Indoor humidity should ideally be 30 to 50%
The U.S. EPA advises keeping indoor relative humidity below 60 percent, and ideally between 30 and 50 percent. This calculator uses 30 to 50% as the target range and treats 30% and 50% as inside it. Readings above 50% and below 60% are flagged as above ideal, and 60% or more is called humid.
Enter your hygrometer's humidity along with the room temperature and the calculator shows more than a single number can. Air at 70°F and 25% humidity starts to leave water on any window or wall surface colder than 32.5°F, and heating that room to 74°F drops the reading to 22%. If you add the outdoor temperature and humidity from a weather app, it also works out whether opening a window would raise or lower the humidity inside.
Why bother measuring humidity?
Relative humidity changes with temperature even when the amount of water in the air stays the same. That is why "45% humidity" alone cannot tell you whether your windows will fog up or whether airing out the room will dry it further. The calculator turns temperature and outdoor air into those answers.
- Dry air while the heat is on: see how far below the range you are, and use the dew point to judge how much moisture you can add before water forms on the glass.
- A humidifier in the nursery: the gauge marks the EPA's 50% ceiling for humidifier use next to the target range.
- A damp closet or bedroom in summer: find out whether opening a window helps or whether you should close up and run the air conditioner or a dehumidifier.
- Drops of water on windows and cold corners: check the dew point against the outdoor temperature to see why condensation appears even when the reading looks fine.
How the calculation works
- Zone: your humidity, to one decimal place, is compared with 30 to 50%. Below 30% is dry, 30 to 50% is in range, above 50% and below 60% is above ideal, and 60% or more is humid.
- Vapor pressure: the saturation vapor pressure at that temperature, ew(t) = 6.112 × exp(17.62t ÷ (243.12 + t)) hPa, is multiplied by the humidity fraction. This is the formula in the WMO Guide to Meteorological Instruments (WMO-No. 8).
- Dew point: the temperature at which that vapor pressure would equal saturation. Any surface colder than this collects water.
- Absolute humidity: grams of water vapor per cubic meter = 216.679 × vapor pressure (hPa) ÷ (temperature in °C + 273.15).
- Changing the temperature or airing out: the water vapor is held constant, and the humidity is recalculated at the new temperature. Airing out assumes the room air is completely replaced by outdoor air that has then reached room temperature.
Formulas e = RH ÷ 100 × 6.112·exp(17.62T ÷ (243.12 + T)) · dew point = 243.12·ln(e ÷ 6.112) ÷ (17.62 − ln(e ÷ 6.112)) · humidity at a new temperature T′ = 100 × e ÷ ew(T′) · T in °C, e in hPa. Temperatures you type in °F are converted first.
What the agencies recommend
Numbers differ by country and purpose. The U.S. and Canadian guidance below is aimed at homes; the Japanese, Spanish and Korean figures come from building rules or health portals and are included for comparison.
| Agency or rule | Humidity | What else it says |
|---|---|---|
| U.S. EPA, mold guide | Below 60%, ideally 30 to 50% | Condensation can be a sign of high humidity; use fans and ventilation, and air conditioners or dehumidifiers when needed |
| U.S. EPA, home humidifiers | No higher than 50% | If water condenses on windows, walls or pictures, move the humidifier, lower its setting or use it less |
| Health Canada | 30 to 50% | Use a humidifier or dehumidifier if necessary; low humidity may aggravate skin allergies and cause respiratory infections, and high humidity can lead to mold |
| Korea, environmental health portal | 40 to 60% | Temperature 18 to 22°C |
| Japan, building hygiene standard | 40 to 70% | For larger buildings with air conditioning |
| Spain, RITE (building thermal rules) | Winter 40 to 50%, summer 45 to 60% | Design indoor conditions, 21 to 23°C in winter and 23 to 25°C in summer |
You may see charts online that list a different target for each season, such as 40% in winter and 60% in summer. The U.S. and Canadian agencies above do not publish a seasonal table, so this calculator keeps one range all year and changes only the suggested next steps with the season.
