Blower Door ACH50 Calculator
Convert blower door CFM50 to ACH50 and natural ACH.
Enter your values
ⓘ Live results
Results
- ACH50
- 4.83
- Building volume
- 18,000 cu ft
- Estimated natural ACH
- 0.242
- Leakage per sq ft
- 0.72 CFM50/sq ft
- Rating
- Tight, code-compliant
Quick answer
ACH50 is CFM50 times 60 divided by the building volume, and natural infiltration is roughly ACH50 divided by 20.
ACH50 = CFM50 x 60 / volume
How to use the Blower Door ACH50 Calculator
- 1Enter the CFM50 result from the blower door test.
- 2Enter conditioned floor area and average ceiling height.
- 3Compare ACH50 against your code target — 3.0 or 5.0 in most current codes.
Worked example
Using the values the calculator loads with:
Inputs
- Blower door result1450 CFM50
- Conditioned area2000 sq ft
- Average height9 ft
Results
- ACH504.83
- Building volume18,000 cu ft
- Estimated natural ACH0.242
- Leakage per sq ft0.72 CFM50/sq ft
- RatingTight, code-compliant
Frequently asked questions
Why divide by 20?
The LBL rule of thumb converts 50 Pa leakage to average natural infiltration; the real divisor varies 15-30 by climate and shielding.
Can a house be too tight?
Only without mechanical ventilation — below 3 ACH50 you need an ERV or HRV.
About this calculator
- Built and reviewed by James Whitfield · How we build and test calculators
- Report an error
Results are estimates for general information. For medical, legal, structural or financial decisions, confirm with a qualified professional.