Understanding your result
The headline is the gallons of water per day the humidifier must add, with the target humidity, the outdoor temperature and the class of humidifier that delivers it in the sub-line. The stats give the output, the outdoor air entering the house in CFM and the indoor dew point. The table shows the chain of the calculation, from the house volume and its air changes to the moisture content of the air indoors and out in grains per pound, the water to add per hour, and the output per day.
The indoor dew point row is the one to watch. It is the temperature at which the indoor air would condense, and on a cold night the inside surface of a double-pane window is not much warmer than 40 °F. If the dew point is above that, the windows will fog or frost and the notes say so. The dew point and humidity calculator explores that relationship for any temperature and humidity.
In summer the problem reverses and the dehumidifier size calculator takes over. If the house has a continuous ventilation fan, its airflow from the whole-house ventilation calculator adds to the air exchange and raises the humidifier load; an ERV recovers some of that moisture.
How we calculate this
The humidity ratio, pounds of water per pound of dry air, comes from the saturation vapour pressure at each temperature by the Magnus approximation, multiplied by the relative humidity, at sea-level pressure; a grain is a seven-thousandth of a pound. The air changes for each tightness level (0.3, 0.5 and 1.0 per hour) are typical natural infiltration rates for tight, average and loose houses in winter. This is the method of AHRI Guideline F for sizing humidifiers, which sizes for the moisture carried out by infiltration at a typical cold-weather condition. The moisture figures for common conditions are listed on the ventilation and exhaust minimums page.
Metric mode accepts °C, m² and metres and shows litres per day.
The assumptions behind the numbers
| Assumption | Value | Where it comes from |
|---|---|---|
| Air changes per hour | 0.3 tight, 0.5 average, 1.0 loose | Typical winter natural infiltration by house type (LBNL and DOE residential data) |
| Outdoor RH | 70% | Typical winter outdoor relative humidity |
| Outdoor design temperature | 20 °F | AHRI Guideline F sizing condition |
| Air density | 0.075 lb per cu ft | Standard air |
| Saturation vapour pressure | Magnus approximation | Alduchov and Eskridge coefficients |
| Target RH | 35% at 70 °F | Common winter comfort guidance; lower it in colder weather |
Assumptions last reviewed October 8, 2026.
The calculator ignores the moisture people, cooking and showers add, the moisture stored in furniture and finishes, and the extra air exchange from a ventilation fan or a fireplace. It does not account for altitude, which changes the humidity ratio slightly. The guide to humidity, dew point and comfort covers the targets in detail, and the guide to dehumidifier sizing covers the summer side.
Two worked examples
An average 2,000 sq ft house
2,000 sq ft, 8 ft ceilings, average tightness (0.5 ACH), 20 °F outdoors, 70 °F indoors, 35% RH target.
- Volume: 16,000 cu ft; air entering 8,000 cu ft/h = 600 lb of air per hour (133 CFM)
- Indoor moisture: 37.9 grains/lb; outdoor 11.2 grains/lb
- Water to add: 600 × 26.7 ÷ 7,000 = 2.29 lb/h
- Output: 6.6 gallons per day, a console or small whole-house unit
- Indoor dew point: 41 °F
At 35% RH the dew point of 41 °F is safely below the inside temperature of most double-pane glass at 20 °F outside. A bypass humidifier on the furnace, rated 12 gallons a day, covers this house with room for colder days.
A tight 1,500 sq ft house in a cold snap
1,500 sq ft, 8 ft ceilings, tight (0.3 ACH), 0 °F outdoors, 70 °F indoors, 30% RH target.
- Volume: 12,000 cu ft; air entering 3,600 cu ft/h = 270 lb/h (60 CFM)
- Indoor moisture: 32.5 grains/lb; outdoor 4.6 grains/lb
- Water to add: 270 × 27.9 ÷ 7,000 = 1.08 lb/h
- Output: 3.1 gallons per day, a tabletop or small console unit
- Indoor dew point: 37 °F
A tight house barely needs a humidifier; its occupants add 1 to 2 gallons a day themselves, so a single small console unit, or none, holds 30%. Lowering the target to 30% for a 0 °F day keeps the dew point at 37 °F, which avoids frost on the windows.
Where to find your inputs
Tightness. A blower-door test gives the answer; otherwise judge by age and condition. Houses built or air-sealed in the last 15 years are usually tight, 1980s to 2000s houses average, and older unimproved houses loose.
Outdoor temperature. A typical cold day for your area, not the record low; 20 °F is the AHRI sizing condition.
Target humidity. 30 to 40%, lower when it is colder outside; a hygrometer shows what you have now.
Area and ceiling. The conditioned area and the average ceiling height.
Common mistakes
- Sizing by the box’s square footage. It assumes an average house; a drafty one needs twice as much.
- Running too high. Above 40% in cold weather, windows fog and frost and walls can grow mould.
- Ignoring air sealing. It cuts the humidifier load and the heating bill.
- A bypass humidifier on a heat pump. The supply air is too cool to evaporate much; use a steam unit.
- Hard water in an ultrasonic unit. It sprays white mineral dust; use distilled water or an evaporative unit.
- Never cleaning the pad. A scaled pad adds little water; replace it every season.
Questions people ask
- What size humidifier do I need for my house?
- For a 2,000 sq ft house of average tightness with 8 ft ceilings, holding 35% relative humidity at 70 °F when it is 20 °F outside, about 6.6 gallons per day, which is a large console or a small whole-house unit. A tight 1,500 sq ft house holding 30% at 0 °F needs only about 3 gallons; a drafty 2,000 sq ft house needs about 13, which is a whole-house bypass or fan-powered unit.
- What humidity should my house be in winter?
- 30 to 40% at 70 °F is comfortable for most people and dry enough to avoid condensation on double-pane windows in moderate cold. As it gets colder outside, lower it, because the inside surface of the glass gets colder. A common guide is 40% at 20 °F outdoors, 35% at 10 °F, 30% at 0 °F and 25% at −10 °F. If windows fog or frost, the humidity is too high for that day.
- Why does a drafty house need a bigger humidifier?
- Because every air change brings in cold outdoor air that holds very little moisture and pushes out indoor air that holds a lot. At 20 °F and 70% RH outdoor air carries about 11 grains of water per pound; at 70 °F and 35% indoor air carries about 38. A house that exchanges its air once an hour loses twice as much moisture as one that exchanges it every two hours. Air sealing shrinks the humidifier and the heating bill at the same time.
- What is the difference between bypass, fan-powered and steam humidifiers?
- All three are whole-house units mounted on the furnace or air handler. A bypass unit uses the furnace blower to push air through a wetted pad and back to the return; it adds 12 to 17 gallons a day and needs no motor of its own. A fan-powered unit has its own fan and adds 15 to 18 gallons. A steam unit boils water with an electric element and adds 10 to 35 gallons regardless of the furnace's temperature, at a higher running cost; it is the choice for large houses and heat pumps.
- Do people and cooking add humidity?
- Yes. A family of four adds about 1 to 2 gallons of water a day through breathing, showers, cooking and drying clothes indoors, which is why a tight house sometimes needs no humidifier at all and a ventilation fan instead. The calculator leaves this out, so its figure is a little high for a full household and right for an empty house on a cold weekday.
- Does a heat pump dry out the house more than a furnace?
- No. Indoor dryness in winter comes from cold outdoor air leaking in, not from the heating equipment, and a heat pump and a furnace dry the house equally for the same leakage. Heat pump supply air is cooler, so it can feel drafty, and bypass humidifiers work less well on it because they rely on warm supply air to evaporate water; a steam humidifier is the usual whole-house choice with a heat pump.