Dew Point Calculator
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Results
Dew point 62.1 °F (16.7 °C) at 77 °F and 60% relative humidity
| Relative humidity | Dew point (°F) | Feels |
|---|---|---|
| 10% | 16.2 | Dry and comfortable |
| 20% | 32.9 | Dry and comfortable |
| 30% | 43.2 | Dry and comfortable |
| 40% | 50.8 | Dry and comfortable |
| 50% | 56.9 | Becoming sticky |
| 60% | 62.1 | Becoming sticky |
| 70% | 66.5 | Oppressive |
| 80% | 70.4 | Oppressive |
| 90% | 73.8 | Oppressive |
| 100% | 77 | Oppressive |
Comfort levels are general guidance from the US National Weather Service for summer weather; how humid air feels varies from person to person.
How to use the dew point calculator
- Choose whether you want the dew point or the relative humidity.
- Enter the air temperature in °F or °C, then either the relative humidity (above 0% and up to 100%) or the dew point in the same unit.
- Read the result, the vapor pressure, the dew point depression and a general comfort label, plus a table of dew points at your temperature.
Dew point formula
γ = ln(RH ÷ 100) + 17.625 × T ÷ (T + 243.04)
Td = 243.04 × γ ÷ (17.625 − γ)
RH = 100 × exp(17.625 × Td ÷ (Td + 243.04) − 17.625 × T ÷ (T + 243.04))
The dew point (Td) is the temperature to which air must be cooled, at constant pressure and moisture content, for it to become saturated and dew or fog to form. The Magnus formula gives the saturation vapor pressure eₛ over water at temperature T (°C). Relative humidity (RH) is the actual vapor pressure divided by eₛ, so solving the formula backwards gives the dew point. The coefficients a = 17.625, b = 243.04 °C and c = 6.1094 hPa are those recommended by Alduchov and Eskridge (1996, Journal of Applied Meteorology) for temperatures from −40 to 50 °C. Fahrenheit inputs are converted to Celsius first.
Worked examples
- Dew point: at 25 °C and 60% relative humidity, the dew point is 16.7 °C. In US units, 77 °F at 60% gives 62.1 °F.
- Relative humidity: air at 30 °C with a dew point of 20 °C has a relative humidity of 55.1%.
Why dew point says more than relative humidity
Relative humidity depends on temperature, so the same amount of moisture gives very different percentages on a cold morning and a hot afternoon. As the NWS La Crosse office points out, air at 30 °F with a dew point of 30 °F has 100% relative humidity, while 80 °F air with a 60 °F dew point has only 51%, yet the warm day feels far more humid because it holds much more water vapor. The dew point measures that moisture directly.
| Dew point | How summer air tends to feel (NWS guidance) |
|---|---|
| 55 °F (13 °C) or lower | Dry and comfortable |
| 55–65 °F (13–18 °C) | Becoming "sticky", with muggy evenings |
| 65 °F (18 °C) or higher | Lots of moisture in the air, becoming oppressive |
These bands are general guidance, not a medical or engineering standard; people used to humid climates often notice humidity at higher dew points.
Dew point table (°F)
| Air temperature | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|---|---|
| 50 °F | 10 | 20 | 27 | 32 | 37 | 41 | 44 | 47 |
| 60 °F | 19 | 28 | 36 | 41 | 46 | 50 | 54 | 57 |
| 70 °F | 27 | 37 | 45 | 51 | 55 | 60 | 64 | 67 |
| 80 °F | 35 | 46 | 54 | 60 | 65 | 69 | 73 | 77 |
| 90 °F | 44 | 54 | 62 | 69 | 74 | 79 | 83 | 87 |
| 100 °F | 52 | 63 | 71 | 78 | 84 | 88 | 93 | 97 |
Dew point table (°C)
| Air temperature | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|---|---|
| 0 °C | −20.3 | −15.5 | −12 | −9.2 | −6.8 | −4.8 | −3 | −1.4 |
| 5 °C | −16.1 | −11.2 | −7.5 | −4.6 | −2.1 | 0 | 1.8 | 3.5 |
| 10 °C | −12 | −6.8 | −3 | 0 | 2.6 | 4.8 | 6.7 | 8.4 |
| 15 °C | −7.8 | −2.4 | 1.5 | 4.7 | 7.3 | 9.6 | 11.6 | 13.4 |
| 20 °C | −3.7 | 1.9 | 6 | 9.3 | 12 | 14.4 | 16.4 | 18.3 |
| 25 °C | 0.5 | 6.2 | 10.5 | 13.9 | 16.7 | 19.1 | 21.3 | 23.2 |
| 30 °C | 4.6 | 10.5 | 14.9 | 18.4 | 21.4 | 23.9 | 26.2 | 28.2 |
| 35 °C | 8.7 | 14.8 | 19.4 | 23 | 26.1 | 28.7 | 31 | 33.1 |
| 40 °C | 12.8 | 19.1 | 23.8 | 27.6 | 30.8 | 33.5 | 35.9 | 38 |
Practical uses
- Condensation: any surface colder than the dew point gets wet: cold water pipes, windows in winter, the outside of a cold drink.
- Fog and dew: when the evening temperature falls to the dew point, expect dew or fog. A small gap between temperature and dew point (the dew point depression) is an early sign.
- Painting and coatings: manufacturers commonly ask for the surface to be a few degrees above the dew point so moisture doesn't form under the coat; follow the product's data sheet.
- Below freezing the formula gives the dew point over water. Frost forms at the slightly warmer frost point.
Use the dew point in the heat index calculator to see how hot humid air feels, or check cold-weather risk with the wind chill calculator.
Frequently asked questions
How do you calculate dew point?
With the Magnus formula: γ = ln(RH/100) + 17.625T/(T + 243.04), then Td = 243.04γ/(17.625 − γ), with T in °C. At 25 °C and 60% relative humidity, the dew point is about 16.7 °C (62.1 °F at 77 °F and 60%).
How do I find relative humidity from the dew point?
RH = 100 × exp(17.625Td/(Td + 243.04) − 17.625T/(T + 243.04)), with both temperatures in °C. Air at 30 °C with a 20 °C dew point has a relative humidity of about 55.1%.
What dew point feels comfortable?
As general NWS guidance for summer, a dew point of 55 °F (13 °C) or lower feels dry and comfortable, 55–65 °F becomes sticky, and 65 °F (18 °C) or higher feels oppressive. Personal comfort varies.
Can the dew point be higher than the temperature?
No. When the air cools to its dew point it is saturated (100% relative humidity) and water condenses out, so the dew point can equal the air temperature but not exceed it. The calculator rejects dew points above the temperature.
Why is dew point a better measure of humidity than relative humidity?
Relative humidity changes with temperature even when the moisture doesn’t. 30 °F air with a 30 °F dew point is at 100% humidity, while 80 °F air with a 60 °F dew point is only about 51%, yet the warm day feels much more humid.