Skip to main content
Tonnage Calculator AC Logo

Air Changes per Hour (ACH) Calculator

Convert CFM to ACH and ACH to CFM with the formula shown, and check whether your air purifier's CADR actually covers your room. Free, instant, no signup.

Air Changes per Hour (ACH) Calculator

ACH = (CFM × 60) ÷ room volume, both directions, plus air purifier CADR

Room volume: 1,152 cubic feet

fan rating, HVAC register, or ERV/HRV flow
This airflow delivers
5.2 ACH
Adequate for low-occupancy spaces; below the 6 ACH living-space target

The ACH Calculation Formula

Air changes per hour and airflow are two views of the same thing, linked by room volume:

  • ACH from airflow: ACH = (CFM × 60) ÷ room volume (ft³)
  • Airflow from a target: CFM = (ACH × room volume) ÷ 60
  • Room volume: length × width × ceiling height

Worked example: a 12×12 bedroom with 8 ft ceilings holds 1,152 ft³. A 100 CFM airflow delivers (100 × 60) ÷ 1,152 ≈ 5.2 ACH - right at the bedroom target. Need the airflow side of this math in more depth (per-ton system CFM, duct capacity, exhaust fans)? That's our HVAC CFM calculator.

How to Calculate Air Changes per Hour, Step by Step

The tool at the top is an air changes per hour calculator, and it runs both ways: as an ACH to CFM calculator when you know the target, and as a CFM to ACH calculator when you know the fan. HVAC people call it an ACH calculator, others an air change per hour calculator, an air changes calculator or an air exchange calculator. By hand, the calculation of air changes per hour takes three steps, and you need only a tape measure and one airflow figure.

  1. Find the room volume. Multiply length by width by ceiling height, all in feet. The answer is in cubic feet.
  2. Find the airflow in CFM. Use the rating of the fan, the purifier or the supply register that serves the room.
  3. Multiply the CFM by 60 and divide by the volume. That is the number of times the air in the room is replaced each hour.

How do you calculate air changes per hour? Multiply the airflow in CFM by 60 and divide by the room volume, which is also how to calculate air change rate, how to calculate ACPH and how to calculate air exchange rate. Air change rate, ACPH and air exchange rate are three more names for ACH, so the air change per hour formula and the ACPH formula are the one written above. Written as the ACH equation or the air changes per hour formula, it is still CFM times 60 over the volume. The ACH to CFM formula is the same line solved for airflow: CFM = ACH × volume ÷ 60.

Worked Example: A Room 20 Feet by 15 Feet With a 10-Foot Ceiling

This is a well known exam question: a room has dimensions of 20 feet by 15 feet with a 10-foot high ceiling, and six 200 CFM registers supply air to it. How many air changes per hour take place? The volume is 20 × 15 × 10 = 3,000 cubic feet. The six registers deliver 6 × 200 = 1,200 CFM. The air change calculation is (1,200 × 60) ÷ 3,000 = 24 air changes per hour.

That is a commercial level of airflow. For a home, turned round, the same room needs 200 CFM for 4 air changes per hour, 300 CFM for 6 and 400 CFM for 8. Tip: double the ceiling height and you halve the ACH from the same fan. Floor area alone never tells you the air change rate per hour, because the fan has to move the whole volume.

ACH to CFM Chart for Common Room Sizes

The airflow each room needs to reach a given number of air changes per hour. Read across from the room to the rate you are aiming for.

RoomVolume2 ACH4 ACH6 ACH8 ACH
10 x 10 ft room, 8 ft ceiling800 cu ft27 CFM53 CFM80 CFM107 CFM
12 x 12 ft room, 8 ft ceiling1,152 cu ft38 CFM77 CFM115 CFM154 CFM
15 x 15 ft room, 8 ft ceiling1,800 cu ft60 CFM120 CFM180 CFM240 CFM
20 x 15 ft room, 10 ft ceiling3,000 cu ft100 CFM200 CFM300 CFM400 CFM
20 x 20 ft room, 8 ft ceiling3,200 cu ft107 CFM213 CFM320 CFM427 CFM
1,000 sq ft, 8 ft ceiling8,000 cu ft267 CFM533 CFM800 CFM1,067 CFM
2,000 sq ft, 8 ft ceiling16,000 cu ft533 CFM1,067 CFM1,600 CFM2,133 CFM

To go from CFM to air changes per hour for a room that is not in the chart, use the calculator at the top. It works as an air change rate calculator in one direction and an air changes per hour to CFM converter in the other.

Recommended ACH by Space

How many air changes per hour do you need? It depends on what the room is for. There is no single standard air change per hour figure for a house. The table gives the usual target for each kind of space.

SpaceRecommended ACHBasis
Bedroom5 - 6comfort ventilation
Living room / office6 - 8comfort ventilation
Kitchen7 - 9cooking moisture & odors
Bathroom6 - 8moisture removal
Basement3 - 4low occupancy
Air purifier target (allergies/asthma)4 - 5AHAM / EPA guidance
Classroom5 - 6ASHRAE 62.1 ventilation basis
Hospital patient room6 total (2 outdoor)ASHRAE 170
Hospital operating room20 total (4 outdoor)ASHRAE 170
Laboratory6 - 12hazard-dependent, per lab safety standards

Air Purifiers: Turning CADR Into ACH

An air purifier's CADR (Clean Air Delivery Rate) is simply CFM of cleaned air, measured per AHAM's AC-1 standard - so the ACH formula applies directly. The industry target for allergies, asthma, smoke, and airborne illness is 4-5 ACH. Two things manufacturers' "coverage" claims hide: the ratings assume 8-foot ceilings(a 10-foot ceiling cuts effective ACH by 25%), and marketing room sizes are often quoted at only 1-2 ACH. AHAM's own rule of thumb - CADR × 1.55 = maximum room square footage - already bakes in roughly 4.8 ACH at 8 ft ceilings, which is why the calculator reports it as the honest coverage number.

