Dew Point Calculator
Enter air temperature and relative humidity to calculate the dew point instantly. Switch modes to reverse-solve for relative humidity or air temperature instead. Supports Celsius, Fahrenheit, and Kelvin. Results include absolute humidity, a human comfort rating, and a dew point chart across the full humidity range.
What is the dew point?
The dew point is the temperature to which air must be cooled, at constant pressure and constant water vapor content, for saturation to occur. At that temperature the air can no longer hold all its moisture and water begins to condense as dew, frost, or fog. A higher dew point means more water vapor is in the air. Unlike relative humidity, the dew point does not change when the air warms or cools without adding or removing moisture, which is why meteorologists and comfort scientists prefer it as a humidity indicator. A dew point of 10 degrees Celsius feels dry and pleasant; one above 21 degrees Celsius starts to feel oppressive because sweat cannot evaporate efficiently.
The Magnus-Tetens formula
This calculator uses the Magnus-Tetens approximation (Alduchov and Eskridge 1996) to compute the dew point. The formula is: alpha = (17.625 x T) / (243.04 + T) + ln(RH / 100), then T_d = (243.04 x alpha) / (17.625 - alpha), where T is the air temperature in Celsius and RH is the relative humidity as a percentage. The formula is accurate to within about 0.35 degrees Celsius for temperatures between -40 and 60 degrees Celsius. The same formula can be rearranged to find RH from temperature and dew point, or to find air temperature from dew point and RH.
Dew point vs. relative humidity
Relative humidity (RH) is the ratio of the actual amount of water vapor in the air to the maximum it could hold at that temperature, expressed as a percentage. Because warm air can hold more vapor, the same physical moisture in the air produces a lower RH reading on a hot day than on a cool one. This makes RH a misleading comfort gauge across seasons. Dew point, by contrast, directly measures how much moisture is present, so it stays constant when temperature changes. A dew point of 15 degrees Celsius on a 30-degree day gives about 43% RH; on a 20-degree day it gives about 73% RH. Both feel identical in terms of the actual vapor load on your body. Dew point wins for comfort assessment; RH wins for knowing how close the air is to saturation.
How to use the three solve modes
This calculator solves all three main humidity relationships. In "dew point" mode you supply air temperature and relative humidity; the calculator returns the dew point, the absolute humidity, and the temperature-dew point spread. In "relative humidity" mode you supply air temperature and dew point; it returns the corresponding RH. In "air temperature" mode you supply dew point and RH; it tells you what dry-bulb temperature that combination implies. All three modes use the same Magnus-Tetens inversion, which is algebraically exact within the formula's validity range.
Dew point comfort scale
| Dew point (°C) | Dew point (°F) | Comfort level |
|---|---|---|
| Below 10 | Below 50 | Dry - a bit dry for some people |
| 10-12 | 50-55 | Comfortable - pleasant |
| 13-15 | 55-60 | Slightly humid - still comfortable for most |
| 16-17 | 61-63 | Getting sticky - somewhat uncomfortable |
| 18-20 | 64-68 | Uncomfortable - lots of moisture |
| 21-23 | 70-74 | Very uncomfortable - oppressive |
| 24 and above | 75 and above | Dangerous - severe heat stress risk |
Human comfort levels based on dew point temperature, widely used in US weather reporting.
Frequently asked questions
What dew point is comfortable?
Most people find dew points below about 13 degrees Celsius (55 degrees Fahrenheit) comfortable and refreshing. Between 13 and 16 degrees it starts to feel slightly humid but still pleasant. Above 21 degrees Celsius (70 degrees Fahrenheit) the air feels oppressive and sticky because sweat cannot evaporate fast enough to cool the body. A dew point above 24 degrees Celsius (75 degrees Fahrenheit) is considered dangerous for prolonged outdoor activity.
Can the dew point ever be higher than the air temperature?
No. The dew point is always equal to or less than the air temperature. If you calculate a dew point that exceeds the air temperature you have an impossible combination: the relative humidity would have to exceed 100%, which cannot happen in practice. When dew point equals air temperature the air is fully saturated (100% RH) and any further cooling causes condensation.
What is the temperature-dew point spread and why does it matter?
The spread (also called the dew point depression) is the air temperature minus the dew point. A small spread means the air is close to saturation. When the spread falls below 2 to 3 degrees Celsius (about 4 to 5 degrees Fahrenheit) fog formation becomes likely, which is why this number is closely watched in aviation weather briefings. A large spread indicates dry air with little risk of fog or condensation.
What is absolute humidity and how does it differ from relative humidity?
Absolute humidity is the actual mass of water vapor per cubic metre of air, measured in grams per cubic metre (g/m3). It tells you how much water physically exists in a volume of air, regardless of temperature. Relative humidity tells you what fraction of the maximum possible moisture the air contains at its current temperature. On a hot day, air with 50% RH can hold - and contain - far more water than air at 10 degrees with 80% RH. Absolute humidity is therefore more useful for engineering and industrial applications where the actual vapor mass matters.
How do I measure dew point without a calculator?
You cannot measure dew point directly with a single instrument. The most common field method uses a sling psychrometer, which has two thermometers: a dry bulb and a wet bulb. Swinging the device lets evaporation cool the wet bulb, and the difference between the two readings can be converted to dew point using psychrometric tables. Modern electronic instruments called chilled-mirror hygrometers measure dew point directly by cooling a mirror surface until condensation forms, but these are expensive laboratory devices.
What was the highest dew point ever recorded?
The highest verified dew point on record is 35 degrees Celsius (95 degrees Fahrenheit), measured on 8 July 2003 in Dhahran, Saudi Arabia. The air temperature at the time was 42 degrees Celsius and the relative humidity was about 68%. Those conditions would be immediately life-threatening for anyone engaged in outdoor physical activity.
Why is the dew point used in aviation?
Aviation weather reports (METARs) include both temperature and dew point because the spread between them determines fog and low-cloud risk. When the spread is 3 degrees Celsius or less, pilots expect restricted visibility. A dew point close to the freezing point also warns of icing risk. Airports use dew point to calculate the pressure altitude and density altitude that affect aircraft performance.