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Blower Door ACH50 Calculator

Convert a blower-door result to air changes per hour at 50 pascals, and estimate the home’s average natural air changes with an N-factor.

Method reviewed October 26, 2026. Version 1.0.0. How it works

Your inputs

Unit system

From the LBNL N-factor table for your climate zone, number of stories and wind shielding (reproduced by BPI and in NYS Clean Heat guidance). The default, 13, is the worked example in that guidance (zone 2, three stories, normal shielding) — look up yours.

Result

Air changes per hour at 50 Pa (ACH50)
7.5
Estimated natural air changes per hour
0.577

7.5 ACH50 — about 0.58 natural air changes per hour with N = 13.

  • Natural infiltration varies with weather from hour to hour; the N-factor gives a seasonal average. Codes and programs set limits in ACH50 — check the one that applies to you.
Estimated natural infiltration
154CFM
Show the working
  1. ACH50 = CFM50 × 60 ÷ volume
    2,000 × 60 ÷ 16,000 = 7.5
  2. Natural ACH ≈ ACH50 ÷ N; natural CFM ≈ CFM50 ÷ N
    7.5 ÷ 13 = 0.5769; 153.8 CFM

ACH50

A blower door depressurises a house to 50 pascals and measures the airflow needed to hold it there: CFM50. Dividing by the house volume and multiplying by 60 gives ACH50 — air changes per hour at 50 Pa — the measure codes and programs use to set airtightness targets.

From test to natural infiltration

Under normal weather the pressures driving air through the leaks are much smaller and change all the time. The Lawrence Berkeley National Laboratory N-factor method estimates the seasonal average: natural ACH ≈ ACH50 ÷ N, and natural CFM ≈ CFM50 ÷ N. N depends on climate, building height and wind shielding; look it up for your home. The default is the worked example in New York State’s Clean Heat infiltration guidance.

Worked example

These are the example values the calculator opens with, and the result the engine calculates for them.

Inputs

Blower-door airflow at 50 Pa (CFM50)
2,000 CFM
Conditioned volume
16,000 ft³
N-factor
13

Result

Air changes per hour at 50 Pa (ACH50)
7.5
Estimated natural air changes per hour
0.577
Estimated natural infiltration
154 CFM

7.5 ACH50 — about 0.58 natural air changes per hour with N = 13.

Assumptions

  • ACH50 = CFM50 × 60 ÷ volume; natural ACH = ACH50 ÷ N (LBNL N-factor method).

Limitations

  • The N-factor depends on climate, height and shielding; a single value is an approximation.

Frequently asked questions

What is a good ACH50?

Lower is tighter. Targets differ by code edition, program and climate; check the one that applies to your project.

Why use an N-factor instead of a fixed number?

Natural infiltration depends on wind and temperature differences, which vary by climate, height and exposure. The N-factor captures those effects as a single divisor.

How is conditioned volume measured?

Floor area × average ceiling height for each conditioned space, including conditioned basements if they were part of the test.

Sources

  1. Infiltration Guidance for Buildings at Design Conditions — NYS Clean Heat (NYSERDA and utilities). LBNL N-factor method: natural ACH = ACH50 ÷ N and CFMn = CFM50 ÷ N (example 10,112 ÷ 13 = 778), with N by climate zone, height and shielding.
  2. Air Change Rates in Rooms — The Engineering ToolBox. ACH = CFM × 60 / room volume.

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