How to Calculate Dew Point (Formula, Steps, and Chart)

Dew point is the temperature air must cool to for its water vapor to start condensing into liquid water. It is the single best number for judging how humid the air actually feels, which is why meteorologists trust it more than relative humidity alone. This guide gives you the exact formula, a worked example you can follow with a calculator, and a comfort chart so you can read any dew point at a glance.

What is dew point, in plain terms?

Dew point is the temperature at which air becomes fully saturated with water vapor. Cool the air to that temperature and dew, fog, or condensation forms. A higher dew point means more moisture is packed into the air.

Here is the key mental model: warm air can hold more water vapor than cold air. Relative humidity only tells you how full the air is relative to its current maximum, so 60% humidity on a cold day and 60% on a hot day feel completely different. Dew point cuts through that. It is an absolute measure of moisture, so a dew point of 22C feels sticky whether it is measured at noon or midnight. One rule always holds: the dew point can equal the air temperature (100% humidity) but it can never be higher than it.

What is the dew point formula?

The most common practical method is the Magnus formula (also called the Magnus-Tetens approximation). It estimates dew point from air temperature and relative humidity with good accuracy across everyday weather conditions.

Work it out in two steps. First compute an intermediate value called gamma:

The constants 17.625 and 243.04 come from fitting the equation to the behavior of water vapor. They are standard values used widely in meteorology, so you can treat them as fixed. If you would rather skip the arithmetic, the dew point calculator does both steps instantly.

How do you calculate dew point step by step?

Let us work a real example: air temperature of 30C with 60% relative humidity. Follow these steps and you will land on the answer.

1. Plug your numbers in: T = 30 and RH = 60. Keep temperature in Celsius and humidity as a whole percent.
2. Compute the first term of gamma: ln(60 / 100) = ln(0.60) = -0.5108.
3. Compute the second term: (17.625 * 30) / (243.04 + 30) = 528.75 / 273.04 = 1.9366.
4. Add them for gamma: -0.5108 + 1.9366 = 1.4258.
5. Apply the dew point equation: Td = (243.04 * 1.4258) / (17.625 - 1.4258) = 346.57 / 16.199 = 21.4C.
6. Read the result: a dew point of about 21.4C, which sits in the oppressive range. Sanity check that it is below the 30C air temperature, and it is.

To convert the answer to Fahrenheit, multiply by 9/5 and add 32: 21.4C becomes about 70.5F. If you want a related comfort metric on hot days, pair this with the heat index calculator, which combines temperature and humidity into a feels-like number.

What does each dew point feel like?

Dew point maps almost directly onto human comfort. Below roughly 10C the air feels dry and crisp; from about 16C to 18C it starts to feel sticky; and above 21C it feels oppressive and hard to cool off in. Use this chart as a quick reference.

This is why two cities can report the same 85% relative humidity yet feel worlds apart. The one with the higher dew point is the one that leaves you dripping. For a fuller picture of how moisture shapes comfort and figurative descriptions of weather, see our notes on imagery in literature, where sensory detail like humidity is used deliberately.

Dew point vs relative humidity: what is the difference?

Relative humidity is a percentage of the air's current capacity, so it swings up and down as temperature changes even when actual moisture is constant. Dew point is an absolute temperature, so it stays put unless real moisture is added or removed.

• Relative humidity rises overnight as the air cools, even though no water was added, which is why a foggy 5am morning can hit 100% RH with the same moisture as a comfortable afternoon.
• Dew point does not jump around like that, so forecasters use it to judge muggy days, storm potential, and overnight low temperatures.
• A neat trick: the overnight low temperature rarely drops much below the afternoon dew point, because once the air cools to the dew point, condensation releases heat and slows further cooling.

According to the US National Weather Service, dew point is a more reliable indicator of comfort than relative humidity precisely because it does not depend on the current air temperature.

Common mistakes when calculating dew point

Most errors come from mixing up units or misreading the formula. Watch for these:

• Using the wrong humidity format. Enter RH as a whole percent (60) inside RH/100, not as a decimal (0.60). If you plug in 0.60 you have effectively divided by 100 twice.
• Mixing temperature units. The Magnus constants 17.625 and 243.04 are calibrated for Celsius. Convert Fahrenheit to Celsius first, then calculate, then convert back if needed.
• Using log base 10 instead of natural log. The formula needs ln (natural logarithm), not log10. On many calculators these are different buttons.
• Getting a dew point above the air temperature. This is physically impossible. If it happens, you made an arithmetic slip or entered RH above 100.
• Forgetting the approximation limits. The Magnus formula is very accurate from about 0C to 60C. At extreme cold or in ice conditions, small errors grow, so treat far-below-freezing results as estimates.

If you enjoy working through formulas like this, you may also like our walkthrough of compound interest explained, which breaks a similar looking equation into plain steps.

Dew point is the honest measure of how humid the air really is. Learn the two step Magnus formula, keep the comfort chart handy, and you can turn any temperature and humidity reading into a number that tells you exactly how sticky the day will feel. When you want the answer in one click, let the calculator above do the math for you.