Dew Point Calculator
Find the dew point temperature from the current air temperature and relative humidity using the Magnus-Tetens approximation.
Example
At an air temperature of 25°C and 60% relative humidity, the Magnus formula gives a dew point of about 16.7°C (62.1°F) — which feels slightly humid.
How it works
Using the Magnus formula, compute the intermediate term gamma = ln(RH/100) + (b·T)/(c+T), then the dew point Td = (c·gamma)/(b-gamma), with constants b = 17.625 and c = 243.04 deg C.
Good to know
This Dew Point Calculator converts two everyday weather readings — air temperature in degrees Celsius and relative humidity in percent — into the dew point temperature, shown in both Celsius and Fahrenheit alongside a plain-language comfort label. It is built for anyone who wants a quick sense of how moist the air actually is: gardeners watching for frost or condensation, HVAC and indoor-air folks checking when surfaces will sweat, runners and cyclists planning around mugginess, and pilots or weather hobbyists who need a fast field estimate.
Reach for it when relative humidity alone is misleading. A "60% humidity" reading feels very different on a cold morning than on a hot afternoon, because humidity is relative to the current temperature. Dew point is an absolute measure of moisture, so it lets you compare days, rooms, or locations directly. It is also the figure you want when you care about condensation: if any surface — a window, a cold drink, a duct, a plant leaf — drops below the dew point, water will form on it.
To read the result, treat the dew point number itself as the comfort cue rather than the humidity percentage. The calculator flags ranges for you, but as a rough guide: below about 10°C the air feels dry, the mid-teens feel comfortable to slightly sticky, and the low 20s and above feel oppressive. The dew point can never exceed the air temperature; if you ever see them equal, the air is fully saturated (100% humidity) and fog, dew, or condensation is likely.
One practical caveat: this tool uses the Magnus-Tetens approximation, which is excellent across normal weather but assumes you feed it accurate inputs. Make sure your humidity reading and temperature come from the same place and moment — a sensor near a heat source, in direct sun, or just brought in from outside can be off by several degrees, and small input errors carry straight through to the dew point.
Frequently asked questions
What is the difference between dew point and relative humidity?
Relative humidity is the percentage of moisture the air holds relative to its maximum at the current temperature, so it changes as temperature changes. Dew point is the actual temperature to which air must cool for water vapor to condense, giving a more direct sense of how moist the air really feels.
Why does this calculator use the Magnus formula?
The Magnus (Magnus-Tetens) approximation is accurate to within about 0.4°C for temperatures from roughly -45°C to 60°C, which covers virtually all weather conditions. It is the standard practical formula used in meteorology for fast, reliable dew point estimates.
Is my data uploaded anywhere?
No — this calculator runs entirely in your browser. Your inputs never leave your device, and it works offline once loaded.
Is this calculator free?
Yes, completely free with no sign-up and no limits.
People also ask
What is a comfortable dew point for indoors?
Indoors, a dew point roughly in the 10°C to 13°C range (about 50°F to 55°F) is generally considered comfortable, while values much above 16°C (60°F) feel sticky and below about 4°C (40°F) can feel dry. Comfort is subjective and also depends on the air temperature and air movement.
At what dew point does fog or dew form?
Fog, dew, or condensation forms when a surface or the air cools to its dew point, because the air can no longer hold all its water vapor. The closer the air temperature is to the dew point, the more likely fog and dew become, and they are essentially certain when the two values are equal.
Can the dew point be higher than the air temperature?
No. The dew point can equal the air temperature, which means the air is fully saturated at 100% relative humidity, but it can never exceed it. If a calculation appears to show a higher dew point, the input humidity or temperature is likely incorrect.
How do I convert dew point from Celsius to Fahrenheit?
Use the standard temperature formula: multiply the Celsius dew point by 9/5 and add 32. For example, a dew point of 16.7°C equals about 62.1°F. This calculator displays both units automatically.
Does a higher dew point mean it will rain?
Not directly. A high dew point indicates the air holds a lot of moisture, which is one ingredient for storms and rain, but rainfall also depends on lifting, instability, and weather systems. A high dew point makes precipitation more possible but does not guarantee it.
How is dew point different from the heat index?
Dew point measures the actual moisture content of the air, while the heat index combines air temperature and humidity to estimate how hot it feels to the human body. A high dew point contributes to a high heat index, but the heat index is specifically about perceived temperature on warm days.
What dew point causes mold or condensation in a house?
Condensation forms on any indoor surface that is colder than the indoor dew point, such as windows, exterior walls, or cold pipes, and persistent moisture there can encourage mold. Keeping indoor humidity lower so the dew point stays below the temperature of cold surfaces reduces the risk.
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