ACH vs. ACH50: Ventilation vs. Airtightness

Don't mix these up. Operating ACH describes air your equipment moves on purpose. ACH50 comes from a blower-door test - the house depressurized to 50 Pascals - and grades how leaky the envelope is: modern energy codes require 3-5 ACH50, older homes commonly test 10-20, and Passive House certification demands 0.6. To estimate real-world infiltration from a test result, divide ACH50 by about 20. That natural leakage number feeds directly into heating and cooling loads - it's one of the inputs in our HVAC load calculator.

Frequently Asked Questions

What is ACH (air changes per hour)?

ACH measures how many times a room's entire volume of air is replaced in one hour. At 6 ACH, all the air in the room is theoretically swapped out every 10 minutes. It's the standard way to express ventilation and air-cleaning rates because it scales with room size - 100 CFM is a lot of air for a closet and almost nothing for a gymnasium, but '6 ACH' means the same turnover everywhere.

How do I calculate ACH?

ACH = (airflow in CFM × 60) ÷ room volume in cubic feet. Example: a 100 CFM fan in a 12×12 room with 8 ft ceilings (1,152 ft³) delivers (100 × 60) ÷ 1,152 ≈ 5.2 ACH. Room volume is just length × width × ceiling height.

How do I convert ACH to CFM?

CFM = (target ACH × room volume) ÷ 60. Example: to hit 8 ACH in a 10×10 bathroom with 8 ft ceilings (800 ft³), you need (8 × 800) ÷ 60 ≈ 107 CFM of exhaust. The ACH → CFM tab above does this and shows the metric equivalents (m³/h and L/s) alongside.

How do I calculate ACH from CFM?

Multiply the CFM by 60 and divide by the room's volume in cubic feet. If your HVAC register delivers 150 CFM into a 1,500 ft³ living room, that's (150 × 60) ÷ 1,500 = 6 ACH. For a whole house, use total system CFM and total conditioned volume.

What is a good ACH for a house?

For comfort ventilation: bedrooms 5-6 ACH, living areas 6-8, kitchens and bathrooms 7-9, basements 3-4. Note these are air-circulation targets used for HVAC design - the code minimum for fresh outdoor air is far lower (ASHRAE 62.2 works out to roughly 0.35 ACH of outdoor air). Circulated air and fresh air are different things; both matter.

How many ACH should an air purifier provide?

4-5 ACH is the widely used target for meaningful air cleaning - allergies, asthma, smoke, and airborne illness. That's the basis of AHAM's room-size ratings. Example: a purifier with a 200 smoke CADR in a 12×12 room (1,152 ft³) delivers about 10.4 ACH - excellent; the same unit in a 500 sq ft open space delivers only 3 ACH. Use the Air Purifier tab above to check your unit against your actual room.

What ceiling height do air purifier ACH calculations assume?

8 feet. AHAM's CADR room-size ratings and virtually every manufacturer's coverage claim are computed at a standard 8-foot ceiling. If your room has 9 or 10-foot ceilings, the real ACH drops proportionally - a room with 10 ft ceilings has 25% more air than the same floor area at 8 ft, so derate the coverage claim by 25% (or just enter your true ceiling height in the calculator above).

How do I calculate ACH without knowing the CFM?

Three options. (1) Look up the equipment: fans, ERVs, and purifiers all publish CFM or CADR ratings. (2) For a building's leakage rate, a blower-door test reports ACH50 (air changes at 50 Pascals depressurization); divide ACH50 by roughly 20 to estimate natural infiltration ACH - a house testing 8 ACH50 leaks about 0.4 ACH naturally. (3) Measure airflow directly with an anemometer at the register (average face velocity × register area = CFM), then convert.

What ACH do hospitals, labs, and clean spaces require?

ASHRAE Standard 170 sets healthcare rates: patient rooms 6 total ACH (2 outdoor), airborne-infection isolation rooms 12, operating rooms 20 total (4 outdoor). Laboratories typically run 6-12 ACH depending on hazard level, and commercial kitchens 15-30. These are code-driven exposure-control numbers - hitting them is about diluting contaminants, not comfort.

How do I convert ACH to m³/s or L/s?

Convert to CFM first (CFM = ACH × volume ÷ 60), then multiply CFM by 0.000472 for m³/s or 0.4719 for L/s. Example: 6 ACH in a 1,152 ft³ room = 115 CFM = 0.054 m³/s = 54 L/s. The ACH → CFM tab shows m³/h and L/s automatically.

What is the difference between ACH and ACH50?

ACH describes normal operating air changes - what a fan or HVAC system actually delivers. ACH50 is a blower-door test result: air changes per hour while the house is depressurized to 50 Pascals, used to grade airtightness (new energy codes require 3-5 ACH50; passive houses hit 0.6). ACH50 is always much higher than natural ACH - divide by about 20 to estimate real-world infiltration.