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Air Changes Per Hour (ACH50) Calculator

Calculate a building's ACH50 airtightness metric from a blower door test's CFM50 reading.

Computed in your browser — nothing you enter is uploaded. Figures are presented for United States against IRC 2024, and every formula is cited under regulatory standards below.

Last verified 2026-08-25 · v1.0.0

Market
Imperial · sales tax
7.87 ft
Schematic, drawn to the proportions you entered — not to scale on screen.

ACH50 (air changes per hour at 50 Pa)

10.62 ACH50

High confidence

Fairly leaky — typical of an older, un-air-sealed home; many energy codes target under 3-5 ACH50 for new construction.

Building volume
8475.52 ft³

With the figures above, the ach50 (air changes per hour at 50 pa) comes to 10.6. This is calculated for United States and cites IRC 2024. Building in another market? Change the selector above so the units and code reference follow.

Add the equipment this sizes

This result is a specification — 10.619 ACH50 — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.

Preliminary estimate, not certified engineering. This tool produces an indicative quantity calculation for planning purposes only — it is not a certified structural analysis, a guaranteed material takeoff, or a substitute for building department approval. Always verify measurements on-site and have a licensed contractor or structural engineer review any load-bearing, code-sensitive, or safety-critical work before purchasing materials or starting construction. Spotted an arithmetic or standards error? Report it to contact@craftquantities.com with your inputs — confirmed fixes become pinned regression tests.

[Schema Verified] Computed in alignment with American Concrete Institute (ACI 318-19) formulas and International Residential Code (IRC 2024) spatial boundaries.

Regulatory standards & verification citations

  • Standard blower door test formula: ACH50 = (CFM50 x 60) / building volume in cubic feet

Your workspace

Most jobs need more than one number. Add the calculators you need next and they open right here, underneath this one — your figures stay on screen and nothing is lost to a page change.

Frequently asked questions

What's a 'good' ACH50 number?
Lower is tighter/better: many modern energy codes target 3-5 ACH50 or lower for new construction, while older, un-air-sealed homes can measure well above 10-15 ACH50.
Can a house be too airtight?
Yes — very tight homes need mechanical ventilation (like an ERV or HRV) to bring in fresh air on purpose, since they no longer get enough incidental fresh air through leaks the way a drafty older home does.
Do I need a professional to get CFM50?
Yes — CFM50 comes from a blower door test performed with calibrated equipment, typically by an energy auditor or HERS rater. This calculator turns that professional reading into the normalized ACH50 metric.