubic feet per minute (CFM) by 60 to get cubic feet per hour, then divide by the room’s volume in cubic feet. The room volume is length × width × ceiling height.
For example, a 15 by 12 foot room with an 8-foot ceiling has a volume of 1,440 cubic feet. A 150 CFM fan moves 150 × 60, or 9,000 cubic feet per hour, so the room gets 9,000 ÷ 1,440, or about 6.3 air changes per hour.
For rooms with vaulted or sloped ceilings, use the average ceiling height to estimate the volume.
It depends on how the room is used. This calculator uses typical targets of about 5 ACH for bedrooms, 6 for living rooms and offices, 8 for kitchens and workshops, and 10 for bathrooms. Rooms with moisture, cooking, fumes or dust are ventilated harder than rooms where people mostly sleep or sit.
These are planning ranges, not code requirements. Many residential codes set minimum exhaust rates in CFM for bathrooms and kitchens rather than an ACH figure, and occupancy, local rules and specific pollution sources can call for more.
Check your local mechanical code or ask an HVAC professional for the exact requirement.
Multiply your target air changes per hour by the room volume in cubic feet, then divide by 60. For a 15 by 12 foot room with an 8-foot ceiling (1,440 cubic feet) and a target of 6 ACH, you need 6 × 1,440 ÷ 60, or 144 CFM.
For a bathroom measuring 8 by 5 feet with an 8-foot ceiling (320 cubic feet) and a target of 10 ACH, you need about 53 CFM, so a fan rated at 50 CFM would fall slightly short once real-world duct losses are counted.
Choose a fan with some headroom above the calculated figure, especially if the duct run is long or has several bends.
A fan’s CFM rating is measured in test conditions with very little resistance. Once it is installed, duct length, bends, small duct diameters, dampers, grilles and the exterior vent cap all add resistance, called static pressure, which reduces how much air the fan actually moves.
A long or flexible duct with several elbows can cut the delivered airflow well below the number on the box. When sizing, check the manufacturer’s airflow at a realistic static pressure rather than the headline figure, keep duct runs short and straight, and use the duct size the fan is designed for.
An HVAC technician can measure the real airflow with a flow hood if you need to know it precisely.
No. This calculator measures ventilation, meaning how many times a fan or air system replaces the air in a room under normal operation.
ACH50 is a measure of airtightness. During a blower door test, a fan depressurizes the whole house to 50 pascals and the airflow needed to hold that pressure is converted into air changes per hour at 50 pascals. A lower ACH50 means a tighter house that leaks less heated and cooled air. Many recent energy codes set a maximum ACH50 for new homes.
Because ACH50 is measured under strong artificial pressure, it is much higher than the natural air leakage a home experiences day to day and cannot be compared directly with a room’s ventilation ACH.
Yes. More ventilation brings in more outdoor air that has to be heated in winter and cooled and dehumidified in summer, which raises energy bills. Very high airflow can also feel drafty, make winter air uncomfortably dry, and add fan noise.
Strong exhaust fans can depressurize a home and pull air backward through fuel-burning appliance flues, so large exhaust systems may need a dedicated make-up air supply. The goal is enough ventilation for the room’s use, not the most possible.
In tight, energy-efficient homes, a heat recovery or energy recovery ventilator can supply fresh air while recovering much of the heat or cooling.
Divide the room volume in cubic feet by the airflow in CFM to get the minutes per air change. A 1,440-cubic-foot room with a 150 CFM fan gets one air change about every 9.6 minutes.
Keep in mind that one air change does not replace every bit of the old air, because fresh air mixes with the existing air as it comes in. With good mixing, about 63 percent of the original air is replaced after one air change, about 86 percent after two, and about 95 percent after three.
That is why a bathroom fan should keep running for a while after a shower to clear the moisture fully.
Estimate only — check your local ventilation code. This calculator works in US units (feet, CFM): air changes per hour = airflow (CFM) × 60 ÷ room volume (ft³), and required airflow = target ACH × volume ÷ 60. The room-type targets of 5 ACH (bedroom), 6 (living room, office), 8 (kitchen, workshop) and 10 (bathroom) are typical planning ranges, not code minimums — local mechanical codes, occupancy and sources such as cooking, moisture and fumes can call for more. A fan’s delivered airflow is often below its rated CFM once ducting is added. Confirm ventilation requirements and fan sizing with your local building department or a qualified HVAC professional, and make sure exhaust fans vent outdoors. For planning purposes only.