What this calculator does
Find the dew point from the temperature and humidity, or the humidity from the dew point. The dew point is how cold the air must get before its water vapor starts turning into droplets (condensing). That assumes nothing else about the air changes, such as its moisture or pressure. So it's a direct measure of how much water the air is carrying. Relative humidity is different. It compares that moisture with the most the air could hold at its current temperature. That's why it rises and falls through the day, even when the amount of moisture stays the same.
This calculator works both ways. Give it the air temperature and relative humidity, and it finds the dew point. Give it the air temperature and dew point, and it finds the relative humidity. Either way, it also shows the vapor pressure and the absolute humidity, in grams of water per cubic meter. And it shows how close the air is to saturation, the point where it can't hold any more water. The results are for general information and planning. They aren't a forecast or a safety guarantee.
Where it's used
Why this math matters outside the classroom:
- Fog forecasts. Forecasters expect fog when the air cools to its dew point overnight, often after clear, calm evenings.
- Muggy days. Many people start to feel muggy once the dew point passes about 18 °C, even when the temperature is moderate.
- Painting outdoors. Painters avoid coating surfaces colder than a few degrees above the dew point, where moisture can condense on them.
- Building walls. Building designers check where wall temperatures fall below the dew point to prevent condensation and mold.
- Worked example. Air at 25 °C and 60% relative humidity has a dew point of about 16.7 °C.
How to use it
- Choose whether you want the dew point or the relative humidity.
- Enter the air temperature in °C or °F, read in the shade.
- Enter the relative humidity as a percentage, or the dew point in either scale.
- Read the answer, the dew point spread and the absolute humidity. Open the working to see each stage of the Magnus calculation.
What the inputs mean
- Air temperature
- The ordinary air temperature, called the dry-bulb temperature. The Magnus numbers used here are reliable from about −45 °C to 60 °C (−49 °F to 140 °F). Readings outside that range are turned away rather than guessed at.
- Relative humidity
- Relative humidity as shown by a hygrometer (humidity meter) or weather station, from just above 0% up to 100%.
- Dew point
- The dew point can never be higher than the air temperature. When the two are equal, the air is saturated and the relative humidity is 100%.
Formula and method
es(T) = 6.1094 × exp(17.625T ÷ (243.04 + T)) (hPa, T in °C) γ = ln(RH ÷ 100) + 17.625T ÷ (243.04 + T) Td = 243.04γ ÷ (17.625 − γ) RH = 100 × es(Td) ÷ es(T) absolute humidity = e × 18.01528 ÷ (8.314462618 × T(K)) (g/m³, e in Pa)
Here's what the symbols mean. T is the air temperature and Td is the dew point. RH is the relative humidity. es is the saturation vapor pressure: the push water vapor makes when the air holds all it can. e is the vapor pressure the air actually has. exp is the exponential function (the eˣ key on a calculator), and ln is the natural log, its reverse. γ (gamma) is a helper number, and T(K) is the temperature in kelvins (°C + 273.15). hPa and Pa are units of pressure.
The Magnus formula is a curve fitted to measured data for that saturation pressure. This calculator uses the numbers Alduchov and Eskridge published in 1996 (17.625 and 243.04 °C). They match steam-table values to within a few tenths of a percent across ordinary weather temperatures.
The air's actual vapor pressure equals the saturation pressure at its dew point. So relative humidity is es(Td) ÷ es(T). Turning that around gives the dew point from the humidity. γ combines the humidity and temperature, and the last step runs the Magnus curve backward.
Absolute humidity treats water vapor as an ideal gas (a simple, well-behaved gas model). The vapor's pressure is multiplied by the weight of one mole of water (18.01528 g). It's then divided by the gas constant (8.314462618) times the temperature in kelvins. That gives the grams of vapor in each cubic meter of air.
Worked example
A room is at 25 °C with 60% relative humidity. What is the dew point?
- γ = ln(0.6) + 17.625 × 25 ÷ (243.04 + 25) = −0.510826 + 1.643878 = 1.133052.
- Td = 243.04 × 1.133052 ÷ (17.625 − 1.133052) = 275.377 ÷ 16.491948 = 16.6977 °C.
- Vapor pressure at the dew point: 18.97 hPa, out of a possible 31.62 hPa at 25 °C.
- Absolute humidity: 1,897.04 Pa × 18.01528 ÷ (8.314462618 × 298.15) = 13.79 g/m³.
Result: The dew point is 16.7 °C (62.1 °F), 8.3 °C below the air temperature. Any surface cooler than 16.7 °C, such as a cold-water pipe, will collect condensation.
Understanding the result
The dew point tells you how humid it feels better than relative humidity does. Air at 10 °C and 90% humidity holds far less water than air at 30 °C and 50%. The dew points (about 8.4 °C versus 18.4 °C) show that right away.
The dew point spread is the gap between the air temperature and the dew point. It shows how close the air is to saturation. A spread of only a degree or two often brings fog, mist or dew. It's also why a cold drink 'sweats' on a humid day.
The general-feel label uses broad, widely used bands. Dew points under about 10 °C (50 °F) feel dry, and the mid-teens feel humid. Above about 21 °C (70 °F), most people find the air oppressive. People differ, so treat the label as a description, not a rule.
Assumptions and limitations
- Saturation is measured over liquid water, even below freezing. Over ice the saturation pressure is lower, so the true frost point is slightly higher than the dew point shown.
- The air is at ordinary atmospheric pressure, and water vapor behaves as an ideal gas.
- The temperature and humidity readings are accurate, and taken at the same place and time.
- The Magnus formula is an approximation. Its errors grow outside about −40 °C to 50 °C, so the calculator stops at −45 °C and 60 °C.
- Home humidity meters are often only accurate to within a few percent. That can shift the dew point by roughly half a degree or more.
- This is an estimate for information only. It isn't a weather forecast or a mold-risk assessment, and it can't promise whether condensation will form.
Frequently asked questions
What is the difference between dew point and relative humidity?
Dew point measures how much moisture the air really contains. Relative humidity measures that moisture as a share of the most possible at the current temperature. Warm the air without adding water, and the relative humidity falls. The dew point stays the same.
Can the dew point be higher than the temperature?
Not in stable air. Once air cools to its dew point, it's saturated. Any more cooling makes water condense out. That keeps the dew point at or below the air temperature.
What is the frost point?
Below freezing, vapor can turn straight into ice. The temperature where that starts is the frost point. Ice has a lower saturation pressure than supercooled water (water still liquid below freezing). So the frost point sits a little above the dew point calculated here.
Why does my window fog up in winter?
The inside of the glass is colder than the room. When the glass gets colder than the indoor dew point, moisture condenses on it. Lowering indoor humidity lowers the dew point, which cuts down the fogging.
How this calculator is checked
Method: Magnus formula for saturation vapor pressure over water with the Alduchov and Eskridge (1996) coefficients a = 17.625, b = 243.04 °C, c = 6.1094 hPa; absolute humidity from the ideal gas law with M = 18.01528 g/mol and R = 8.314462618 J/(mol·K). The calculation is covered by automated tests with known reference values and edge cases. Last updated 2026-10-18. Found a problem? Tell us.
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