Dew Point Calculator
This dew point calculator finds the dew point temperature from the air temperature and the relative humidity. Enter the two values and the dew point calculator returns the temperature at which the air becomes saturated and moisture starts to condense. It uses the standard dew point calculation formula, so you can compute dew point for comfort, condensation, and drying problems in seconds.
Results
The Dew Point Calculator is part of our Cooling & Heating collection. Browse more cooling & heating to size, convert, and check your work alongside this tool.
What the dew point calculator does
The dew point is the temperature at which air can hold no more moisture, and this dew point calculator finds it from the current temperature and relative humidity. Cool air to its dew point and water condenses, which is why glasses fog and pipes sweat. Enter the two readings and the tool will compute dew point directly, so you can determine dew point without reading a psychrometric chart. It applies the same dew point calculation formula meteorologists use.
The dew point calculation formula
The dew point calculation formula uses the Magnus approximation, which relates temperature and relative humidity to the dew point:
with a = 17.27 and b = 237.7 for temperatures in Celsius. The dew point calculator evaluates this to compute dew point, and because relative humidity is the driver, the same formula lets you calculate dew point from relative humidity and temperature at any condition. Higher humidity pushes the dew point closer to the air temperature.
Dew point calculation example
Air at 25°C with 60% relative humidity. The dew point calculation formula gives:
So any surface below about 16.7°C will collect condensation. Change the humidity to 80% and the dew point calculator returns about 21.3°C, much closer to the air temperature. Enter your own readings and the tool will compute dew point the same way.
How to use the dew point calculator
Enter the air temperature and the relative humidity as a percentage, then read the result. The dew point calculator applies the dew point calculation formula and returns the dew point temperature. To calculate dew point from humidity at different conditions, change either input and the dew point cal updates. This is the fastest way to find dew point temperature without a lookup table.
Reading the dew point calculator result
The dew point tells you how moist the air really feels and when condensation will form. A dew point below 10°C feels dry, 10 to 16°C is comfortable, and above 20°C feels muggy. For condensation control, any surface colder than the dew point will sweat, so compare the value to your window, wall, or pipe temperature. When you determine dew point for a building, keeping surfaces above it prevents mold and damp.
When to use the dew point calculator
- Judging comfort and mugginess by dew point rather than humidity alone
- Preventing condensation by comparing surface temperature to the dew point
- Drying and painting work, where you calculate dew point from humidity first
- HVAC sizing, where you determine dew point for latent load
Common dew point calculation mistakes
- Trusting relative humidity alone. The same RH at two temperatures gives different dew points; compute dew point to compare fairly.
- Ignoring surface temperature. Condensation depends on the coldest surface versus the dew point, not the room average.
- Mixing units. Enter temperature consistently so the dew point calculation formula returns the right result.
Tips for accurate results
- Measure temperature and humidity in the same spot before you compute dew point.
- Compare the dew point to your coldest surface, not the room air.
- Lower indoor humidity through ventilation to bring the dew point down.
Why the dew point calculation formula matters
Relative humidity alone is misleading because it changes with temperature, but the dew point does not. Two rooms can both read 50% relative humidity yet feel completely different if their temperatures differ, because the actual moisture content, which the dew point reflects, is different. This is why forecasters and HVAC engineers prefer dew point: it is an absolute measure of how much water the air holds. When you compute dew point with this calculator, you get a number you can compare directly across conditions, unlike relative humidity, which shifts as the air warms or cools through the day.
Using the dew point cal to stop condensation
The most practical use of a dew point calculator is preventing condensation and the mold that follows it. Condensation forms on any surface colder than the dew point, so a single-glazed window at 12°C in a room whose air has a 16°C dew point will run with water. The fix is either to warm the surface, with insulation or better glazing, or to lower the dew point by reducing indoor humidity through ventilation. Determine dew point for your space, measure your coldest surfaces, and keep them above that line. The same logic protects cold-water pipes, ductwork, and wall cavities from hidden damp.
Dew point chart
Dew point temperature for common conditions, a quick dew point chart from the calculator (air temperature across the top, relative humidity down the side):
| RH / Temp | 20°C | 25°C | 30°C |
|---|---|---|---|
| 40% | 6.0 | 10.5 | 14.9 |
| 60% | 12.0 | 16.7 | 21.4 |
| 80% | 16.4 | 21.3 | 26.2 |
| 100% | 20.0 | 25.0 | 30.0 |
Dew Point Calculator FAQ
How do I compute dew point?
Enter the air temperature and relative humidity. The dew point calculator applies the dew point calculation formula (the Magnus approximation) to compute dew point in one step.
How do I calculate dew point from humidity?
The dew point depends on both temperature and relative humidity, so enter both. The tool will calculate dew point from relative humidity and temperature using the standard formula.
What dew point feels comfortable?
Below 10°C feels dry, 10 to 16°C is comfortable, and above 20°C feels muggy. Use the dew point cal to see how your conditions compare.
When does condensation form?
Whenever a surface is colder than the dew point. Determine dew point with this calculator, then keep windows, walls, and pipes above it to avoid sweating and mold.