SettingsSettings for this calculationUS
Work a result from a test, or the flow a test rig has to reach for a target.
Before the test, the target mode tells the tester what the fans must move; after it, the measured mode turns their flow into the certificate's figure.
Every internal surface bounding the tested volume — walls, the underside of the roof, and the ground floor.
The ground floor is the part a first take-off leaves out, and on a single-storey shed it is close to a quarter of the total. Agree the boundary with whoever will accept the certificate before the day.
The volume inside the tested envelope, for the air change rate of the same test.
Air changes at 50 Pa (n50) divide the same flow by the volume instead of the area; a large building reads far better that way.
The fan flow that held the building at 50 pascals, from the test report.
Read it at 50 Pa from the report's fitted line rather than a single reading.
The exponent of the test's own multi-point fit, to carry the result to 75 Pa.
US whole-building tests report at 75 Pa (0.3 in. w.g.), UK tests at 50 Pa. The flow at the other pressure is the flow times the ratio of the pressures raised to n, and n belongs to the building's leaks: without the test's own figure the conversion is a guess, so the page leaves it out.
Air permeability q50
0.2735 CFM/ft²
Air permeability is the test flow divided by the envelope area; the air change rate divides the same flow by the volume. They are one test described two ways.
- Measured flow at 50 Pa
- 735.72 CFM
- Air changes per hour at 50 Pa (n50)
- 5 per hour
They open the calculator with your figures already in it
Air Permeability (q50) and Test Flow Calculator: 0.2735 CFM/ft² — 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)
- BS EN ISO 9972:2015, Thermal performance of buildings — Determination of air permeability of buildings — Fan pressurization method: air permeability = the flow at the reference pressure ÷ the envelope area; air change rate = the flow ÷ the internal volume
- ATTMA Technical Standards L1 (dwellings) and L2 (non-dwellings), the UK measurement standards behind Approved Document L submissions, at 50 Pa
- The power law of the fan pressurization method, Q = C × Δpⁿ, which carries a flow from one test pressure to another only with the exponent n from the test's own fit
Inputs used
- What You Want
- The permeability a measured test flow gives
- Envelope Area
- 2690 sq ft
- Internal Volume
- 8829 ft³
- Measured Flow at 50 Pa
- 735.72 CFM
- Target Air Permeability
- 0.27 CFM/ft²
- Flow Exponent n From the Test (0 to skip)
- 0
Intermediate steps
- Measured flow at 50 Pa
- 735.72 CFM
- Air changes per hour at 50 Pa (n50)
- 5 per hour
Confidence note: Air permeability is the test flow divided by the envelope area; the air change rate divides the same flow by the volume. They are one test described two ways.
What this calculation does not cover
- The envelope area is a take-off, and the published result moves in proportion to it. Agree the boundary — ground floor included, separate zones excluded — with whoever will accept the certificate.
- A result at 50 Pa (0.2 in. w.g.) is not a result at 75 Pa (0.3 in. w.g.) with a unit change. The 75 Pa figures appear only when the test's own flow exponent is entered.
- It is arithmetic on the test's figures; the measurement itself follows BS EN ISO 9972 under ATTMA TSL1 for dwellings or TSL2 for other buildings.
Add the equipment this sizes
This result is a specification — 0.2735 CFM/ft² — 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 2026-09-22 · v1.0.0
Regulatory standards & verification citations3
- BS EN ISO 9972:2015, Thermal performance of buildings — Determination of air permeability of buildings — Fan pressurization method: air permeability = the flow at the reference pressure ÷ the envelope area; air change rate = the flow ÷ the internal volume
- ATTMA Technical Standards L1 (dwellings) and L2 (non-dwellings), the UK measurement standards behind Approved Document L submissions, at 50 Pa
- The power law of the fan pressurization method, Q = C × Δpⁿ, which carries a flow from one test pressure to another only with the exponent n from the test's own fit
Which documents these citations point at
- Approved Documents to the Building Regulations (England) — L (United Kingdom)Practical guidance on meeting the Building Regulations, one lettered part per subject — Part A structure, Part B fire, Part K protection from falling, Part L conservation of fuel and power, and the rest.
Standards referenced: ISO 9972 (International Organization for Standardization, International).
A code or standard has force only where a jurisdiction has adopted it, usually with local amendments. This site holds no adoption data for any authority, so check what is in force with the authority where you build. Any section cited above without an edition should be checked against the edition in force where you build. What it would take to know.
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