ACH50 to Natural ACH Calculator
Estimate natural infiltration ACH from a blower door ACH50 result.
Enter your values
Enter the ach50 from blower door test you are working with.
Choose one of: 15 - Very exposed, tall building, 17 - Windy climate, 20 - Typical (LBNL default), 25 - Sheltered, mild climate.
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.
- Estimated natural ACH
- 0.25
- n-factor used
- 20
- Air changes per day
- 6
Quick answer
Natural ACH is estimated by dividing the tested ACH50 by an empirical n-factor that reflects climate, building height and shielding.
ACH_natural = ACH50 / n
At a glance
| What it does | Estimate natural infiltration ACH from a blower door ACH50 result. |
|---|---|
| Category | HVAC & Airflow |
| Inputs needed | ACH50 from blower door test, n-factor (climate/height) |
| Main output | Estimated natural ACH |
| Formula | ACH_natural = ACH50 / n |
| Cost | Free — no sign-up, no download |
How to use the ACH50 to Natural ACH Calculator
- 1Enter the ACH50 result from a blower door test.
- 2Choose an n-factor appropriate for the climate, building height and exposure (20 is a common simplified default).
- 3Read the estimated natural (non-pressurized) infiltration rate.
Inputs explained
- ACH50 from blower door test
- Enter the ach50 from blower door test you are working with.
- n-factor (climate/height)
- Choose one of: 15 - Very exposed, tall building, 17 - Windy climate, 20 - Typical (LBNL default), 25 - Sheltered, mild climate.
Worked example
Using the values the calculator loads with:
Inputs
- ACH50 from blower door test5.0
- n-factor (climate/height)20 - Typical (LBNL default)
Results
- Estimated natural ACH0.25
- n-factor used20
- Air changes per day6
Frequently asked questions
Is 20 always right?
No — it is a widely-used simplification; taller and more exposed buildings often use a lower n-factor (more infiltration per ACH50).
Why do I need natural ACH at all?
Natural ACH better represents everyday infiltration for load calculations than the test-pressure ACH50 figure.
What do I need to enter into the ACH50 to Natural ACH Calculator?
Just 2 values: ach50 from blower door test, n-factor (climate/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 ACH50 to Natural ACH Calculator work out the answer?
Natural ACH is estimated by dividing the tested ACH50 by an empirical n-factor that reflects climate, building height and shielding. It applies the formula ACH_natural = ACH50 / n and shows the working so you can check each step by hand.
Is the ACH50 to Natural ACH 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.