Should I Open the Windows? Humidity Calculator
Enter the indoor and outdoor temperature and humidity to see whether outside air is actually drier, even when its relative humidity looks higher. The calculator also estimates the indoor RH after ventilation and the air changes needed for your target.
Open or keep closedOpen the windows
- Indoor dew point (°F)
- 57.37
- Outdoor dew point (°F)
- 39.21
- Moisture difference (g/kg dry air)
- 4.99
- Outdoor air RH at room temperature
- 30.36%
- Projected indoor RH
- 41.32%
- Air changes needed for target
- 0.41
- Target feasibility
- Outdoor air is dry enough to reach the target
Assumes well-mixed air, standard sea-level pressure, steady conditions, and no moisture added inside during ventilation.
How to use this calculator
- Choose Fahrenheit or Celsius, then enter the indoor temperature and relative humidity from a room reading.
- Enter the outdoor temperature and relative humidity expected while the windows are open.
- Set the air changes you expect; one air change replaces a room-volume of air with outdoor air.
- Choose the indoor RH target, then compare the recommendation, projected RH, and required air changes.
Why outdoor relative humidity can be misleading
Relative humidity is a percentage of how much vapor the air can hold at its current temperature. Cold outdoor air at 80% RH can carry much less water than indoor air at 60% RH. Once that cold air enters a heated room, its temperature rises but its water content stays nearly the same, so its relative humidity drops. For deciding whether ventilation will dry a room, compare humidity ratio or dew point, not RH alone.
How this calculator works
The calculator first converts temperatures to Celsius and uses the Buck equation over water for saturation vapor pressure:
eₛ(T) = 0.61121 × exp[(18.678 − T ÷ 234.5) × T ÷ (257.14 + T)]
Vapor pressure is e = RH ÷ 100 × eₛ. At standard sea-level pressure, each reading becomes a humidity ratio: w = 0.621945 × e ÷ (101.325 − e). The difference between indoor and outdoor w is shown in grams of water per kilogram of dry air. A positive difference means the outdoor air is drier and ventilation can remove moisture from the room air.
For the projected result, the room is treated as one well-mixed air volume. After N air changes, its humidity ratio is wN = wo + (wi − wo) × exp(−N). The calculator turns that result back into RH at the indoor temperature. It also solves that equation backward for the air changes needed to meet the selected target. If the outdoor air itself is not dry enough at room temperature, ventilation alone cannot reach the target.
What changes the result most
Outdoor dew point and indoor temperature are usually more useful than the outdoor RH percentage. A warm humid day can add moisture even at a modest RH, while cool winter air is often dry after it warms indoors. The number of actual air changes matters too: a cracked window may exchange little air in a sheltered room, while cross-ventilation and wind can exchange much more.
What this calculator leaves out
The model assumes perfect mixing, constant outdoor conditions, and no moisture from people, cooking, showers, drying laundry, wet materials, or leaks. It uses sea-level pressure, so humidity-ratio results shift slightly at elevation. Below freezing, real air can be influenced by frost and ice-saturation conventions; the calculation consistently uses the stated Buck equation over water.
Worked example
At 72°F and 60% RH, indoor air has a dew point of about 57.37°F. Outdoor air at 45°F and 80% RH sounds damp, but its dew point is only about 39.21°F. Its humidity ratio is about 4.99 g/kg lower than the indoor air, and it would be about 30.36% RH after warming to 72°F.
With one ideal air change, the model projects the room at about 41.32% RH. A 50% target needs about 0.41 air changes, so outdoor air is dry enough to reach it under the model assumptions.
Common questions
Why can 80% outdoor humidity be drier than 60% indoor humidity?
Cool air holds less water vapor than warm air. At 45°F, air at 80% RH can contain less vapor than 72°F air at 60% RH. When the outdoor air warms indoors without adding water, its RH falls even though its actual moisture content has not changed.
Does opening windows reduce humidity in winter?
Often, but not automatically. Winter air is commonly dry in terms of humidity ratio because it is cold, and warming it indoors lowers its relative humidity. Compare outdoor and indoor dew point or use this calculator with the local temperature and RH before relying on ventilation.
Should I open windows on a warm, humid day?
Usually not for moisture control if the outdoor humidity ratio is higher than indoors. A lower outdoor RH percentage does not guarantee drier air when the outdoor temperature is much higher. The calculator recommends keeping windows closed when outdoor air contains at least as much water vapor as indoor air.
Does rain mean outdoor air will make the house wetter?
Rain often accompanies humid air, but rain itself is not the comparison. Outdoor air can still be drier than indoor air when it is cool enough, even at high RH. Avoid water entering through open windows, and use local readings rather than assuming rain decides the moisture balance.
What is an air change?
One air change means outdoor air equal to the room volume has entered and mixed with the room air. It does not mean every original molecule has left; mixing is gradual, which is why the model uses an exponential curve. Actual air changes depend on wind, temperature difference, openings, and whether another opening provides a path through the room.
Is dew point better than relative humidity for comparing indoor and outdoor air?
For comparing moisture content across different temperatures, dew point is usually more useful because it follows actual vapor pressure. Humidity ratio is even more direct for the calculation. Relative humidity remains useful for comfort and condensation risk at a particular temperature, but it cannot by itself show which air is drier.