SettingsSettings for this calculationUS
The total conditioned (heated/cooled) floor area of the building.
Use the total finished, conditioned square footage.
The average ceiling height across the conditioned space.
8 ft (2.4 m) is standard for most homes.
Your airtightness goal, e.g. from an energy code requirement or certification program.
Many energy codes target 3-5 ACH50 or lower for new construction; Passive House-style standards target well under 1.
Target CFM50
432 CFM50 (target maximum)
Your actual blower door test result should come in at or below this target — this is the inverse of the Air Changes Per Hour Calculator, useful for setting a goal before construction or air sealing work rather than evaluating a completed test.
- Building volume
- 8,640 ft³
They open the calculator with your figures already in it
Blower Door CFM50 Target Calculator: 432 CFM50 (target maximum) — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
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How this was calculated
Formula source(s)
- Standard blower door formula rearranged: target CFM50 = (target ACH50 x building volume in cubic feet) / 60
Inputs used
- Conditioned Floor Area
- 1080 sq ft
- Average Ceiling Height
- 8 ft
- Target ACH50
- 3
Intermediate steps
- Building volume
- 8,640 ft³
Confidence note: Your actual blower door test result should come in at or below this target — this is the inverse of the Air Changes Per Hour Calculator, useful for setting a goal before construction or air sealing work rather than evaluating a completed test.
What this calculation does not cover
- Building volume here is nothing more than floor area multiplied by average ceiling height. That ignores the thickness of intermediate floor assemblies, flattens stairwells and double-height spaces into an average, and omits any conditioned basement or attic whose area you did not enter. The target moves in direct proportion to the volume, so a volume measured ten per cent differently from the tester's gives a CFM50 limit ten per cent different — agree the volume basis with whoever will run the test before you build to this number.
- Nothing here defines the tested envelope: whether an attached garage, an unheated basement, a conditioned crawl space or a plant room sits inside or outside it. That single decision changes both the volume in the denominator and the leakage the fan has to move. Two people can test the same building to two different ACH50 figures purely by drawing the boundary in two different places.
- ACH50 normalises leakage against volume. UK and much European testing, and some North American programmes, instead specify air permeability or leakage per unit of envelope area. This page never asks for envelope area, so a target handed to you in those terms cannot be met by hitting this CFM50 figure.
- The number assumes a properly conducted test at 50 Pa. It carries no correction for wind, stack effect or baseline pressure, no averaging of a depressurisation result against a pressurisation one, and no allowance for how the building is prepared — which trickle vents, flues, extract fans and drain traps get sealed and which are left as found. Those are the usual reasons two tests of the same building disagree.
- This is a target airflow, not a ventilation or combustion-air check. A building tightened towards a low ACH50 needs mechanical ventilation sized and commissioned for it, and any open-flued or naturally aspirated appliance inside the envelope needs its own combustion air and spillage assessment. Hitting this figure says nothing about either, and those are the parts that carry a safety consequence.
Add the equipment this sizes
This result is a specification — 432 CFM50 (target maximum) — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.
Sources checked 2026-09-02 · in the site-wide review of 2026-09-06 · v1.0.1
Regulatory standards & verification citations1
- Standard blower door formula rearranged: target CFM50 = (target ACH50 x building volume in cubic feet) / 60
Cite this page
Your workspace
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