In short: dew point is the temperature air must cool to before it becomes saturated and moisture starts condensing out as dew, fog or frost. Unlike relative humidity, it does not change with temperature swings, which is why forecasters use it as the real measure of mugginess — a dew point of 13–16 °C (55–60 °F) feels comfortable, while above about 21 °C (70 °F) most people find the air oppressive no matter what the thermometer reads.
This is a planning aid based on a standard meteorological approximation. For agricultural, HVAC or safety-critical decisions, confirm with local weather station data.
How to calculate dew point
Dew point depends on two things: the air temperature and how much water vapor the air is actually holding, expressed as relative humidity. This calculator uses the Magnus formula, the standard approximation meteorologists use to convert temperature and relative humidity into dew point without needing a lookup table.
Worked example. Take air at 25 °C and 50% relative humidity — the calculator's defaults. First, α = ln(0.5) + 17.62×25/(243.12+25) = −0.6931 + 1.6429 = 0.9498. Then Td = 243.12×0.9498/(17.62−0.9498) = 230.91/16.67 ≈ 13.9 °C (56.9 °F). That single calculation — one logarithm and one division — is all the tool is doing behind the scenes.
Dew point vs. frost point. Below 0 °C, the same air can condense either as supercooled dew or as frost, and the true frost point runs a little higher than the dew point calculated here because ice has a lower saturation vapor pressure than liquid water. For sub-freezing work, treat the figure above as a close approximation rather than an exact frost point.
Why dew point beats relative humidity for comfort
Relative humidity is a ratio: it compares how much moisture the air holds to how much it could hold at the current temperature. Because warm air can hold far more moisture than cold air, the same 50% RH reading means roughly six times more actual water vapor at 30 °C than at 0 °C. That makes RH alone a poor way to judge how muggy the air will feel from one day, or one season, to the next.
Dew point sidesteps that problem. It is an absolute measure — grams of water vapor per volume of air, expressed as a temperature — so a dew point of 18 °C means the same amount of moisture whether the air temperature is 20 °C or 35 °C. That is why the U.S. National Weather Service and most meteorologists report dew point, not just relative humidity, when describing how the air will actually feel.
Dew point comfort scale
How the dew point this calculator returns is generally interpreted, based on the widely used National Weather Service comfort reference.
| Dew point | °F | How it feels |
|---|---|---|
| Below 10 °C | Below 50 °F | Dry |
| 10.0–12.7 °C | 50–54 °F | Very comfortable |
| 12.8–15.5 °C | 55–59 °F | Comfortable |
| 15.6–18.2 °C | 60–64 °F | OK, some notice the humidity |
| 18.3–20.9 °C | 65–69 °F | Humid, somewhat uncomfortable |
| 21.1–23.8 °C | 70–74 °F | Very humid, quite uncomfortable |
| 23.9 °C and above (muggiest) | 75 °F and above | Extremely uncomfortable, oppressive |
Quick reference: dew point at 50% relative humidity
How much the dew point shifts with air temperature, holding relative humidity fixed at 50%. Notice the spread between temperature and dew point stays roughly 9–12 °C across this range.
| Air temperature | Dew point |
|---|---|
| 0 °C / 32 °F | −9.2 °C / 15.4 °F |
| 10 °C / 50 °F | 0.0 °C / 32.1 °F |
| 20 °C / 68 °F | 9.3 °C / 48.7 °F |
| 25 °C / 77 °F | 13.9 °C / 56.9 °F |
| 30 °C / 86 °F | 18.4 °C / 65.2 °F |
| 35 °C / 95 °F | 23.0 °C / 73.4 °F |
| 40 °C / 104 °F | 27.6 °C / 81.7 °F |
Worked examples
A humid summer afternoon
25.8 °C
32 °C air at 70% RH gives a dew point of about 25.8 °C (78.5 °F) — squarely in the "oppressive" band, even before factoring in the heat itself.
A dry desert afternoon
6.9 °C
38 °C air at just 15% RH gives a dew point of about 6.9 °C (44.4 °F). Despite the high temperature, the air itself carries very little moisture and feels dry.
A pleasant spring day
15.1 °C
22 °C air at 65% RH gives a dew point of about 15.1 °C (59.2 °F) — right in the "comfortable" band on the scale above.
What else affects the result
Elevation and pressure. The Magnus formula assumes standard atmospheric pressure at sea level. At high altitude, where atmospheric pressure is meaningfully lower, the true saturation point shifts slightly; the effect is small below about 1,500 m (5,000 ft) but grows at extreme elevations.
Instrument accuracy. Consumer humidity sensors are commonly accurate to only ±3–5% RH, which can shift a calculated dew point by roughly 1–2 °C. For anything safety-critical, use a calibrated hygrometer or a nearby weather station reading.
Sub-zero air. Below freezing, condensation can form as frost rather than liquid dew. The true frost point runs slightly above the dew point value calculated here — see the callout above.
Frequently asked questions
What is dew point, in plain terms?
Dew point is the temperature air would need to cool to for it to become fully saturated with the moisture it already holds. At that temperature, water starts condensing out of the air as dew, fog or frost. A higher dew point means the air is carrying more actual moisture.
How is dew point different from relative humidity?
Relative humidity is a percentage relative to what the current temperature can hold, so 50% RH means something very different on a cold day than on a hot day. Dew point is an absolute measure of moisture in the air and does not change just because the air warms up or cools down, which is why meteorologists treat it as the better comfort indicator.
What dew point feels comfortable?
Most people find a dew point between about 13 and 16 degrees Celsius (55 to 60 degrees Fahrenheit) comfortable. Below 10 degrees Celsius (50 degrees Fahrenheit) air feels dry. Above about 21 to 24 degrees Celsius (70 to 75 degrees Fahrenheit) it starts to feel muggy or oppressive, regardless of the air temperature.
Can the dew point be higher than the air temperature?
No. The dew point can equal the air temperature, which happens at 100% relative humidity, but it can never exceed it. If a reported dew point is higher than the air temperature, one of the two readings is in error.
How accurate is this dew point calculator?
This calculator uses the Magnus formula with the Alduchov–Eskridge coefficients, which is accurate to within about 0.1 degrees Celsius across the typical atmospheric range of roughly -40 to 50 degrees Celsius. Outside that range the approximation becomes less reliable.
Sources & references
- Alduchov, O. A., & Eskridge, R. E. (1996). Improved Magnus form approximation of saturation vapor pressure. Journal of Applied Meteorology, 35(4), 601–609.
- World Meteorological Organization (WMO). Guide to Instruments and Methods of Observation.
- National Weather Service (NOAA). Dew point and human comfort reference.
John Mac, PE
Water Systems Specialist · Austin, Texas
Texas-licensed Professional Engineer with 13+ years in residential water systems and 180+ completed projects. Writes and reviews the calculators on watercalculator.us. Read full bio
Texas PE (2016) · BS Civil Engineering, Hydraulics (2013)