Air Changes Per Hour (ACH50) Calculator

SettingsSettings for this calculationUS

Market

Imperial · sales tax

Conditioned Floor Area

The total conditioned (heated/cooled) floor area of the building.

Use the total finished, conditioned square footage, not including unconditioned garages or unfinished basements.

Average Ceiling Height

The average ceiling height across the conditioned space.

8 ft (2.4 m) is standard for most homes; vaulted ceilings or multiple stories need a blended average.

Blower Door CFM50 Reading

The airflow reading from a blower door test at 50 Pascals of pressure difference.

This comes from a professional blower door test — a fan is sealed into an exterior door and depressurizes the house while measuring the airflow needed to maintain 50 Pa.

ACH50 (air changes per hour at 50 Pa)

10.42 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: 8,640 ft³

Then change the inputs to see how far the answer moves.

Show calculation logic

How this was calculated

Formula source(s)

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

Inputs used

Conditioned Floor Area: 1080 sq ft
Average Ceiling Height: 8 ft
Blower Door CFM50 Reading: 1500

Intermediate steps

Building volume: 8,640 ft³

Final result10.42 ACH50

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

What this calculation does not cover

ACH50 is leakage measured while a fan holds the building at 50 Pa, not the rate at which air actually changes in service. Real infiltration is a fraction of this figure and varies with wind, stack effect, exposure, shielding and building height, so this result is not a ventilation rate and not a number to feed straight into a heat-loss or plant-sizing calculation.
The test behind the reading is run with trickle vents, extract terminals and other intentional openings closed or sealed, so a low result is not evidence the building is ventilated. A dwelling driven down towards the tight end without mechanical ventilation added at the same time has been sealed, not improved.
The calculation divides whatever CFM50 you type; it does not check the reading. Temperature and wind correction on the day, a single-point reading versus a multi-point fit, pressurisation versus depressurisation, and how thoroughly the crew sealed intentional openings all move the flow figure, and none of them are visible here.
Volume is taken as floor area times average ceiling height, and the fields only reach dwelling scale — a floor area above 1,000 m² (about 10,760 sq ft) or a ceiling above 6 m (about 20 ft) is replaced with the limit. Whether a conditioned basement, a room in the roof or the space between storeys belongs inside the tested volume is settled by the test protocol and by the two numbers you enter, so the ACH50 on a certificate for the same house can differ from this one.
This is not a compliance check. It applies no code target, no climate zone and no certification scheme's pass mark, and it does not produce air permeability in m3/(h.m2) of envelope area, which is the figure a UK or European air-leakage test is judged against. The flow field is CFM only, so a certificate quoting m3/h at 50 Pa has to be converted before it goes in the box.

Add the equipment this sizes

This result is a specification — 10.42 ACH50 — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.

QtyItemPrice each ($)

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

Cite this page

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.

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How to calculate air changes per hour (ACH50) in 4 steps

Conditioned Floor AreaThe total conditioned (heated/cooled) floor area of the building.

Average Ceiling HeightThe average ceiling height across the conditioned space.

Blower Door CFM50 ReadingThe airflow reading from a blower door test at 50 Pascals of pressure difference.

ACH50 (air changes per hour at 50 Pa)The tool computes the ACH50 (air changes per hour at 50 pa) from those figures and shows the formula, its sources, and a confidence rating alongside it.

Conditioned Floor Area	ACH50 (air changes per hour at 50 Pa) (ACH50)
500 sq ft	22.9
1,000 sq ft	11.4
1,500 sq ft	7.62
2,000 sq ft	5.72

Conditioned Floor Area

ACH50 (air changes per hour at 50 Pa) (ACH50)

500 sq ft

22.9

1,000 sq ft

11.4

1,500 sq ft

7.62

2,000 sq ft

5.72

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.