What is the dew point?
The dew point is the temperature at which the water vapor in the air can no longer stay a vapor and starts to condense into liquid droplets. The amount of vapor air can hold shrinks as the air gets colder. If you cool a parcel of air without adding or removing any moisture, it eventually becomes "full" (relative humidity 100%), and the temperature at which that happens is the dew point. Once something is colder than the dew point, moisture collects on it: dew forms on grass at dawn, drops appear on a cold glass, and in the open air the result is fog. Condensation on a winter window and sweat on a cold water pipe in summer are the same effect.
How it is calculated: the Magnus formula
The tool uses a widely taught approximation of the Magnus formula with the coefficients a = 17.27 and b = 237.7 (°C).
α = a×T / (b + T) + ln(RH/100)
Dew point Td = b×α / (a − α)
T is the air temperature in °C and RH is the relative humidity in percent. The first term reflects the saturation vapor pressure curve at the air temperature, and ln(RH/100) accounts for how far the humidity falls short of 100%. At RH = 100% the logarithm is ln(1) = 0, so the dew point equals the air temperature. This approximation has a small error across everyday temperatures, which is plenty for household use, but precise meteorological observation or extreme conditions call for professional instruments and corrected formulas. Below freezing, the dew point differs from the frost point; the value here is the dew point with respect to liquid water.
Dew point table by temperature and humidity (°C)
This table is calculated with the formula above when the page is built. Columns are relative humidity; rows are air temperature.
| Temp \ RH | 30% | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|---|
| 0°C | -15.5 | -12.0 | -9.2 | -6.8 | -4.8 | -3.0 | -1.4 |
| 5°C | -11.1 | -7.5 | -4.6 | -2.1 | -0.0 | 1.8 | 3.5 |
| 10°C | -6.8 | -3.0 | 0.1 | 2.6 | 4.8 | 6.7 | 8.4 |
| 15°C | -2.4 | 1.5 | 4.7 | 7.3 | 9.6 | 11.6 | 13.4 |
| 20°C | 1.9 | 6.0 | 9.3 | 12.0 | 14.4 | 16.4 | 18.3 |
| 25°C | 6.2 | 10.4 | 13.8 | 16.7 | 19.1 | 21.3 | 23.2 |
| 30°C | 10.5 | 14.9 | 18.4 | 21.4 | 23.9 | 26.2 | 28.2 |
| 35°C | 14.8 | 19.4 | 23.0 | 26.0 | 28.7 | 31.0 | 33.1 |
At the same 60% humidity, the dew point is about 2.6°C when the air is 10°C but about 21.4°C when the air is 30°C. That is why a relative humidity number alone is a poor way to compare how muggy two situations feel.
Using the dew point to judge comfort
- Dew point of 10°C (50°F) or below: dry and comfortable
- Dew point of 10 to 16°C (50 to about 61°F): comfortable
- Dew point of 16 to 20°C (about 61 to 68°F): somewhat humid
- Dew point above 20°C (68°F): very humid and uncomfortable
These bands are a commonly used rule of thumb, not an official or legal standard, and people with different activity levels or acclimatization will feel them differently. The dew point is often preferred over relative humidity because it tracks how humid the air actually feels. To combine temperature and humidity into a single discomfort score, try the Discomfort Index Calculator.
Worked examples
Example 1. Air at 20°C and 50% humidity. α = 17.27×20/257.7 + ln(0.5) ≈ 1.3404 − 0.6931 ≈ 0.647, so Td = 237.7×0.647/(17.27 − 0.647) ≈ 9.3°C. That is a comfortable indoor condition.
Example 2. Air at 30°C and 70% humidity gives a dew point of about 23.9°C, well above 20°C, which is a sticky day. The dew point explains the discomfort better than the air temperature alone.
Example 3 (condensation). Indoors at 20°C and 60% humidity, the dew point is about 12.0°C. If cold outside air pulls the inside surface of a window below 12°C, water will collect on that surface. Enter a value in the surface temperature box and the tool makes this comparison for you.
Ways to use the dew point
- Preventing condensation and mold: if the surface of a window, an exterior wall corner or a cold pipe is colder than the room's dew point, moisture will collect there. Ventilating or dehumidifying lowers the dew point itself and removes the cause.
- Setting humidifier or dehumidifier goals: the same relative humidity target means different amounts of moisture at different temperatures. A dew point of roughly 10 to 16°C is a handy reference for a comfortable range.
- Anticipating fog and frost: when the overnight low is expected to approach the evening dew point, dew or fog becomes likely, and on clear nights where the dew point is below 0°C frost becomes more likely.
- Exercise and outdoor work: the higher the dew point, the more slowly sweat evaporates, so the same air temperature feels hotter.
Common misunderstandings and limits
- A low dew point does not mean it is cold. The dew point measures moisture content, not heat. A desert afternoon can be scorching with a low dew point.
- A room hygrometer does not show your coldest surface. It measures the air near where it sits, but condensation appears first on the coldest wall or glass. Measure that surface separately, for instance with an infrared thermometer.
- Sensor error carries through. Inexpensive hygrometers can be off by 5 percentage points or more, which can translate into a dew point error of a degree or two.
- Humidity above 100% or at or below 0% cannot be entered. Those values have no physical meaning here, so the tool shows an error.
Reading the dew point through the seasons
On a humid summer day the air temperature may be only 25 to 28°C, yet the dew point can sit above 22°C. Sweat then hardly dries and laundry takes longer to dry. On such days the dehumidify mode of an air conditioner or a dehumidifier helps, and bringing the indoor dew point down toward 16 to 18°C is one reasonable goal. In winter the opposite happens: heating can keep a room above 20°C while the dew point is only 0 to 5°C, so the air feels dry. Humidifying raises the dew point, but you should watch that the surfaces of windows and walls touching the outside do not get close to it, because over-humidifying causes condensation and then mold. In spring and autumn the dew point is often low and pleasant, though if the morning low falls close to the previous evening's dew point, fog is likely, which is useful to know before an early commute.
Frequently Asked Questions
Can the dew point be higher than the air temperature?
No. The dew point is always lower than or equal to the current air temperature, and the two are exactly equal when relative humidity reaches 100%.
How is dew point different from relative humidity?
Relative humidity is a percentage of the most vapor the air could hold at its current temperature, so the same air gives a different value as it warms or cools. The dew point expresses the actual amount of vapor as a temperature, which makes it better for comparing how humid it is across seasons and temperatures.
How can I tell in advance whether a window will get condensation?
Work out the dew point from the room's temperature and humidity, then check whether the inside surface of the window is colder than that. The surface temperature box in this tool performs exactly that comparison.
At what dew point should I run a dehumidifier?
There is no fixed rule. Many people consider dehumidifying when the indoor dew point stays above about 16 to 18°C and the room feels clammy, or whenever condensation or mold appears. Personal preference and health conditions matter.
Can I use it for temperatures below freezing?
You can enter them, and the result is the dew point with respect to liquid water. Below 0°C, vapor often turns directly into frost, so the true frost point differs slightly. Treat the number as an everyday reference.
This calculator uses an approximation formula for reference. It does not replace a building diagnosis or precise meteorological measurement.