Blower Door ACH50 Calculator
Air changes per hour at 50 pascals from door fan airflow and house volume.
What it does
A blower door test pressurizes the house to 50 pascals, roughly a 20 mph wind on every wall, and measures how much air the fan must push to hold that pressure. Divide by the house volume and you get ACH50, the score every energy audit quotes.
The tool above takes the fan airflow and the conditioned house volume. It returns ACH50, a common natural infiltration estimate as ACH50 over 20, and the field guide's tightness band from very tight to extremely loose.
The defaults describe a 1,500 square foot ranch with 8 foot ceilings and a fan pulling 1,500 CFM50: 7.5 ACH50, squarely in the very loose band, where a draft hunt pays well. The same house sealed to 500 CFM50 would score 2.5 and land in the tight range.
The formulas and bands follow the Alexandria, Virginia guide to test results and sample test forms, with the divide by 20 natural infiltration rule from the Air Infiltration and Ventilation Centre's discussion of normalized leakage.
How to use
- Pick imperial or metric units.
- Enter the airflow the blower door held at 50 pascals.
- Enter the conditioned house volume, not just floor area.
- Press Calculate for ACH50 and the tightness band.
- Read natural ACH as a rough weather average, then decide on air sealing.
Formula
- ACH50
- Air changes per hour at 50 pascals of pressure; the standard tightness score.
- CFM50
- Fan airflow in cubic feet per minute at the 50 pascal test pressure.
- volume
- Conditioned house volume: square footage times average ceiling height.
- divide by 20
- Rule of thumb converting ACH50 to an average natural air change rate.
The 50 pascal pressure standardizes the test so houses can be compared, and it is far above natural pressures, so ACH50 is always much larger than everyday infiltration. The divide by 20 shortcut comes from normalized leakage work and is a screening figure; site exposure and climate push the real number around.
Worked example
Example: 1500 CFM50 in a 12,000 cubic foot home
Defaults: 1,500 square feet with 8 foot ceilings, fan airflow 1,500 CFM50.
- Volume: 1,500 sq ft x 8 ft = 12,000 cu ft.
- ACH50: 1,500 x 60 / 12,000 = 7.5.
- Natural estimate: 7.5 / 20 = 0.38 ACH.
- Band: 7.5 sits in very loose, between loose and extremely loose.
- Sealed to 500 CFM50 the same house would score 2.5, tight.
Reference table
| ACH50 | Band | What it feels like |
|---|---|---|
| 0 to 1.5 | Very tight | New passive and high performance builds |
| 1.5 to 3 | Tight | Recent energy code construction |
| 3 to 5 | Moderately tight | Weatherized older homes |
| 5 to 7 | Loose | Typical unimproved older home |
| 7 to 10 | Very loose | Drafty; leaks are easy to find |
| Above 10 | Extremely loose | Major shell problems or open paths |
Limits
- Divide by 20 is a rule of thumb, not a measurement.
- ACH50 ignores where the leaks are.
- Volume definitions vary between auditors.
- Wind and stack effect vary natural leakage daily.
Common mistakes
- Using floor area where volume belongs.
- Comparing ACH50 between homes of very different size as if it were efficiency.
- Reading natural ACH as a guaranteed average.
- Skipping the pre test walkthrough that finds big holes.
- Targeting the number alone instead of comfort and durability.
Try it yourself
Quick checks. Answers sit tucked under each question.
600 CFM50, 24,000 cu ft?
600 x 60 / 24,000 = 1.5 ACH50, very tight.
3000 CFM50, 18,000 cu ft?
10.0 ACH50, the top of very loose.
2550 m3/h in 340 m3?
7.5 ACH50, the same house in metric.
Questions
What is a blower door test?
A calibrated fan seals into a doorway, pressurizes the house to 50 pascals, and measures the airflow needed to hold that pressure.
What is a good ACH50?
New tight homes land near 1.5 to 3; older homes often run 5 to 10. Programs like Passive House push under 0.6.
Why 50 pascals?
It standardizes the test far above natural pressures, so results compare across houses, weather, and auditors.
Is ACH50 the same as natural air changes?
No; natural leakage runs roughly 10 to 30 times smaller, which is why the divide by 20 shortcut exists.
Should I seal a very tight house?
Past a point, mechanical ventilation matters more than more sealing; a very tight home needs planned fresh air.
Does ACH50 say where the leaks are?
No; it sizes the problem. Smoke pencils and thermal imaging find the actual paths.