What is the dew point?

The dew point is the temperature to which air must be cooled, at constant pressure, for water vapor to condense into liquid water (dew). A higher dew point means more moisture in the air and a muggier feel.

How accurate is this formula?

This is a simple linear approximation that works reasonably well when relative humidity is above roughly 50%. For drier air or extreme temperatures, more precise formulas such as the Magnus-Tetens equation give better results.

Why is the spread (temperature minus dew point) useful?

The smaller the spread between air temperature and dew point, the closer the air is to saturation, increasing the chance of fog, dew, or precipitation. When they are equal, relative humidity is 100%.

Dew Point Calculator

What the Dew Point Calculator Does

This dew point calculator estimates the temperature at which the air becomes saturated and water vapor starts to condense into dew, fog, or frost. You enter the current air temperature in Celsius and the relative humidity as a percentage, and it returns the dew point in degrees Celsius.

It is handy for anyone who cares about moisture and comfort: HVAC technicians sizing dehumidifiers, gardeners watching for overnight frost, painters and woodworkers checking whether a surface will sweat, brewers and homebrewers, pilots, and weather hobbyists. The dew point tells you how 'muggy' the air really feels, often more reliably than humidity alone.

How It Works: The Formula

This tool uses the widely cited fast approximation:

dew point (C) = T - (100 - RH) / 5

Here T is the air temperature in Celsius and RH is the relative humidity in percent. The logic is simple: for every 5 percentage points that humidity sits below 100%, the dew point drops roughly 1 C below the air temperature. At 100% humidity the air is fully saturated, so the dew point equals the air temperature.

This is a rule-of-thumb shortcut for the more rigorous Magnus formula. It is most accurate when relative humidity is above about 50%. Below that, the simple version increasingly overestimates the dew point, so treat low-humidity results as ballpark figures.

Worked Example With Real Numbers

Suppose it is a warm summer evening at 30 C with 70% relative humidity. Plug the values in:

dew point = 30 - (100 - 70) / 5 = 30 - (30 / 5) = 30 - 6 = 24 C.

A dew point of 24 C is high, which is why such an evening feels sticky and oppressive: sweat evaporates poorly. For contrast, at the same 30 C but only 40% humidity, the estimate gives 30 - (60 / 5) = 18 C, a far more comfortable, drier feel.

How to Read the Result

Dew point is an absolute measure of moisture, so the same number means the same comfort level regardless of the air temperature. A common comfort scale:

Below 10 C: dry and comfortable
10 to 15 C: pleasant
16 to 18 C: starting to feel humid
19 to 21 C: noticeably muggy
22 C and above: oppressive, sticky air

Tips, Factors, and Common Mistakes

Keep a few practical points in mind so your reading stays useful:

Trust it most above 50% RH; below that, the simple formula runs warm, so use a Magnus-based estimate for dry air.
Match the units: this version expects Celsius. Convert Fahrenheit first (C = (F - 32) x 5/9) or the answer will be wrong.
Dew point can never exceed the air temperature. If it does in your math, you entered RH above 100% or mixed up a value.
When the dew point falls at or below 0 C, expect frost rather than liquid dew, which matters for gardeners and drivers.
Humidity changes through the day, but the dew point stays fairly steady; a rising dew point signals incoming moisture and possible fog.

Why Dew Point Beats Humidity Alone

Relative humidity only tells you how close the air is to saturation at its current temperature, so 60% humidity at 10 C holds far less actual moisture than 60% at 30 C. Because dew point reports the real moisture content, it is the better single number for judging comfort, condensation risk, and weather changes.

Use this calculator for quick estimates and planning. For safety-critical work such as industrial coating or precise meteorology, confirm with a calibrated hygrometer or a full Magnus-formula computation.