Guide
How to Calculate Dew Point
You don’t need a weather station or an app to figure out the dew point. If you know the temperature and relative humidity, you can calculate it yourself in under a minute. Below are two methods: a simplified formula you can do in your head, and the full equation the pros use.
Quick answer
Td ≈ T − ((100 − RH) ÷ 5). For precision, use the Magnus equation — or skip the math with the dew point calculator.The Simplified Formula (Mental Math)
For most everyday situations — humidity above 50%, temperature between 50°F and 90°F — this shortcut is accurate to within about 1°F:
Dew Point (°F) ≈ Temperature (°F) − ((100 − Relative Humidity) ÷ 5)In plain English: take 100 minus the relative humidity, divide by 5, then subtract that number from the temperature.
Example (Simplified)
It’s 80°F outside, and the humidity is 65%.
- 100 − 65 = 35
- 35 ÷ 5 = 7
- 80 − 7 = 73°F dew point
Result: Dew point ≈ 73°F. That’s firmly in the “oppressive” zone — the air feels heavy and sticky. (Check the dew point comfort chart to see where this number lands, or read what is a comfortable dew point for the short comfort range.)
Rule of thumb: every 5% drop in humidity lowers the dew point by about 1°F. At 80°F and 40% humidity, the math gives you 80 − 12 = 68°F — still uncomfortable, but noticeably different from 73°F.
The Full Formula (Magnus Equation)
The simplified formula works well enough for daily use, but when humidity drops below 40% or you need serious precision, the Magnus equation is the standard:
γ = ln(RH ÷ 100) + (b × T) ÷ (c + T)
Td = (c × γ) ÷ (b − γ)Where:
- T = air temperature in °C
- RH = relative humidity (%)
- b = 17.625
- c = 243.04 °C
- Td = dew point temperature in °C
Why This Formula Exists
The relationship between temperature, humidity, and dew point isn’t linear — the simplified version cuts corners by assuming it is. The Magnus equation accounts for how water vapor behaves differently at different temperatures. It’s the same formula built into every dew point calculator (including ours) and weather model.
Full Example (Magnus Equation)
Same conditions as before: 80°F (26.7°C) and 65% humidity.
- Convert RH to a decimal: 65 ÷ 100 = 0.65
- Calculate the first term: ln(0.65) = −0.4308
- Calculate the second term: (17.625 × 26.7) ÷ (243.04 + 26.7) = 470.59 ÷ 269.74 = 1.7445
- γ = −0.4308 + 1.7445 = 1.3137
- Td = (243.04 × 1.3137) ÷ (17.625 − 1.3137) = 319.26 ÷ 16.3113 = 19.57°C
- Convert back to Fahrenheit: 19.57 × 9/5 + 32 = 67.2°F
Full formula gives 67.2°F. Simplified gave 73°F. The 5.8°F gap is typical at moderate humidity — the simplified version overestimates slightly. For humidity above 60%, both methods agree closely enough for comfort decisions.
When to Use Each Method
| Situation | Use | Accuracy |
|---|---|---|
| Checking if it'll feel sticky outside | Simplified | ±2°F |
| RH below 40% or temp above 95°F | Full formula | ±0.5°F |
| HVAC / engineering / lab work | Full formula | ±0.5°F |
| Don't want to do math at all | Dew point calculator → | Exact |
Frequently Asked Questions
Can I calculate dew point from temperature alone?
No. Dew point requires both temperature and relative humidity. A single temperature reading tells you nothing about how much moisture is in the air. That's why two 80°F days can feel completely different — one at 40% humidity (dew point ≈ 56°F, comfortable) and another at 75% (dew point ≈ 71°F, oppressive).
Why does the simplified formula work?
The simplified formula (T − ((100 − RH) / 5)) is a linear approximation of the Magnus equation over the range where most people live: 50–90°F and 40–100% humidity. Outside that range — very dry or very hot conditions — it drifts and the full equation is needed.
What's the dew point if humidity is 100%?
When relative humidity is 100%, the dew point equals the current temperature. The air is fully saturated — it can't hold any more water vapor, and condensation (or fog) forms. In the simplified formula: 100 − 100 = 0, so T − 0 = T.
Is there an app for this?
This page is the app. Bookmark it. But if you want dew point on your phone’s home screen, check out TraceWeather — our weather app that puts dew point front and center alongside temperature and humidity.
How do I calculate dew point in Fahrenheit?
You're already there—the simplified formula above is built for Fahrenheit, which is why US readers can skip any unit conversion. The result comes out in °F directly. If you're working in Celsius, use the Magnus equation instead.
How do I calculate dew point without relative humidity?
You can't get there from temperature alone—dew point needs a moisture reading. But if you have a wet-bulb thermometer instead of a hygrometer, you can still do it: measure both the wet-bulb and dry-bulb temperatures and read the dew point off a psychrometric chart. The smaller the gap between the two readings, the closer the dew point sits to the air temperature.
How do I calculate dew point with a hygrometer?
A hygrometer gives you the relative humidity—that's the missing input. Read the humidity and the air temperature, then run either formula above. Let an analog hygrometer sit in the room for a few minutes to stabilize before you take the reading.
Can I calculate dew point in Excel?
Yes. Put the temperature in A2 and the relative humidity in B2, then use the simplified formula:=A2-((100-B2)/5)
Prefer a chart to a formula? See the dew point temperature chart →
← Back to the dew point chart | What is a comfortable dew point? →