Whole House Ventilation Calculator

The old answer to ventilation was that a house breathes through its own leaks. That was true of houses built before air sealing was taken seriously, and it is not true of anything built to a modern energy code. It was never a good answer anyway: leakage is driven by wind and temperature difference, so a house ventilates hardest in January and stops entirely on a mild still day. ASHRAE 62.2 replaces that with a flow rate — three hundredths of a CFM per square foot plus seven and a half per bedroom plus one — delivered by a fan. This calculator works it out and sizes the fan for how much of the time it will actually run.

Whole-house ventilation required: 90 CFM continuous 0.03 × 2,000 ft² + 7.5 × (3 bedrooms + 1) = 60.0 + 30.0. The bedroom term is a proxy for occupancy — the standard assumes two people in the first bedroom and one in each of the rest.
Fan size, running continuously: 90 CFM A continuously running fan delivers its rated flow against the required rate directly. It is the simplest arrangement and usually the quietest, because a small fan running all the time beats a large one cycling.
Local exhaust is separate: Kitchen 100 CFM, bath 50 CFM Intermittent figures; continuously running versions may be 25 and 20 CFM. These do not count toward the whole-house rate and the whole-house rate does not replace them. 2 bathrooms at 50 CFM each, plus the kitchen.
Infiltration is not ventilation: Zero on a mild day Air leakage is driven by temperature difference and wind. On a still 65 degree day — exactly when nobody opens a window because the house feels fine — a leaky house exchanges almost no air at all. A modern tight house exchanges very little on any day. Neither is a ventilation strategy, which is why the standard asks for a fan.
Exhaust only system: Unbalanced An exhaust-only system depressurises the house, so replacement air arrives through whatever leaks exist — including, potentially, a chimney. It is cheap and it is the most common arrangement, and it is the riskiest one in a tight house with combustion appliances.
Air changes per hour: 0.34 ACH Assuming 8 ft ceilings. The standard is written as a flow rate rather than an air change rate because what matters is contaminant dilution per person and per square foot of house, not how many times the volume turns over.
Where the air comes in matters: Not the garage A depressurised house draws replacement air from wherever it can, and an attached garage, a crawl space or a chimney are all easier paths than a purpose-made inlet. This is the practical argument for balanced ventilation even where the code accepts exhaust-only.
Ventilate first, then filter: Different problems Ventilation dilutes what a filter cannot catch — moisture, carbon dioxide, volatile compounds from finishes and furnishings, and cooking by-products. A good filter handles particles and does nothing about any of those. Houses need both and they are not substitutes.

The formula and what is behind it

The whole-house rate has two terms. The area term covers what the building itself emits: moisture, and volatile compounds from finishes, adhesives, cabinetry and furnishings. The bedroom term is a proxy for occupancy, assuming two people in the first bedroom and one in each additional one, and it covers what people emit — carbon dioxide, moisture, and everything associated with cooking and washing.

The result for a typical two thousand square foot three bedroom house is about ninety CFM, running continuously. That is a small fan. It is also a fan that has to run, which is the part that gets designed out.

Running intermittently does not scale linearly. Contaminants accumulate between cycles, so a fan running a quarter of the time needs more than four times the flow to deliver the same result — the standard applies an efficacy factor on top of the duty cycle.

A small continuous fan is almost always better than a large intermittent one. It is quieter, it uses less power at any given moment, and it does not produce the pressure swings that a large fan cycling creates.

Local exhaust is a separate requirement

Kitchen and bathroom fans deal with point sources at the moment they are produced: steam from a shower, moisture and grease and combustion products from cooking. They are required in addition to the whole-house rate, not instead of it.

The figures are a hundred CFM for a kitchen and fifty for a bathroom when operated intermittently, or lower rates if they run continuously. A range hood has to be ducted outside to count — a recirculating hood with a charcoal filter removes some grease and no moisture and no combustion products at all.

Running a bathroom fan continuously at twenty CFM is one legitimate way to provide part of the whole-house ventilation, if the fan is rated for continuous duty and quiet enough that nobody switches it off. Most bathroom fans are neither.

The common failure is a whole-house rate provided entirely by "the bathroom fans, when someone remembers". Occupant-operated ventilation is not ventilation.

Which way the air moves

An exhaust-only system is a fan pulling air out. Replacement air arrives through whatever leaks the house has, which means the house runs slightly negative. It is cheap, it is the most common arrangement, and in a tight house with a naturally vented water heater or furnace it can reverse the flue draft and pull combustion products indoors.

A supply-only system pushes filtered outdoor air in and lets it leave through the leaks. The house runs slightly positive, which keeps soil gas, garage fumes and radon out. In a cold climate it also pushes indoor humidity into the wall assembly, where it condenses.

A balanced system moves equal amounts both ways, so the house stays neutral. Adding a heat recovery core reclaims most of the temperature difference from the outgoing air; an energy recovery core also moves some humidity, which matters in both very humid and very dry climates.

Balanced systems cost more to install and are the only arrangement that works properly in a cold climate or alongside atmospheric combustion appliances. Where an exhaust-only system is used with any naturally vented appliance, a combustion safety test is not optional.

What this is based on

ASHRAE 62.2 — whole-house ventilation rate, 0.03 CFM/ft² + 7.5 CFM per (bedrooms + 1)
ASHRAE 62.2 local exhaust rates — 100 CFM kitchen and 50 CFM bathroom intermittent

A screening estimate against ASHRAE 62.2. The intermittent efficacy factor here is simplified; the standard gives a table. It does not address duct design, fan static pressure, filtration, or combustion appliance zone testing, and it does not cover jurisdictions that have adopted a different ventilation standard or amended this one.

Frequently asked questions

How much ventilation does a house need?

Under ASHRAE 62.2, 0.03 CFM per square foot of floor area plus 7.5 CFM per bedroom plus one. A 2,000 square foot three bedroom house works out to about 90 CFM running continuously.

Does my house not ventilate itself through leaks?

Not reliably, and not at all when you need it. Infiltration is driven by wind and temperature difference, so it stops on a mild still day. Modern air-sealed construction leaks very little on any day. Neither is a ventilation strategy.

Do bathroom fans count toward whole-house ventilation?

Only if they are rated for continuous duty and actually run continuously. Local exhaust for kitchens and bathrooms is a separate requirement from the whole-house rate — the standard asks for both, and occupant-operated fans do not satisfy the whole-house part.

Is exhaust-only ventilation safe?

In a house with no naturally vented combustion appliances, generally yes. With an atmospheric water heater or furnace it can reverse the flue draft and pull combustion products indoors, which needs a combustion safety test at minimum and is better solved with balanced ventilation.

Is an HRV worth it?

In a cold climate, and in any house with atmospheric combustion appliances, yes. It recovers most of the heat from the air being exhausted and keeps the house at neutral pressure. In a mild climate the payback on the heat recovery alone is slow, but the pressure balance still matters.