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Calculator · Air changes
Air changes per hour calculator
Work out air changes per hour for a room or a house from CFM and volume, find the CFM a space needs for a target ACH, or turn a blower door CFM50 reading into ACH50 and see it next to the 2021 or 2024 IECC limit for the climate zone.
- Choose ACH from airflow, CFM for a target ACH, or a blower door ACH50 check.
- Type the CFM, target ACH or CFM50, then the length, width and ceiling height, or the volume in cubic feet.
- For a blower door check, pick the adopted IECC edition and the climate zone, then read the ACH50 against the limit.
Example result for the default inputs
14 ft x 12 ft x 8 ft = 1,344 cu ft, 150 CFM.
Air changes per hour
6.70 ACH
| Volume | 1,344 cu ft |
|---|---|
| Airflow | 150 CFM |
| ACH = CFM x 60 / volume | 6.70 |
| Minutes for one air change (60 / ACH) | 9.0 |
For estimation purposes only. Consult a licensed engineer for final designs.
Free without signup, 3 questions a day
How air changes per hour are calculated
Every mode on this page runs off one identity. Airflow is in CFM, volume in cubic feet, and the 60 converts minutes to hours. A blower door reading is the same math at a 50 Pa test pressure, so the result is ACH50, not the air change rate the house sees on a normal day.
- Volume
volume = length x width x ceiling heightOr enter the volume directly. The calculator accepts 50 to 1,000,000 cu ft.- Air changes per hour
ACH = CFM x 60 / volumeHow many times the airflow would replace the room's volume in an hour, assuming full mixing.- Airflow for a target ACH
CFM = ACH x volume / 60Same identity, solved for airflow. Use it for a shop, a cure room or any space with an air change target.- Blower door ACH50
ACH50 = CFM50 x 60 / volumeCFM50 is the fan flow that holds the house at 50 Pa. Use the volume the rater used, since a different volume changes ACH50 for the same leak.- 2021 IECC limit
5.0 ACH50 in zones 0 to 2, 3.0 in zones 3 to 8Set by Section R402.4.1.3 for homes complying with Section R401.2.1. Under any compliance path, R402.4.1.2 caps leakage at 5.0 ACH50 or 0.28 cfm per sq ft of dwelling unit enclosure area.- 2024 IECC limit
4.0 ACH50 in zones 0 to 2, 3.0 in zones 3 to 5, 2.5 in zones 6 to 8Section R402.5.1.3, item 1, where complying with Section R401.2.1. Item 2 sets 4.0 ACH50 or 0.22 cfm per sq ft of enclosure area for Section R401.2.2 or R401.2.3.- Rough natural infiltration
ACH natural, roughly ACH50 / 20A rule of thumb from blower door research. Jones, Persily and Sherman write that 20 does not fit all buildings and climates and should be adjusted for height, shielding, leak size and climate. Treat it as a ballpark only.
Worked examples for rooms and houses
The first two rows are room math. The last four put the same house through two code editions, which is where a blower door number can pass one year and miss the next.
| Example | Volume | Result | IECC limit, ACH50 |
|---|---|---|---|
| 12x14 bedroom, 8 ft ceiling, 150 CFM | 1,344 cu ft | 6.70 ACH | N/A |
| 20x30 shop, 10 ft ceiling, 6 ACH target | 6,000 cu ft | 600 CFM | N/A |
| 16,000 cu ft house, 750 CFM50, 2021 IECC, zone 4 | 16,000 cu ft | 2.81 ACH50 | Meets 3.0 |
| 16,000 cu ft house, 750 CFM50, 2024 IECC, zone 6 | 16,000 cu ft | 2.81 ACH50 | Exceeds 2.5 |
| 16,000 cu ft house, 1,200 CFM50, 2021 IECC, zone 2 | 16,000 cu ft | 4.50 ACH50 | Meets 5.0 |
| 16,000 cu ft house, 1,200 CFM50, 2024 IECC, zone 2 | 16,000 cu ft | 4.50 ACH50 | Exceeds 4.0 |
At 750 CFM50 the house tests at 2.81 ACH50. That meets the 2021 zone 4 limit of 3.0 and exceeds the 2024 zone 6 limit of 2.5. At 1,200 CFM50 it's 4.50 ACH50, which meets the 2021 zone 2 limit of 5.0 and exceeds the 2024 limit of 4.0 in the same zone.
