Free relative humidity calculator
Enter the air temperature and dew point to get the relative humidity as a percentage, with a read on whether the air is dry, comfortable, or muggy, using the inverse Magnus formula, updated live, as you type.
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Relative humidity via inverse Magnus formula (Sonntag 1990). Dew point must be ≤ air temperature.
Results are estimates. Consult a professional.
What is relative humidity?
Relative humidity is the amount of water vapor the air is holding compared with the most it could hold at its current temperature, written as a percentage. At 60% relative humidity the air is carrying 60% of its maximum moisture for that temperature; at 100% it is saturated and water begins to condense. This relative humidity calculator takes an air temperature and a dew point and returns the relative humidity that pairs with them.
The "relative" part is the catch. Because warm air can hold far more vapor than cold air, the same physical amount of moisture reads as a high humidity on a cool morning and a much lower humidity by a warm afternoon — even though no water entered or left the air. That is why a single relative humidity figure only makes sense once you know the temperature it was measured at.
How the relative humidity calculator works
The calculator inverts the Magnus formula. The Magnus relation links three quantities — air temperature, dew point, and relative humidity — so given any two it can solve for the third. Here you supply the air temperature and the dew point, and the calculator returns the relative humidity directly, with no intermediate steps to track.
The inputs explained
Two readings go in, and both are everyday weather values. Knowing how each one moves the result makes the relative humidity easy to reason about.
Air temperature
The ordinary dry-bulb temperature from a thermometer or weather report, entered in °C. It sets the air's moisture capacity: at a fixed dew point, a warmer air temperature means a lower relative humidity, because the warmer air could hold more than it currently is.
Dew point
The temperature to which the air would have to cool before its moisture condenses, also in °C. It fixes the actual water content. The closer the dew point sits to the air temperature, the higher the relative humidity — and when the two are equal, the air is saturated and humidity reads 100%.
A worked example using the relative humidity calculator
A hygrometer reads an air temperature of 25 °C (77 °F) and a dew point of 15 °C (59 °F). Those are the calculator's default inputs. The dew point alone says the air is moderately moist, but to know whether the room sits in the comfortable band you need the relative humidity.
Step 1 — Enter the two readings
Air temperature 25 °C, dew point 15 °C. The dew point is below the air temperature, as it must be, so the result will land below 100%.
Step 2 — Evaluate the two Magnus terms
The dew-point term is (17.625 × 15) / (243.04 + 15) = 1.0246 and the temperature term is (17.625 × 25) / (243.04 + 25) = 1.6439. Their difference, the exponent, is −0.6193.
Step 3 — Solve for the relative humidity
Exponentiating and scaling gives R = 100 × exp(−0.6193) = 53.8%.
What is a comfortable indoor humidity?
For occupied indoor spaces, a relative humidity of roughly 40–60% is the range most people find comfortable and that limits the problems at either extreme — dry air and static below it, mustiness and mold above it. The table below shows what happens as you move through the bands.
| Indoor relative humidity | How it feels | What to expect |
|---|---|---|
| Below 30% | Too dry | Dry skin, chapped lips, irritated airways, and frequent static shocks; wood floors and furniture can crack. |
| 30–40% | Dry but acceptable | Comfortable for most people; a humidifier helps in winter if static or dry sinuses appear. |
| 40–60% | Comfortable | The target band — air feels neither dry nor clammy, and mold and dust-mite growth are kept in check. |
| 60–70% | Slightly humid | Air starts to feel sticky; condensation appears on cold windows and mold risk begins to rise. |
| Above 70% | Too humid | Persistent dampness, condensation, and a real risk of mold, mildew, and dust-mite proliferation. |
The 40–60% target follows ASHRAE guidance for occupied spaces; the upper limit aligns with the EPA's advice to keep indoor humidity below 60% (cited below). Individual comfort varies with temperature, clothing, and activity.
Relative humidity vs dew point: which to trust
Relative humidity is intuitive indoors, where temperature is roughly steady, but it is a poor way to compare moisture across different temperatures. Because the percentage is measured against a ceiling that rises and falls with temperature, the same air can post wildly different relative humidity numbers through a single day while its actual water content never moves.
For judging how muggy outdoor air will feel, the dew point is the more reliable gauge precisely because it does not drift with temperature. The U.S. National Weather Service makes the same point: to compare how humid two days feel, look at the dew point rather than the relative humidity. If you have the temperature and humidity instead, the dew point calculator runs the conversion the other way.
National Weather Service (La Crosse, WI) — guidance that the dew point, not relative humidity, is the better indicator of how humid the air feels because relative humidity shifts with temperature.Humidity, mold, static, and health thresholds
The 40–60% band is not only about comfort — it is where the main moisture-related risks are smallest. The EPA advises keeping indoor relative humidity below 60%, and ideally between 30% and 50%, because relative humidity above 60% promotes condensation and mold growth.
- Above 60% — mold and dust mites. Sustained high humidity lets mold, mildew, and dust mites flourish on walls, fabrics, and around windows. The EPA names 60% as the line to stay under for mold control.
- Below 30% — dry-air effects. Very dry air dries out skin and airways, worsens static electricity, and can crack wood. It also helps some airborne viruses persist, which is why winter indoor air often feels harsh.
- 40–60% — the sweet spot. Inside this band, mold and mite growth slow, static eases, and most people breathe comfortably. It is the band ASHRAE recommends for occupied spaces and the one to aim a humidifier or dehumidifier at.
Common relative humidity mistakes to avoid
- Comparing humidity readings at different temperatures. A 70% reading in a cool basement and a 50% reading in a warm living room can hold similar moisture. Compare dew points, not raw percentages, when temperatures differ.
- Assuming high humidity means a lot of water in the air. Cold air at 90% relative humidity holds little moisture in absolute terms. The percentage describes how full the air is, not how much water it carries.
- Forgetting that heating dries the air. Warming a room without adding moisture raises its capacity and drops the relative humidity — the reason indoor air turns dry and static-prone in winter even when outdoor humidity is high.
- Treating the comfort band as exact. The 40–60% range is guidance, not a hard line. A reading of 62% is not a crisis; a persistent 75% is the real signal to run a dehumidifier.
For related views of the same moist air, the heat index calculator shows how hot high humidity makes it feel, and the dew point calculator converts temperature and humidity into the absolute moisture measure.
How accurate is this relative humidity calculator?
The calculator uses the inverse Magnus formula with the Sonntag (1990) coefficients (a = 17.625, b = 243.04 °C), the standard way to relate temperature, dew point, and relative humidity. Across the everyday range of −40 to +50 °C the Magnus relation is accurate to within a fraction of a percent of relative humidity — far tighter than any home hygrometer. The result is clamped to 0–100%, so a dew point entered above the air temperature returns 100% rather than an impossible value.
The arithmetic is reliable; the inputs are the weak link. A dew point taken from a cheap sensor, or two readings measured minutes and meters apart, will shift the humidity. And the comfort and health bands are general guidance — acclimation, airflow, and activity all change how a given humidity feels. Treat the percentage as a solid number and the comfort labels as rules of thumb.
Frequently asked questions about the free relative humidity calculator
About this relative humidity calculator
This relative humidity calculator runs entirely in your browser — nothing you enter is sent anywhere. It solves relative humidity from the air temperature and dew point with the inverse Magnus formula and reports the result as a percentage, clamped to a physical 0–100% range.
It pairs with the dew point calculator and the heat index calculator. Browse every weather tool on the calculators index.