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Calculation Machine

Blower Door ACH50 Calculator

Convert blower door CFM50 to ACH50 and natural ACH.

Reviewed by the Calculation Machine team · How we check our maths

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.

The formula

ACH50 = CFM50 x 60 / volume

Worked example

Worked example

Using the values the calculator loads with.

Inputs

  • Blower door result1450 CFM50
  • Conditioned area2000 sq ft
  • Average height9 ft

Result

  • ACH504.83
  • Building volume18,000 cu ft
  • Estimated natural ACH0.242
  • Leakage per sq ft0.72 CFM50/sq ft
  • RatingTight, code-compliant

How to use the Blower Door ACH50 Calculator

  1. 1Enter the CFM50 result from the blower door test.
  2. 2Enter conditioned floor area and average ceiling height.
  3. 3Compare ACH50 against your code target — 3.0 or 5.0 in most current codes.

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