Dew point by room temperature and humidity
Each cell is the dew point of that room air in °F. If a window pane, frame or corner of an exterior wall is colder than this, water forms on it.
| Room temp＼Humidity | 30% | 40% | 50% | 60% | 70% |
|---|---|---|---|---|---|
| 64°F | 31.9 | 39.2 | 45.0 | 49.8 | 54.0 |
| 68°F | 35.4 | 42.8 | 48.7 | 53.6 | 57.8 |
| 72°F | 38.9 | 46.3 | 52.3 | 57.4 | 61.7 |
| 76°F | 42.3 | 49.9 | 56.0 | 61.1 | 65.5 |
| 80°F | 45.8 | 53.5 | 59.7 | 64.9 | 69.3 |
At 72°F, raising humidity from 40% to 60% lifts the dew point from 46.3°F to 57.4°F, more than 11 degrees. That is why heavy humidifying makes cold windows drip. On days when the outdoor temperature is below the dew point, the first places to collect water are poorly insulated windows, frames and corners of exterior walls, which sit close to the outdoor temperature.
Why heating dries the air
Warm air can hold more water vapor, so heating air without adding water lowers its relative humidity, and cooling it raises the reading. Here is what happens to 68°F air at 50% when only the temperature changes.
| Temperature | 60°F | 64°F | 68°F | 72°F | 76°F | 80°F |
|---|---|---|---|---|---|---|
| Humidity | 66% | 57% | 50% | 44% | 38% | 33% |
Above 64°F, every 4 degrees of warming takes off roughly 5 to 7 points. If the air feels dry in winter, turning the thermostat down a few degrees can help as much as adding a humidifier. The "Try a different room temperature" slider in the calculator lets you test this with your own numbers.
Will opening a window raise or lower humidity?
Outdoor air changes in humidity as it warms or cools to room temperature, so what matters is how much water vapor it carries compared with your indoor air, not the outdoor humidity reading. The table shows the humidity you would get once outdoor air has been warmed or cooled to 72°F.
| Outdoor temp＼Outdoor humidity | 60% | 70% | 80% |
|---|---|---|---|
| 14°F | 6% | 8% | 9% |
| 23°F | 9% | 11% | 13% |
| 32°F | 14% | 16% | 18% |
| 41°F | 20% | 23% | 26% |
| 50°F | 28% | 32% | 37% |
| 59°F | 38% | 45% | 51% |
Even 80% humidity outdoors in freezing weather ends up at 18% once that air is warmed to 72°F, so winter ventilation dries a heated home. In warm, damp weather the opposite is common.
- Outdoor air at 75°F and 95% (a rainy day), brought to a 78°F room: 86%
- Outdoor air at 90°F and 60%, brought to a 78°F room: 88%
- Outdoor air at 86°F and 80%, cooled to a 78°F room: above 100%, so water condenses (on an air-conditioner coil or a cold wall)
When the room is already humid and the outdoor air is even more humid, close the windows and run the air conditioner or a dehumidifier.
What to check in each room
- Nursery: no public humidity target specific to a nursery was found, so the same 30 to 50% range applies. If you use a humidifier, the EPA says not to humidify above 50%, to empty portable units, wipe them dry and refill with fresh water daily, and to clean them every third day.
- Bedroom: measure before bed and after waking, in the same spot. A humidifier that sprays onto bedding or walls wets only that area.
- Living room: readings right beside a humidifier, at a window or near the kitchen differ a lot from the rest of the room. Measure where people actually sit, away from those spots.
- Closet: the inside of a closet and the side along an exterior wall can be more humid than the middle of the room. Put clothes away dry and leave a gap between furniture and the wall.
Raising and lowering humidity
- To raise it: a humidifier or hanging damp laundry are the simplest options. If water condenses on windows, walls or pictures, the EPA says to move the humidifier, lower its setting or use it less. The EPA also suggests bottled water labeled “distilled”, noting that distillation is the most effective way to remove minerals.
- To lower it: run the bathroom fan or open the window when showering, use exhaust fans while cooking or running the dishwasher, and use an air conditioner or dehumidifier when needed, all of which the EPA lists. On days when the outdoor air is drier than the indoor air, airing out is the cheapest dehumidifier.
- By changing the temperature: with the same amount of water in the air, cooling raises the reading and warming lowers it. The dew point does not move, so the risk of condensation on cold glass stays the same.
Common mistakes
- Reading the meter next to a humidifier: the spot where the mist lands reads far higher than the room. Keep the hygrometer away from humidifiers, windows and the kitchen.
