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Reviewed · Climate: NOAA hourly records, 2015-2024 · How we calculate

Whole-House Ventilation Calculator: How Much Fresh Air (CFM)?

Enter the floor area and the number of bedrooms. You get the continuous fresh-air rate the building code calls for, the ASHRAE 62.2 rate, and the size of HRV, ERV or fan that delivers it.

Inputs

sq ft

All conditioned floors.

bedrooms

Your ventilation

Updates as you type
50CFM

CFM of continuous fresh air by the International Residential Code for 2,000 sq ft and 3 bedrooms. ASHRAE 62.2, which many programs and some codes use, asks for 90 CFM in total.

Ventilation rates
IRC table method60 CFM
ASHRAE 62.2 (total)90 CFM
Show the math
IRC: 0.01 × 2,000 + 7.5 × (3 + 1) = 50 CFMASHRAE 62.2: 0.03 × 2,000 + 7.5 × (3 + 1) = 90 CFM

Estimate only. Check the code your town has adopted and the fan's rated CFM at its installed pressure.

The short answer

A 2,000 sq ft house with 3 bedrooms needs 50 CFM of continuous fresh air by the International Residential Code (0.01 CFM per sq ft plus 7.5 per person, people = bedrooms + 1), or 90 CFM by ASHRAE 62.2. Size the HRV, ERV or exhaust fan to deliver that at its installed pressure.

IRC ventilation rates

IRC 2021 Table M1505.4.3(1): continuous whole-house ventilation, CFM, by floor area and bedrooms.

IRC whole-house ventilation rates in CFM by floor area and bedrooms
Floor area (sq ft)0 to 1 bedrooms2 to 3 bedrooms4 to 5 bedrooms6 to 7 bedroomsover 7 bedrooms
Under 1,5003045607590
1,501 to 3,00045607590105
3,001 to 4,500607590105120
4,501 to 6,0007590105120135
6,001 to 7,50090105120135150
Over 7,500105120135150165

How it works

The formula

IRC: CFM = 0.01 × floor area + 7.5 × (bedrooms + 1)
ASHRAE 62.2: CFM = 0.03 × floor area + 7.5 × (bedrooms + 1)

Assumptions

  • IRC 2021 M1505.4.3, Equation 15-1 and its table (as adopted). The code allows 30% less for a ducted, balanced system, and higher rates for a fan that runs part of the time.
  • ASHRAE 62.2's total rate from its addendum r, adopted in the 2013 standard; bedrooms count as at least one. An infiltration credit can lower it for leaky houses; we show the full rate.
  • This is fresh air for the whole house; bathroom and kitchen exhaust are sized separately (see our exhaust fan calculator).

Worked example: 2,000 sq ft, 3 bedrooms

  1. IRC: 0.01 × 2,000 = 20 + 7.5 × 4 = 30 → 50 CFM
  2. ASHRAE 62.2: 0.03 × 2,000 = 60 + 30 = 90 CFM

Too big or too small

  • Too little: stale air and moisture. Tight new houses need mechanical ventilation; old leaky ones get some by accident.
  • Too much: higher heating and cooling bills. An HRV or ERV recovers much of that heat.
  • Check the rating at real pressure: a fan's CFM drops as duct resistance rises.

Questions people ask

How much fresh air does a house need?

The International Residential Code asks for 0.01 CFM per sq ft plus 7.5 CFM per person, counting bedrooms + 1 as people: 50 CFM for a 2,000 sq ft, 3 bedroom house. ASHRAE 62.2 (2013 and later) uses 0.03 CFM per sq ft, so 90 CFM.

What size HRV or ERV do I need?

One that delivers the continuous rate at its listed external static pressure: about 50 to 90 CFM for the example house, 73 to 143 for a 3,500 sq ft, 4 bedroom home. The IRC lets a balanced, ducted system run 30% less.

Why is ASHRAE 62.2 higher than the code?

Its 2013 revision raised the floor-area part from 0.01 to 0.03 CFM per sq ft and made the rate a total, which an allowance for natural leakage can reduce. Many efficiency programs use it; the IRC kept the older figure.

Can the fan run part of the time?

Yes, if it runs harder while it's on: the IRC multiplies the rate by 1.3 for 75% run time, 1.5 for 66%, 2 for 50%, 3 for 33% and 4 for 25%.