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The calculator converts your air temperature to Celsius if needed, then applies the Magnus formula: it computes gamma = ln(RH/100) + (17.625 × T)/(243.04 + T) and solves for the dew point as 243.04 × gamma / (17.625 - gamma). The dew point is the temperature air must cool to, with no change in moisture content, before water vapor starts condensing as dew, fog or droplets on cold surfaces. Because it measures the actual moisture in the air rather than a ratio to the current temperature, it is a steadier, more honest comfort indicator than relative humidity. Each result comes with a comfort band, from dry to oppressive, plus the number in both Celsius and Fahrenheit.
The dew point is the temperature at which air becomes fully saturated with moisture. Cool the air any further, and water vapor condenses into dew, fog or droplets on a cold surface, which is also why meteorologists pair dew point readings with the wet bulb temperature and the heat index when judging humidity. Unlike relative humidity, which only describes how close the air is to saturation at its current temperature, the dew point directly reflects how much water vapor is in the air. That is why it is the number meteorologists trust for humidity comfort: a dew point of 16°C feels roughly the same whether the air temperature is 24°C or 34°C. One rule holds always: the dew point can never be higher than the air temperature. When the two are equal, relative humidity is 100 percent and the air is holding all the moisture it can.
The standard approximation is the Magnus formula with the constants recommended by Alduchov and Eskridge: b = 17.625 and c = 243.04. First compute gamma = ln(RH/100) + (b × T)/(c + T) with T in Celsius, then dew point = c × gamma / (b - gamma). Worked example: at 25°C and 60% relative humidity, gamma = ln(0.60) + (17.625 × 25)/(243.04 + 25) = -0.5108 + 1.6439 = 1.1331, so the dew point is 243.04 × 1.1331 / (17.625 - 1.1331) = 16.7°C (62.1°F). A hotter example: at 30°C and 80% humidity the same steps give 26.2°C (79.1°F), deep in the oppressive band. The formula is accurate to a fraction of a degree across normal weather and indoor conditions, which is why the calculator accepts temperatures from -40°C to 60°C.
Use these bands, a widely used meteorological rule of thumb, to read your result. Below 10°C (50°F): dry to comfortable, fresh air that can feel dry indoors in winter. 10 to 13°C (50 to 55°F): very comfortable. 13 to 16°C (55 to 61°F): comfortable. 16 to 18°C (61 to 64°F): slightly humid. 18 to 20°C (64 to 68°F): humid, sweat evaporates slowly and exercise feels harder. Above 20°C (68°F): oppressive, heavy and muggy. Above about 24°C (75°F): extremely uncomfortable. Sensitivity varies from person to person, and wind and sunshine also shape how humidity feels, so treat the bands as guidance rather than a medical threshold.
Relative humidity can be misleading because warm air holds far more water vapor than cold air. Compare a cold misty morning at 5°C and 90% relative humidity, which has a dew point of only 3.5°C, with a hot afternoon at 32°C and 40% humidity, which has a dew point of 16.7°C and feels far more humid despite the lower percentage. This is why a high humidity reading in winter can coexist with dry-feeling air, and why 40% humidity on a hot summer day can feel sticky. Relative humidity also swings through the day as temperatures rise and fall, even when the actual moisture in the air stays exactly the same. The dew point stays steady, which makes it the better number for comparing humidity between places, days and seasons.
For a fast mental estimate when relative humidity is above about 50 percent, subtract (100 - RH) / 5 from the air temperature in Celsius. At 25°C and 60% humidity that gives 25 - 8 = 17°C, close to the exact 16.7°C. A related rule of thumb: every 1°C gap between the air temperature and the dew point lowers relative humidity by roughly 5 to 6 percentage points when the air is fairly humid. These shortcuts are handy in the field, but for precise comfort readings, fog forecasting or condensation decisions, use the full calculation.
Condensation forms on any surface colder than the surrounding air's dew point, which is why a glass of iced water sweats in a warm room and why bathroom mirrors fog. Fog forms when the air temperature drops to meet the dew point, most often on clear, calm nights, and checking the forecast low against the evening dew point is the basis of every fog forecast; overnight lows frequently bottom out near the evening dew point because condensation releases heat. High dew points also feed summer storms: plenty of moisture means plenty of fuel for showers and thunderstorms, and high absolute humidity, the grams of water vapor per cubic metre of air, is what makes the air feel heavy. Below freezing the physics shifts slightly: the value over ice, called the frost point, sits a touch higher than the dew point over water, so frost can appear on surfaces even when the calculated dew point is just below 0°C. Air conditioners and dehumidifiers remove moisture by cooling air across a coil that sits below its dew point, which is also why refrigerant dehumidifiers lose effectiveness in cold rooms and why HVAC installers size equipment against the dew point.
Indoors, the dew point warns of damp and mold. A room at 20°C and 60% relative humidity has a dew point of 12°C, so any window, wall or pipe colder than that will run with water, and surfaces that stay damp encourage mold. To stay safe, keep indoor relative humidity around 30 to 50 percent and below 60 percent, the range recommended for limiting condensation and mold risk; at 20°C that is a dew point of roughly 2 to 9°C. Ventilate kitchens and bathrooms, dry laundry outside or in a vented dryer, and keep surfaces warmer than the dew point with insulation, double glazing and steady heating, since steel, concrete and glass can be much colder than the room air, especially early in the morning. Trades use a stricter version of this rule for painting and coatings: the surface temperature should stay at least 3°C (5°F) above the dew point during application and drying, or coatings can blush, blister or fail to bond.