Standards and data sources
- 2021 International Energy Conservation Code, Sections R402.4.1.2 and R402.4.1.3 - tested air leakage at 50 Pa (0.2 in. w.c.). 5.0 air changes per hour in Climate Zones 0, 1 and 2, and 3.0 in Climate Zones 3 through 8, where complying with Section R401.2.1, and no more than 5.0 under any compliance path.
- 2024 International Energy Conservation Code, Sections R402.5.1.2 and R402.5.1.3 - 4.0 air changes per hour in Climate Zones 0, 1 and 2, 3.0 in Climate Zones 3 through 5, and 2.5 in Climate Zones 6 through 8 where complying with Section R401.2.1, and 4.0 where complying with Section R401.2.2 or R401.2.3. Two exceptions allow 0.27 cfm per sq ft, one for attached or R-2 dwelling units tested on their own or with adjacent units, and one for buildings with 1,500 sq ft or less of conditioned floor area.
- U.S. DOE Building Energy Codes Program, Energy Savings Analysis, 2024 IECC for Residential Buildings - lists the 2024 change in climate zones 0, 1 and 2 from 5.0 to 4.0 ACH50. A proposal for 2.0 ACH50 in climate zones 3 through 8 was adjusted, and the prescriptive limits stayed at 3.0 ACH50 in zones 3 through 5 and became 2.5 ACH50 in zones 6 through 8.
- Benjamin Jones, Andrew Persily and Max Sherman, The Origin and Application of Leakage-Infiltration Ratios, ASHRAE and AIVC IAQ 2016 conference - background for the divide-by-20 rule and its limits. The rough natural infiltration row uses it.
Limitations of this calculator
- ACH from a supply CFM assumes that air fully mixes with the room. It counts total airflow, and most HVAC supply air is recirculated, so it's not a count of outdoor air changes.
- The IECC check covers the ACH50 limits only. It doesn't check the cfm per sq ft of enclosure area alternatives or the 2024 exceptions at 0.27 cfm per sq ft for attached or R-2 units and for buildings of 1,500 sq ft or less of conditioned floor area. Those need areas this page doesn't ask for.
- The pass or fail line compares the unrounded ACH50 with the limit. A reading exactly at the limit meets it, since the code says not exceeding in 2021 and not greater than in 2024.
- Local amendments can change the numbers. Check the edition and any amendments your jurisdiction adopted before you tell a builder the house passed.
- The natural infiltration row divides ACH50 by 20. The source paper says that ratio has to be adjusted for building height, shielding, leak size and climate, so don't size equipment or ventilation from it.
- BuildSolver's whole-house load engine doesn't take a blower door number. It assumes natural infiltration of 0.1, 0.18 and 0.3 ACH on cooling and 0.3, 0.55 and 1 ACH on heating for tight, average and leaky homes. Those are BuildSolver's calibrated assumptions, not code values.
- Whole-dwelling ventilation under ASHRAE 62.2 is a separate calculation. This page doesn't convert an air change rate into a 62.2 ventilation rate.
Common air change and blower door mistakes
- Mixing up ACH50 and natural ACH. A house at 3.0 ACH50 doesn't trade 3 air changes an hour on a mild day. The test forces air through the leaks at 50 Pa. Comparing ACH50 to a ventilation target or a load assumption mixes two different numbers.
- Using floor area times 8 ft for every house. Vaulted ceilings, a conditioned basement or a two-story foyer can add a lot of volume. A bigger volume makes ACH50 look better for the same CFM50, so use the volume the rater measured.
