Calculate air changes per hour (ACH or ACH50) from blower door airflow in CFM and building volume, or solve for the CFM or volume from ACH.

Volume inside the tested air barrier: conditioned floor area × average ceiling height.

Use CFM50 for a 50 Pa blower-door test.

This choice labels the result. It does not convert airflow between pressures.

Optional settings

Blower Door Calculator Formula

The following equations are used to calculate the missing parameter in the Blower Door Calculator.

ACH = (CFM × 60) / HV (if calculating ACH) HV = (CFM × 60) / ACH (if calculating House Volume) CFM = (HV × ACH) / 60 (if calculating Air Flow)
  • Where ACH is the air changes per hour (1/hr)
  • HV is the house volume (ft³)
  • CFM is the air flow (cubic feet per minute)

Select the missing parameter, provide the two displayed values and choose the measurement condition. This labels the result; it does not convert between pressures.

What is a Blower Door Calculator?

Definition:

A blower door calculator is a tool used to determine one of the key parameters in a blower door test by computing the missing value from the provided inputs, helping assess a building’s airtightness and energy efficiency.

How to Use the Blower Door Calculator?

Example Problem:

The following example outlines the steps and information needed to calculate the missing parameter.

First, determine the house volume. In this example, the house volume is 20,000 ft³.

Next, use an illustrative ACH50 of 5 for the same 50 Pa test condition. This example is not a ventilation recommendation or a compliance threshold.

Finally, calculate the air flow using the formula above:

CFM = (HV * ACH) / 60

CFM = (20000 * 5) / 60

CFM = 100000 / 60

CFM ≈ 1666.67 CFM

FAQ

What is a blower door test?

A blower door test assesses a building’s airtightness by measuring the amount of air leakage, which helps in evaluating energy efficiency and overall building performance.

How does the blower door calculator work?

Select enclosed volume, airflow or air changes per hour and enter the two displayed measurements. The calculator applies the corresponding equation; it does not infer a missing field from a blank.

Why is measuring airtightness important?

Airtightness testing can help locate leakage and guide air sealing. ACH50 is a test-pressure leakage metric, not natural infiltration or a measure of ventilation adequacy. Indoor air quality and required ventilation need separate assessment.