Dew Point Calculator | Calculate Dew Point from Temp and Humidity

Formula: alpha = ln(RH/100) + a*T/(b+T); Td = b*alpha/(a - alpha) with a = 17.625, b = 243.04 (°C)

How dew point is calculated

The dew point is the temperature to which air must cool for its water vapor to start condensing. It is computed from temperature and relative humidity with the Magnus-Tetens approximation.

Example

At 25 °C and 60% RH: alpha = ln(0.60) + 17.625 x 25 / (243.04 + 25) = -0.511 + 1.644 = 1.133. Td = 243.04 x 1.133 / (17.625 - 1.133) = 275.4 / 16.49 = 16.7 °C.

Unlike relative humidity, dew point is an absolute measure of moisture, so it is a better gauge of how muggy it actually feels. When dew point equals the air temperature, the air is saturated and dew, fog, or rain becomes likely.

What is the Dew Point Calculator?

The Dew Point Calculator is a free, browser-based everyday calculator tool that helps you Finds the dew point from air temperature and relative humidity. Enter the temperature in Celsius or Fahrenheit and the humidity percentage, and the tool applies. Instead of working through the math by hand or in a spreadsheet, you enter your values and the calculator returns an accurate result instantly — while still showing the formula and the steps so you can verify the reasoning. It is designed for quick everyday use: no sign-up, no installation, and everything runs locally in your browser for complete privacy.

How to use the Dew Point Calculator

1. Enter the required values into the input fields.
2. Press the calculate button — the result appears immediately, updated live as you change any value.
3. Read the step-by-step breakdown below the result to see exactly how the calculation was performed.
4. Use the interpretation guide to understand what the result means for your situation, and try different inputs to see how they change the outcome.

How to use the Dew Point Calculator

Enter the air temperature and the relative humidity, and the tool computes the dew point with the Magnus formula: alpha = ln(RH/100) + a·T/(b + T), then Td = b·alpha/(a − alpha), where a = 17.625 and b = 243.04 with temperature in °C. The result is the temperature at which the air becomes saturated and condensation begins. For example, 25°C air at 60% humidity has a dew point near 16.7°C.

Interpreting your result

The dew point is the most honest measure of how humid the air actually feels: a dew point under 10°C feels dry, 10–15°C comfortable, 15–20°C humid and sticky, and above 20°C oppressive — regardless of the relative humidity number, which swings with temperature. When the air cools to the dew point, fog, dew, or condensation forms — which is why the calculator is the go-to for predicting frost risk, condensation on windows and pipes, and fog. The difference between air temperature and dew point (the dew-point spread) tells you how much cooling is needed before moisture condenses.

Common mistakes to avoid

The most common error is using relative humidity as the measure of comfort — 90% humidity at 10°C feels fine, while 60% at 30°C feels sticky; the dew point resolves the confusion. Second, entering the temperature in Fahrenheit when the formula expects Celsius — the tool converts, but manual use must too. Third, forgetting that the dew point can never exceed the air temperature. Finally, ignoring the dew point when assessing frost risk — a forecast dew point near freezing with clear skies predicts frost.

Tips for best results

Use the dew point, not relative humidity, for comfort comparisons — it is stable and directly interpretable. When the temperature forecast drops toward the dew point, expect condensation or fog. For greenhouse and storage planning, keep the dew point below the surface temperature to avoid moisture damage. For running and outdoor activity, high dew points (>20°C) mean poor evaporative cooling. Check the dew-point spread when predicting overnight cooling and frost formation.

Authoritative source: timeanddate.com