Dew Point Calculator
Enter the air temperature and relative humidity to find the dew point — the temperature at which moisture starts to condense. Results include dew point in °C, °F and K, vapour pressures and a condensation risk assessment.
Find the Temperature at Which Moisture Starts to Condense
Dew Point Formula
Where:
- \(T\) — air temperature (°C)
- \(RH\) — relative humidity (%)
- \(T_d\) — dew point temperature (°C)
- \(a = 17.625\) and \(b = 243.04\,\text{°C}\) — constants of the Magnus-Tetens approximation (Alduchov & Eskridge, 1996)
This is the Magnus-Tetens formula: it finds the temperature at which the air becomes saturated, using the fact that relative humidity equals the ratio of the actual vapour pressure to the saturation vapour pressure. The related saturation vapour pressure used in the results is \(e_s = 6.112 \, \exp\!\left(\frac{aT}{b+T}\right)\) hPa.
Dew Point – Calculation Example
Example: the air in a room is 25 °C with a relative humidity of 60 %. At what temperature does moisture start to condense?
\[ \gamma = \ln\!\left(\frac{60}{100}\right) + \frac{17.625 \times 25}{243.04 + 25} = -0.5108 + 1.6438 = 1.1330 \]
\[ T_d = \frac{243.04 \times 1.1330}{17.625 - 1.1330} = \frac{275.37}{16.492} \approx 16.70 \,\text{°C} \]
Result: the dew point is 16.70 °C (62.06 °F, 289.85 K). Any surface — a window, a wall, a cold drink — cooled below this temperature will collect condensation. The dew point spread is 25 − 16.70 = 8.3 °C, which indicates fairly dry, comfortable air.
How to Use the Calculator
- Enter the Air temperature (T) of the surrounding air.
- Pick the matching Unit — °C, °F or K. The result is converted back into the same unit.
- Enter the Relative humidity (RH) as a percentage between 0 and 100. Many weather apps, hygrometers and HVAC reports show this value directly.
- Read the results: the Dew point temperature (Td) in all three temperature units, plus the vapour pressures, the dew point spread and a short condensation risk assessment.
What Is the Dew Point?
The dew point is the temperature to which air must be cooled, at constant pressure, for the water vapour in it to become saturated and begin to condense into liquid water — dew, fog, or droplets on a cold surface. Unlike relative humidity, which changes as the temperature changes, the dew point is a direct measure of how much moisture the air actually holds. Two rooms with very different temperatures but the same dew point contain exactly the same amount of water vapour.
This makes the dew point the most useful single number for judging how muggy or dry the air feels. A dew point below 10 °C feels dry, 10–16 °C feels comfortable, 16–21 °C noticeably humid, and above 21 °C most people describe the air as oppressive. Weather forecasts increasingly quote the dew point rather than relative humidity precisely because it maps so cleanly onto perceived comfort.
In practice, the dew point tells you the condensation threshold: moisture will form on any surface whose temperature falls to or below \(T_d\). That is why a cold glass of water "sweats" in summer, why bathroom mirrors fog up during a shower, and why uninsulated walls and single-glazed windows collect water in winter. The dew point spread (\(T - T_d\)) shown in the results summarises the risk: a spread under 2 °C means the air is nearly saturated and even slight cooling produces condensation, while a spread above 5 °C means fairly dry conditions with little condensation risk.
The calculator also reports the saturation vapour pressure at the air temperature, the actual vapour pressure in the air, and the saturation pressure at the dew point. At the dew point the last two are equal — that is the very definition of saturation, and it is a good consistency check on the result.
Accuracy, Assumptions and Practical Tips
- Valid temperature range. The Magnus formula with \(a = 17.625\), \(b = 243.04\) °C is accurate to within about 0.35 °C for air temperatures from −40 °C to +50 °C at normal atmospheric pressure. Outside this range the result is only an approximation, and the calculator will flag it.
- One atmosphere assumed. The formula assumes air near standard sea-level pressure. For high altitudes or pressurised spaces the true dew point shifts slightly, but for most everyday purposes the difference is negligible.
- Relative humidity must be above 0 %. At exactly 0 % humidity the dew point is undefined (the air contains no water vapour); the calculator requires a value between 0 (exclusive) and 100 %.
- Frost point below 0 °C. When the dew point is below freezing, water vapour in practice deposits directly as frost rather than liquid dew. The true frost point is slightly higher than the computed dew point, and the calculator notes this whenever it applies.
- Measuring humidity. Cheap hygrometers can drift by several percentage points; for a reliable dew point, check the sensor against a known reference (for example, saturated salt solutions) and recalibrate if needed.
- Condensation inside buildings. To prevent mould and moisture damage, keep indoor surfaces warmer than the dew point — typically by ventilating, insulating cold bridges (window frames, corners, external walls), and not letting indoor relative humidity climb much above 60 % in cold weather.
Frequently Asked Questions
How do I calculate the dew point by hand without a calculator?
Use the Magnus-Tetens formula. First compute \(\gamma = \ln(RH/100) + 17.625T/(243.04 + T)\) with \(T\) in °C, then \(T_d = 243.04\,\gamma/(17.625 - \gamma)\). You need a scientific calculator or a log table for the natural logarithm. For example, at 20 °C and 50 % RH: \(\gamma = -0.693 + 1.335 = 0.642\), giving \(T_d \approx 10.0\) °C.
What is a comfortable dew point temperature?
A dew point below 10 °C (50 °F) feels dry, 10–16 °C (50–60 °F) is comfortable, 16–21 °C (60–70 °F) feels noticeably humid, and above 21 °C (70 °F) most people find the air muggy or oppressive. Unlike relative humidity, the dew point does not change when the temperature changes during the day, so it is a more reliable comfort indicator.
Can I find the dew point from temperature in Fahrenheit?
Yes — convert first. Change Fahrenheit to Celsius with \(T_{°C} = (T_{°F} - 32) \times 5/9\), apply the Magnus formula, then convert the result back with \(T_{°F} = T_{°C} \times 9/5 + 32\). Or simply enter the Fahrenheit value in the calculator above and select °F as the unit — both conversions are done for you.
Why is the dew point always lower than the air temperature?
Because relative humidity below 100 % means the air is not yet saturated. Cooling the air lowers its capacity to hold water vapour, and the dew point is the temperature at which that shrinking capacity finally equals the amount of vapour present. The only case where dew point equals air temperature is exactly 100 % relative humidity — air at the saturation point.
What is the difference between dew point and relative humidity?
Relative humidity says how close the air is to saturation as a percentage, but that percentage depends on temperature: warm air holds more vapour. The dew point is the actual amount of moisture expressed as a temperature. Two rooms at 15 °C and 30 °C with 50 % RH hold very different amounts of water, yet rooms with the same dew point hold exactly the same amount.
Is dew point the same as wet-bulb temperature?
No, though all three are related. The dew point is the temperature of saturation by cooling at constant pressure with no evaporation. The wet-bulb temperature is what a thermometer reads when evaporation cools it, and it always lies between the dew point and the air temperature. The three values converge only at 100 % relative humidity.
What dew point should I keep indoors to avoid condensation and mould?
Keep indoor surfaces warmer than the dew point, which usually means holding relative humidity between about 30 % and 60 % in cold weather. In winter, single-glazed windows and uninsulated wall corners can fall below the indoor dew point and collect water. Ventilate kitchens and bathrooms, insulate cold bridges, and if mould persists despite moderate humidity, consult a building professional.