How much ventilation does a 3-bedroom house need?

ASHRAE 62.2 sets the whole-house rate at 0.03 CFM per square foot of floor area plus 7.5 CFM for each bedroom plus one. A 2,000 sq ft house with three bedrooms needs 0.03 × 2,000 + 7.5 × 4 = 90 CFM of continuous outdoor air. If the fan runs only part of the time, the rate rises in proportion; a fan running half the time must move 180 CFM.

What size dehumidifier does a basement need?

Dehumidifier capacity is rated in pints removed per day under the AHAM test conditions that changed in 2019, which is why a unit sold today as 35 pints removed about 50 under the old test. For a 1,000 sq ft basement that feels damp and smells musty, about 30 pints per day is the usual recommendation; a very wet basement with visible seepage needs 40 to 50. Larger, drier spaces need less per square foot.

What size humidifier do I need?

Humidifier output is rated in gallons per day. A 2,000 sq ft house of average tightness needs about 6.6 gallons per day to hold 35% relative humidity at 70 °F when it is 20 °F outside; a tight house of the same size about 4, and a drafty older one about 13, because every air change carries moisture out. Whole-house bypass units deliver 12 to 18 gallons per day; console units 3 to 9.

What CADR does an air purifier need for a bedroom?

The AHAM rule is a smoke CADR of at least two-thirds of the room area in square feet, for an 8 ft ceiling. A 12 × 14 ft bedroom (168 sq ft) needs a CADR of about 112 CFM, which gives roughly 4.8 air changes per hour through the filter. Higher ceilings and the wish for 5 or more air changes push the figure up; the calculator shows both ways of arriving at it.

What size ceiling fan does a room need?

Fan size follows room area: up to 75 sq ft takes a 29 to 36 inch fan, 76 to 144 sq ft a 36 to 42 inch fan, 144 to 225 sq ft a 44 inch fan, 225 to 400 sq ft a 50 to 54 inch fan, and rooms above 400 sq ft a 60 inch or larger fan or two fans. Mounting height matters as much: blades should hang 8 to 9 ft above the floor, which means a downrod on any ceiling above 9 ft.

Calculators

Ventilation, humidity and air-quality calculators

Whole-house ventilation by ASHRAE 62.2, dehumidifiers and humidifiers, ERV and HRV sizing, air purifier CADR, ceiling and whole-house fans.

A tight house keeps heat in, and it keeps moisture, cooking smells and everything the occupants exhale in with it. Fresh air that used to leak through the walls now has to be measured and moved on purpose, and the moisture that an old drafty house dried out on its own now has to be removed by a machine. The calculators in this group put numbers on that: how many cubic feet per minute of outdoor air a house needs, how many pints of water a basement gives up each day, how many gallons a humidifier must add in a dry winter, and how much clean air an air purifier must deliver to matter.

Most of these figures come from published standards and industry test methods rather than from physics alone: ASHRAE 62.2 for whole-house ventilation, the AHAM test conditions behind dehumidifier capacities and air purifier CADR ratings, and the industry sizing charts for ceiling and whole-house fans. Each calculator names its source and says where the figure is a judgement.

7 calculators

Whole-House Ventilation Calculator
The continuous outdoor air a house needs under ASHRAE 62.2 from its area and bedrooms, continuous or cycled.
Dehumidifier Size Calculator
Pints per day for a basement or room from its area and dampness under the 2019 rating, with the energy to run it.
Humidifier Size Calculator
Gallons per day a humidifier must add to hold a winter humidity, from the house volume, tightness and outdoor temperature.
ERV / HRV Size Calculator
The recovery ventilator size for a ventilation rate, ERV or HRV for your climate, and the heat it recovers.
Air Purifier CADR Calculator
The clean air delivery rate a room needs by the AHAM 2/3 rule or a target of 4 to 6 air changes per hour.
Ceiling Fan Size Calculator
Blade span for the room and the downrod for the ceiling height, with typical airflow and clearances.
Whole-House Fan Calculator
Fan CFM by the 2 to 3 CFM per sq ft or 15 to 20 ACH rules, the attic vent area it needs and the windows to open.

Guides for this topic

Ducts & air
Whole-house ventilation: the ASHRAE 62.2 formula
Tight houses save energy and trap whatever is in the air. ASHRAE 62.2 says how much outdoor air a home needs, and the arithmetic fits on one line.
Ducts & air
Bathroom and kitchen exhaust: the code minimums and the better numbers
The code sets a floor for exhaust fans. The HVI rules set a better target, and the duct decides whether the fan ever gets there.
Ducts & air
Dehumidifier sizing: pints per day, really
A "50-pint" dehumidifier bought in 2018 and one bought today are not the same size. Here is what the rating measures, how to size for a damp space, and why the old numbers still circulate.
Ducts & air
Humidity, dew point and comfort at home
Relative humidity changes with the temperature; dew point does not. Once you read the air by its dew point, comfort, condensation and mould all make more sense.
Ducts & air
MERV filters: filtration vs airflow, and the replacement schedule
A better filter catches more and restricts more. The answer is rarely the cheapest or the highest MERV; it is the right rating in a filter deep enough for the system.
Sizing basics
Why an oversized AC or furnace costs you comfort and money
Bigger feels safer and works worse. Here is how an oversized system leaves a house clammy, cycles itself to an early end, and what to check before the next one is the same size.

Questions people ask

How much ventilation does a 3-bedroom house need?: ASHRAE 62.2 sets the whole-house rate at 0.03 CFM per square foot of floor area plus 7.5 CFM for each bedroom plus one. A 2,000 sq ft house with three bedrooms needs 0.03 × 2,000 + 7.5 × 4 = 90 CFM of continuous outdoor air. If the fan runs only part of the time, the rate rises in proportion; a fan running half the time must move 180 CFM.
What size dehumidifier does a basement need?: Dehumidifier capacity is rated in pints removed per day under the AHAM test conditions that changed in 2019, which is why a unit sold today as 35 pints removed about 50 under the old test. For a 1,000 sq ft basement that feels damp and smells musty, about 30 pints per day is the usual recommendation; a very wet basement with visible seepage needs 40 to 50. Larger, drier spaces need less per square foot.
What size humidifier do I need?: Humidifier output is rated in gallons per day. A 2,000 sq ft house of average tightness needs about 6.6 gallons per day to hold 35% relative humidity at 70 °F when it is 20 °F outside; a tight house of the same size about 4, and a drafty older one about 13, because every air change carries moisture out. Whole-house bypass units deliver 12 to 18 gallons per day; console units 3 to 9.
What CADR does an air purifier need for a bedroom?: The AHAM rule is a smoke CADR of at least two-thirds of the room area in square feet, for an 8 ft ceiling. A 12 × 14 ft bedroom (168 sq ft) needs a CADR of about 112 CFM, which gives roughly 4.8 air changes per hour through the filter. Higher ceilings and the wish for 5 or more air changes push the figure up; the calculator shows both ways of arriving at it.
What size ceiling fan does a room need?: Fan size follows room area: up to 75 sq ft takes a 29 to 36 inch fan, 76 to 144 sq ft a 36 to 42 inch fan, 144 to 225 sq ft a 44 inch fan, 225 to 400 sq ft a 50 to 54 inch fan, and rooms above 400 sq ft a 60 inch or larger fan or two fans. Mounting height matters as much: blades should hang 8 to 9 ft above the floor, which means a downrod on any ceiling above 9 ft.

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