Blower Door ACH50 Calculator
Convert blower door CFM50 to ACH50 and natural ACH.
Enter your values
Enter the blower door result in CFM50.
Enter the conditioned area in sq ft.
Enter the average height in ft.
Results update instantly as you type — no submit needed. Your values are remembered on this device, and the shareable link reopens the calculator with exactly these numbers.
- 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
At a glance
| What it does | Convert blower door CFM50 to ACH50 and natural ACH. |
|---|---|
| Category | HVAC & Airflow |
| Inputs needed | Blower door result, Conditioned area, Average height |
| Main output | ACH50 |
| Formula | ACH50 = CFM50 x 60 / volume |
| Cost | Free — no sign-up, no download |
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.
Inputs explained
- Blower door result(CFM50)
- Enter the blower door result in CFM50.
- Conditioned area(sq ft)
- Enter the conditioned area in sq ft.
- Average height(ft)
- Enter the average height in ft.
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.
What do I need to enter into the Blower Door ACH50 Calculator?
Just 3 values: blower door result, conditioned area, average height. Every field starts with a realistic example, so you can change one number at a time and watch the result update instantly.
How does the Blower Door ACH50 Calculator work out the answer?
ACH50 is CFM50 times 60 divided by the building volume, and natural infiltration is roughly ACH50 divided by 20. It applies the formula ACH50 = CFM50 x 60 / volume and shows the working so you can check each step by hand.
Is the Blower Door ACH50 Calculator free, and do I need an account?
It is completely free with no sign-up, no download and no usage limit. Everything is calculated in your browser, so the numbers you type never leave your device.
Results are estimates for general information. For medical, legal, structural or financial decisions, confirm with a qualified professional.