Step Code airtightness calculator: ACH50 to CFM50.
Know your leakage number before the blower door does. Put in your target and your latest test, and see the leakage the target allows and how much has to come out.
Check the highlighted field. Results update when every number is in range.
Your numbers
- Interior volume
- 21,600 ft³
- Leakage the target allows
- 540 CFM50
- Leakage today
- -
- Has to come out
- -
To reach the Step 4 working target of 1.5 ACH50, this house can leak no more than 540 CFM50 at the final test.
Every gap in the envelope added together comes to about 56 sq.in of equivalent leakage area (EqLA at 10 Pa).
A missed test, on your numbers
Fill in any of the cost fields to see the total.
- Crew time-
- Retests-
- Schedule slip-
- Other-
- Total-
A measured seal brings the leakage down live, with the blower door running, and stops at your target before your energy advisor runs the compliance test.
Worked example: a 2,400 sq.ft house
With 9 ft average ceilings the interior volume is 21,600 cubic feet. Here is what each working target allows.
| Step | Working target | Max leakage | EqLA at 10 Pa |
|---|---|---|---|
| Step 3 | 2.5 ACH50 | 900 CFM50 | about 93 sq.in |
| Step 4 | 1.5 ACH50 | 540 CFM50 | about 56 sq.in |
| Step 5 | 1.0 ACH50 | 360 CFM50 | about 37 sq.in |
How the numbers work
ACH50 is air changes per hour at 50 pascals: how many times the blower door replaces the air in the house in an hour while holding that pressure. CFM50 is the same leakage as a flow, in cubic feet per minute. They convert through the volume: CFM50 = ACH50 × volume ÷ 60.
EqLA (equivalent leakage area) is the size of one sharp-edged hole that would leak as much as every gap combined, at 10 pascals, the figure printed on most energy advisor reports. This page assumes a typical flow exponent of 0.65; your report uses the exponent measured in the test, so its EqLA will differ a little. Real leakage is spread across hundreds of small gaps, which is why it is hard to chase by hand.
Volume here is floor area times average ceiling height. Your energy advisor calculates the official volume from the drawings and confirms the airtightness your energy model needs, so treat these results as planning numbers. See the Step Code airtightness guide for the targets and the cost guide for the full comparison.
Common questions
How do I convert ACH50 to CFM50?
Multiply ACH50 by the interior volume in cubic feet and divide by 60. A 2,400 sq.ft house with 9 ft ceilings is 21,600 cubic feet, so 1.5 ACH50 is 540 CFM50.
What CFM50 does a Step 4 house need?
It depends on the volume. Step 4 works to about 1.5 ACH50. For a 21,600 cubic foot house that is 540 CFM50 or less. Your energy advisor confirms the exact airtightness your energy model needs.
Is this the official compliance number?
No. Your energy advisor calculates the official volume from the drawings and runs the compliance test. Use this as a planning number before the test, not in place of it.
What is EqLA?
Equivalent leakage area: the size of one sharp-edged hole that would leak as much as all the gaps in the house combined, at 10 pascals. Most energy advisor reports print it. This page estimates it with a typical flow exponent; your report uses the measured one.
Built by Sebastian Motora, owner-operator. Last reviewed October 8, 2026.
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