- Assuming humid outdoor air makes the room humid: freezing air at 80% humidity ends up at 18% once it is warmed to 72°F. Compare the water vapor, not the percentages.
- Assuming a reading in range means no condensation: 72°F air at 50% leaves water on any surface below 52.3°F.
- Treating a seasonal chart as an official standard: tables such as "40% in winter, 60% in summer" have no public agency source behind them.
- Reading 50% as "half the air is water": air at 72°F and 50% holds only about 9.8 g of water vapor per cubic meter.
Limits and cautions
- The airing-out figure assumes outdoor air fully replaces room air. In a real home, moisture from people, cooking and laundry, and moisture stored in walls and furniture, bring the result somewhere in between.
- Consumer hygrometers can be off by several points. A trend measured in the same spot at the same time of day is more useful than one reading.
- The condensation alert compares only the outdoor temperature with the dew point. The real temperature of a pane of glass depends on the glazing, frame, curtains and where the heat sources are.
- This calculator does not diagnose mold or breathing problems. If you have symptoms, talk with a doctor.
Sources
- 01U.S. EPA: A Brief Guide to Mold, Moisture and Your HomeKeep indoor humidity below 60 percent, ideally between 30 and 50 percent; condensation can be a sign of high humidity (checked 2026-10-17)
- 02U.S. EPA: Use and Care of Home HumidifiersDo not humidify above 50 percent; empty, dry and refill portable units daily; clean every third day; use distilled water
- 03Health Canada: Healthy home guideKeep humidity between 30 and 50%, using a humidifier or dehumidifier if necessary
- 04Spain, BOE: RITE, IT 1.1.4.1.2Design indoor conditions: summer 23 to 25°C and 45 to 60%, winter 21 to 23°C and 40 to 50%
- 05Japan, MHLW: building environmental hygiene standardRelative humidity 40% or more and 70% or less in occupied rooms of buildings with air conditioning
- 06Korea, environmental health portal: indoor air quality managementTemperature 18 to 22°C and humidity 40 to 60% as comfortable, healthy conditions
- 07NCAR/EOL: Water Vapor Pressure FormulationsWMO-No. 8 (CIMO Guide) saturation vapor pressure over water, ew = 6.112 exp(17.62 t / (243.12 + t)), t in °C and ew in hPa
- 08Vaisala: Absolute humidity calculation (HMT120 user guide)Absolute humidity a = 216.679 × Pw (hPa) ÷ (T + 273.15) g/m³
Frequently asked questions
What is the ideal indoor humidity?
The U.S. EPA says to keep indoor relative humidity below 60 percent, ideally between 30 and 50 percent. Health Canada also advises 30 to 50%. This calculator treats 30% and 50% as inside the range and calls readings from above 50% to under 60% above ideal.
What should indoor humidity be in winter?
The range stays the same all year: 30 to 50%. In winter, heating pulls humidity down quickly, and humidifying more raises the dew point. If water beads on windows, the EPA says to move the humidifier, lower its setting or use it less.
What humidity is right for a baby's room?
No public target specific to a nursery was found, so the same 30 to 50% range applies. If you use a humidifier, the EPA says not to go above 50%, and to empty portable units, wipe them dry and refill them with fresh water daily.
Why does water form on my windows when humidity is in range?
Air at 72°F and 50% has a dew point of 52.3°F. A pane or frame colder than that collects water even though the reading is fine. It happens most on cold days at poorly insulated windows and corners of exterior walls.
Why does heating make the air feel dry?
The amount of water vapor stays the same, but warm air can hold more of it, so relative humidity falls. Air at 68°F and 50% drops to 38% when heated to 76°F, and to 33% at 80°F.
Does opening a window in winter dry out the room?
Yes. Outdoor air at 32°F and 70% humidity becomes 16% once it warms to 72°F. Fresh air is still worth having, so air the room out, measure again afterward and add moisture if the reading falls below the range.
Will opening a window on a rainy day raise the humidity?
It can, if the outdoor air carries more water vapor than the indoor air. Outdoor air at 75°F and 95% brought to a 78°F room gives 86%. If the room is already humid, close the windows and run the air conditioner or a dehumidifier.
Does 50% humidity mean half the air is water?
No. Relative humidity is the share of the water vapor that air at that temperature could hold at most. At 72°F and 50%, a cubic meter of air carries only about 9.8 g of water vapor.