- Checking against the wrong code edition. The 2024 IECC dropped zones 0 to 2 from 5.0 to 4.0 ACH50 and set 2.5 in zones 6 to 8. A house at 4.5 ACH50 in zone 2 meets the 2021 prescriptive limit and misses the 2024 one.
- Reading supply CFM as fresh air. A 150 CFM supply register in a bedroom can show more than 6 ACH, but nearly all of that air is recirculated through the air handler. Outdoor air needs its own number.
- Treating ACH50 divided by 20 as a design value. The divide-by-20 rule is a ballpark. Wind exposure, building height and where the leaks are all move the real number, and the research behind the rule says so.
Common questions about air changes per hour
How many air changes per hour does 150 CFM give a 12x14 room?
With an 8 ft ceiling the room holds 1,344 cu ft. 150 x 60 / 1,344 = 6.70 ACH. That counts all the air moving through the room, recirculated air included.
What CFM gives 6 air changes an hour in a 20x30 shop with a 10 ft ceiling?
The shop is 6,000 cu ft, so CFM = 6 x 6,000 / 60 = 600 CFM.
What blower door result passes the 2021 IECC?
Where complying with Section R401.2.1, 5.0 ACH50 or less in Climate Zones 0, 1 and 2, and 3.0 ACH50 or less in Climate Zones 3 through 8. Section R402.4.1.2 also caps any compliance path at 5.0 ACH50 or 0.28 cfm per sq ft of dwelling unit enclosure area.
What changed for air leakage in the 2024 IECC?
The Section R401.2.1 limit is 4.0 ACH50 in Climate Zones 0, 1 and 2, 3.0 in Climate Zones 3 through 5, and 2.5 in Climate Zones 6 through 8. Where complying with Section R401.2.2 or R401.2.3 the limit is 4.0 ACH50 or 0.22 cfm per sq ft of enclosure area. The section moved from R402.4.1 to R402.5.1.
Does a house at exactly 3.0 ACH50 pass in zone 4?
Yes. The 2021 text says not exceeding and the 2024 text says not greater than, so a reading at the limit meets it. A 16,000 cu ft house at 800 CFM50 tests at 3.00 ACH50 and meets the zone 4 limit in both editions. This calculator compares the unrounded number, so a result that prints as 3.00 but is slightly over shows extra decimals.
Can I turn ACH50 into a natural air change rate?
Only roughly. The common rule divides ACH50 by 20, so 3.0 ACH50 is about 0.15 ACH. Research by Jones, Persily and Sherman notes the 20 has to be adjusted for building height, shielding, leak size and climate.
What volume should I use for a blower door check?
Use the volume on the rater's test report. A different volume gives a different ACH50 from the same CFM50, and the rater's number is the one the code official sees.
Is this the same as the ASHRAE 62.2 ventilation rate?
No. ASHRAE 62.2 sets a whole-dwelling ventilation rate from floor area and bedrooms. Use the ASHRAE 62.2 ventilation calculator for that number.
Can the BuildSolver chat calculate ACH?
No. The chat has no air change or blower door tool, so use this page for those numbers. The chat runs a free whole-house Manual J cooling and heating load.
Used on the same job
Where ACH meets ventilation and load
A tight house still needs whole-dwelling ventilation. That rate comes from floor area and bedrooms under ASHRAE 62.2. Run it on the ASHRAE 62.2 ventilation calculator.
BuildSolver's load engine doesn't read a blower door result. It picks natural infiltration from the air tightness you describe, assuming 0.1, 0.18 and 0.3 ACH on cooling and 0.3, 0.55 and 1 ACH on heating for tight, average and leaky homes. Those presets are BuildSolver's calibrated assumptions, not code numbers. The Manual J calculator uses them.
Run the whole-house load in chat
The chat has no air change or blower door tool, so ACH and ACH50 come from this page. Describe the house in chat, including how tight it is, and the assistant runs the free whole-house Manual J cooling and heating load for the same job.
Open the chat