A dew point calculator finds the temperature at which air becomes saturated and dew forms, using the air temperature and relative humidity. Formula: α = (a·T)/(b+T) + ln(RH/100); Td = (b·α)/(a-α) with a=17.27, b=237.7. At 25 °C (77 °F) and 60% humidity, the dew point is about 16.7 °C (62 °F).
On this page
How to use the dew point calculator
- Enter the air temperature in °C.
- Enter the relative humidity in percent.
- Read the dew point in °C.
What is the dew point formula?
The dew point is the temperature air must cool to, at constant pressure and moisture, to become saturated. Below it, water vapor condenses into dew, fog or cloud. The dew point calculator uses the Magnus approximation with constants a = 17.27 and b = 237.7 °C:
- α = a × T ÷ (b + T) + ln(RH ÷ 100)
- Dew point = b × α ÷ (a − α)
T is the air temperature in °C and RH is relative humidity in percent. For ordinary weather temperatures, the dew point calculator result is accurate to within a few tenths of a degree.
Dew point calculator worked example
At 25 °C (77 °F) and 60% relative humidity: α = 17.27 × 25 ÷ 262.7 + ln(0.60) = 1.6435 − 0.5108 = 1.1327. Dew point = 237.7 × 1.1327 ÷ (17.27 − 1.1327) = 16.7 °C, or about 62 °F. If the air cooled overnight to 16.7 °C, dew would start to form.
| Air temperature | Relative humidity | Dew point |
|---|---|---|
| 20 °C (68 °F) | 40% | 6.0 °C (43 °F) |
| 15 °C (59 °F) | 90% | 13.4 °C (56 °F) |
| 20 °C (68 °F) | 80% | 16.4 °C (62 °F) |
| 25 °C (77 °F) | 60% | 16.7 °C (62 °F) |
| 30 °C (86 °F) | 50% | 18.4 °C (65 °F) |
| 35 °C (95 °F) | 40% | 19.4 °C (67 °F) |
| 30 °C (86 °F) | 70% | 23.9 °C (75 °F) |
Dew point and comfort
The National Weather Service explains that dew point is a better measure of how humid it feels than relative humidity, because relative humidity changes with temperature while dew point reflects the actual amount of moisture in the air. Its guide for summer:
- 55 °F or lower: dry and comfortable
- 55 °F to 65 °F: becoming sticky, with muggy evenings
- 65 °F or higher: lots of moisture, oppressive
In the table, 30 °C at 50% and 30 °C at 70% have the same temperature, but the second has a dew point of 75 °F and feels far more uncomfortable, because sweat evaporates slowly.
Dew point vs relative humidity
Relative humidity is the amount of water vapor in the air compared with the most it could hold at that temperature. Cold air holds less, so the same air has a higher relative humidity on a cool morning than on a hot afternoon, even though the dew point barely changes. That is why a 90% humidity reading on a cool day can feel fine, while 50% on a hot day can feel sticky. When the temperature falls to the dew point, relative humidity reaches 100%.
Practical uses
- Forecasting: when the evening temperature-dew point spread is small, fog or dew is likely by morning. Overnight lows rarely fall far below the dew point.
- Frost: if the dew point is below freezing, moisture deposits as frost instead of dew.
- Homes: condensation forms on windows, pipes and walls colder than the indoor dew point, which can lead to mold. Lowering indoor humidity lowers the dew point.
- Painting and coatings: manufacturers often say not to apply coatings unless the surface is a few degrees above the dew point.
Enter Celsius into the dew point calculator; convert Fahrenheit with °C = (°F − 32) × 5 ÷ 9 first, and convert the result back with °F = °C × 9 ÷ 5 + 32.
Indoor dew point
Indoors, a relative humidity of about 30% to 50% is a common target. At 21 °C (70 °F), 50% humidity gives a dew point of roughly 10 °C (50 °F), so any surface colder than that, such as a single-pane window in winter or a cold water pipe in summer, will collect condensation. Use the dew point calculator with your indoor temperature and humidity readings to see how cold a surface can get before it sweats. A dehumidifier, exhaust fans and better insulation all help keep surfaces above the dew point.
Pilots and weather watchers use the same idea outdoors: when the temperature and dew point are within a couple of degrees, fog and low cloud are likely. A quick check with the dew point calculator in the evening helps you plan for dew on the car or grass the next morning.
Related converters and calculators
Related calculators:
- Heat index calculator: heat
- Temperature converter: °F/°C
- Unit converter: units
- Scientific calculator: math
Sources
National Weather Service: Dew point vs humidity.
Questions about this calculator
What is the dew point?
The temperature at which air becomes saturated and condensation forms.
What dew point feels comfortable?
55 °F or lower, according to the National Weather Service.
What is the dew point at 25 °C and 60%?
About 16.7 °C (62 °F).
Can the dew point be higher than the temperature?
No, at most it equals the temperature at 100% humidity.
What formula is used?
The Magnus approximation with a = 17.27 and b = 237.7 °C.
Check the assumptions and units before relying on a result. Financial, tax and health estimates do not replace advice for your individual circumstances.
Report a calculation issue