The result is only as good as the readings behind it. Inexpensive digital hygrometers are often off by 3 to 5 percent relative humidity, which shifts the dew point by about a degree. For reliable numbers, place the sensor away from direct sun, radiators, cooking, showers and outside walls, and give it 15 to 30 minutes to settle after moving it. You can check your meter with the salt test: seal it in a container with a small dish of damp table salt for 8 to 12 hours. A saturated salt solution holds the air at about 75% humidity, so any difference from 75 is your meter's error. Note also that pressure affects the secondary moisture figures only slightly under normal conditions, and this calculator does not adjust for altitude.
Weather reports often give the temperature and dew point rather than the relative humidity, and the same Magnus formula works in reverse. Dividing the saturation vapor pressure at the dew point by the saturation vapor pressure at the air temperature and multiplying by 100 gives the relative humidity. Example: 30°C with a dew point of 20°C works out to roughly 55% humidity, while 22°C with a dew point of 10°C is about 47%. In Fahrenheit, simply enter your temperature in °F and the calculator converts, computes and returns the dew point in °F; 77°F at 60% humidity has a dew point of 62.1°F.
The most common mistake is judging humidity by relative humidity alone without considering the temperature, which is how people end up surprised that 90% humidity on a cold morning feels dry. Another is mixing up indoor and outdoor readings, since indoor air is usually warmer with a different humidity. Always double check that the temperature unit matches the scale you read, a frequent slip after converting between Celsius and Fahrenheit. Do not ignore cold spots: window frames and outside corners reach the dew point first and are where condensation and mold start. Finally, remember that a relative humidity of exactly 0% has no dew point at all, since air with no water vapor cannot saturate, so the formula needs a humidity above zero.
With the Magnus formula: first compute gamma = ln(RH/100) + (17.625 × T)/(243.04 + T) where T is the air temperature in Celsius and RH is the relative humidity percentage, then the dew point is 243.04 × gamma / (17.625 - gamma). At 25°C and 60% humidity this gives 16.7°C. The calculator does all of this automatically and converts Fahrenheit inputs to Celsius first.
A dew point below 16°C (61°F) feels comfortable for most people: below 10°C feels dry, 10 to 13°C is very comfortable and 13 to 16°C is comfortable. From 16 to 20°C the air starts to feel humid, and above 20°C (68°F) it feels oppressive and sticky, with sweat evaporating slowly. Above about 24°C (75°F) most people find it extremely uncomfortable. With a dew point above 20°C, take extra care with exercise and outdoor work.
No. By definition the dew point can never exceed the current air temperature. It can only equal it, which happens at 100% relative humidity when the air is saturated and holding all the moisture it physically can. If your numbers suggest otherwise, the temperature and humidity readings came from different places or times.
No. Relative humidity is the percentage of moisture the air holds relative to the maximum it could hold at its current temperature, so it changes as the temperature changes. The dew point is the actual temperature at which the current moisture would saturate the air, so it measures real moisture content directly. That is why the dew point is the better number for comparing humidity across different times, places and temperatures.
Because cold air holds very little moisture. A misty morning at 5°C and 90% relative humidity has a dew point of only 3.5°C, so there is hardly any water vapor in the air and it does not feel humid. A hot afternoon at 32°C and just 40% humidity has a dew point of 16.7°C, far more moisture, and feels noticeably sticky. The dew point reveals the truth that the percentage hides.
Yes. Any surface colder than the room's dew point will gather condensation. At 20°C and 60% humidity the dew point is 12°C, so glass colder than that will mist up. This is also why the painting rule exists: keep surface temperatures at least 3°C above the dew point when applying coatings. Watch window frames and outside corners first, since they are usually the coldest spots.
Keep indoor relative humidity between about 30 and 50 percent, and always below 60 percent, to limit condensation and mold. At 20°C that corresponds to a dew point of roughly 2 to 9°C. If your indoor dew point stays above 12 to 15°C, improve ventilation in kitchens and bathrooms, fix cold spots with insulation, or run a dehumidifier in damp rooms.
Yes. When relative humidity is above about 50 percent, subtract (100 - RH) / 5 from the air temperature in Celsius. At 25°C and 60% humidity that gives 25 - 8 = 17°C, close to the exact 16.7°C. It is a handy field shortcut, but use the full Magnus calculation for fog forecasting, condensation decisions or precise comfort readings.
The calculator gives the dew point over liquid water. Below 0°C, ice is the relevant surface, and the value is called the frost point, which sits slightly higher than the water-based dew point. In practice this means frost can form on surfaces even when the calculated dew point is just below freezing, so treat sub-zero results as a frost warning rather than an exact number.
Select Fahrenheit as the temperature unit and enter the air temperature in °F; the calculator converts it to Celsius internally, runs the Magnus formula and converts the answer back to °F. For example, 77°F at 60% humidity gives a dew point of 62.1°F. Comfort bands work the same way: below 50°F feels dry, 50 to 61°F comfortable, and above 70°F oppressive.
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