In winter, cold outdoor air holds very little water. Once it leaks into a house and warms up, its relative humidity drops, often below 25%, which dries skin, irritates the nose and throat, causes static shocks and makes wood floors shrink. A humidifier puts that moisture back. This calculator estimates how many gallons of water a day it must evaporate.
The formula
Water per hour = volume of air × air changes per hour × (indoor absolute humidity − outdoor absolute humidity)
Absolute humidity is the mass of water in a cubic meter of air. It depends on temperature and relative humidity: air at 70 °F and 40% holds about 7.4 g of water per m³, while air at 30 °F and 70% holds only about 3.1 g/m³. Every cubic meter of outdoor air that replaces indoor air must be topped up by the difference.
Air changes per hour (ACH) measure how fast the air in a home is replaced. A tight modern home is around 0.3, an average house about 0.5, and an older, leaky house 1.0 or more.
Example
A 1,500 sq ft home with 8 ft ceilings holds 12,000 cu ft of air. At 0.5 ACH, keeping 40% humidity at 70 °F when it is 30 °F and 70% outside takes about 4.6 gallons a day, roughly 24 fl oz an hour.
What humidity to aim for
| Outdoor temperature | Maximum indoor humidity to avoid window condensation |
|---|---|
| 20 to 40 °F | 40% |
| 10 to 20 °F | 35% |
| 0 to 10 °F | 30% |
| below 0 °F | 25% or less |
Between 30 and 50% is comfortable and limits dust mites and mold. Above that, water condenses on cold windows and walls.
Choosing a humidifier
- Room units are rated in gallons per day and usually cover one room or an open area.
- Whole-house humidifiers attach to the furnace and can deliver 12 to 18 gallons a day.
- Ratings are often measured in ideal conditions: pick a model slightly above the calculated output.
Limits
Cooking, showers, plants and people add moisture too, often 1 to 2 gallons a day for a family, so the real need can be lower. Sealing air leaks reduces it the most. Clean the tank regularly and use distilled water in ultrasonic models to avoid white dust.