About
Dew point is the temperature at which air becomes saturated (water condenses). Better humidity indicator than RH because it's absolute. Below 50°F: dry. 50-60: comfortable. 60-70: sticky. 70+: oppressive. 75+: dangerous heat stress.
What is Dew Point Calculator?
A Dew Point Calculator computes dew point from the inputs you provide. It applies the standard formula to the values you enter and returns the result instantly, without sending any data to a server. Free Dew Point Calculator. The tool.
Dew Point Calculator
Above 70°F: muggy. Above 75: oppressive. 65-70: comfortable.
Inputs
°F
%
Dew Point
-
Breakdown
In Celsius
-
Spread (T - DP)
-
Comfort level
-
Note
-
What dew point means
The dew point is the temperature to which air must be cooled, at constant pressure and constant water-vapour content, for the water in it to begin condensing. It is the single best number for describing how humid the air really feels, because unlike relative humidity it does not change as the temperature rises and falls. When the air temperature drops to the dew point, relative humidity hits 100 percent and you get dew on the grass, fog in the valley, or condensation on a cold drink.
Meteorologists prefer dew point over relative humidity for comfort because it is absolute. Air at 60 percent relative humidity feels bone dry on a cool 60-degree morning but oppressively sticky on a 90-degree afternoon, even though the percentage is identical, because warm air holds far more moisture. The dew point cuts through that: a 70-degree dew point is muggy whether the thermometer reads 75 or 95.
This tool takes the air temperature and relative humidity, computes the dew point with the Magnus formula, reports it in both Fahrenheit and Celsius, shows the spread between temperature and dew point, and rates the comfort level on the standard meteorological scale.
How the calculation works
The calculator uses the Magnus formula, the standard meteorological approximation, rather than the crude rule of thumb:
Quick rule (high RH only): DP = T - (100 - RH) / 5 Magnus formula (used here, T in Celsius): gamma = ln(RH / 100) + (a x T) / (b + T) DP = (b x gamma) / (a - gamma) with a = 17.625, b = 243.04 C
- T is the air (dry-bulb) temperature; the tool converts Fahrenheit input to Celsius internally.
- RH is relative humidity as a percentage from 1 to 100.
- The Magnus constants a = 17.625 and b = 243.04 give an accuracy of better than 0.4 degrees Celsius over the normal weather range, far better than the divide-by-5 shortcut.
- Spread (T minus dew point) shrinks toward zero as the air approaches saturation; a small spread means fog or rain is likely.
Worked example
The thermometer reads 80 degrees Fahrenheit and relative humidity is 60 percent. Find the dew point.
- Convert temperature: T = (80 - 32) x 5/9 = 26.7 degrees Celsius.
- Compute gamma: gamma = ln(0.60) + (17.625 x 26.7) / (243.04 + 26.7) = -0.511 + 1.744 = 1.233.
- Compute dew point in Celsius: DP = (243.04 x 1.233) / (17.625 - 1.233) = 299.7 / 16.39 = 18.3 degrees Celsius.
- Convert back: 18.3 x 9/5 + 32 = 64.9 degrees Fahrenheit.
- Spread: 80 - 64.9 = 15.1 degrees Fahrenheit.
Result: The dew point is about 65 degrees Fahrenheit (18 degrees Celsius), right at the edge of feeling sticky. The 15-degree spread means the air is not close to saturation, so no fog is expected.
Reference: dew point comfort scale
| Dew point (F) | Dew point (C) | How it feels |
|---|---|---|
| Below 50 | Below 10 | Dry, crisp, very comfortable |
| 50 - 60 | 10 - 16 | Comfortable for most people |
| 60 - 65 | 16 - 18 | Becoming noticeable, slightly humid |
| 65 - 70 | 18 - 21 | Sticky, uncomfortable |
| 70 - 75 | 21 - 24 | Oppressive, sweaty |
| Above 75 | Above 24 | Dangerous; high heat-stress risk |
Thresholds are the conventional scale used in US weather reporting. Individual tolerance varies, but a 65-degree dew point is the common dividing line between pleasant and muggy.
Common pitfalls
- Trusting the divide-by-5 shortcut at low humidity. DP = T - (100 - RH)/5 is only reasonable above about 50 percent humidity. It diverges badly in dry air; the Magnus formula this tool uses stays accurate across the range.
- Confusing dew point with relative humidity. A high relative humidity on a cold day can still mean a low, comfortable dew point. Judge mugginess by dew point, not the percentage.
- Mixing temperature units. The Magnus constants are calibrated for Celsius. Feed Fahrenheit straight in and the answer is wrong; convert first (this tool does it for you).
- Ignoring the spread for fog. When the temperature and dew point converge to within a couple of degrees, expect dew, fog, or condensation. A large spread means clear, drying conditions.
- Expecting dew point above air temperature. It is physically impossible; the maximum is equal to the air temperature at 100 percent humidity.
Related tools
Frequently asked questions
What is the dew point in simple terms?
The dew point is the temperature to which air must be cooled, at constant pressure, for the water vapour in it to start condensing into liquid. It is an absolute measure of how much moisture is in the air. The closer the dew point is to the current air temperature, the more humid it feels; when air temperature falls to the dew point, relative humidity reaches 100 percent and dew, fog, or condensation forms.
Why is dew point a better comfort indicator than relative humidity?
Relative humidity is a percentage that changes as the temperature changes, even when the actual moisture content does not, so 60 percent humidity at 60 degrees feels nothing like 60 percent at 90 degrees. Dew point is absolute: it tells you the real amount of water vapour present regardless of temperature. A dew point above 65 degrees Fahrenheit feels sticky to almost everyone, while one below 55 feels comfortably dry, no conversion needed.
What dew point feels comfortable?
As a rule of thumb in Fahrenheit: below 55 is dry and pleasant, 55 to 60 is comfortable, 60 to 65 is becoming noticeable, 65 to 70 is sticky and uncomfortable for many people, 70 to 75 is oppressive, and above 75 is dangerous because sweat evaporates poorly and heat stress rises. These thresholds are widely used by meteorologists to describe muggy conditions.
How is dew point calculated from temperature and humidity?
This calculator uses the Magnus formula. It first computes an intermediate term gamma = ln(RH/100) + (a x T)/(b + T), using constants a = 17.625 and b = 243.04 with temperature in Celsius, then the dew point equals (b x gamma)/(a - gamma). The simpler approximation, dew point is roughly air temperature minus (100 minus RH) divided by 5, is only accurate at high humidity.
Can the dew point ever be higher than the air temperature?
No. The dew point can equal the air temperature (when relative humidity is 100 percent and the air is fully saturated) but it can never exceed it. If conditions try to push the dew point above the air temperature, the excess water vapour condenses out as dew, fog, or rain, bringing the air back to saturation. So dew point is always less than or equal to the measured temperature.