💧 Dew Point Calculator (Temperature and Humidity)
By Shihab Mia · Updated 2026-07-02
This dew point calculator finds the temperature at which the air becomes saturated and water vapor starts to condense. Enter the air temperature and the relative humidity, and it returns the dew point in both Celsius and Fahrenheit using the Magnus formula. For the default example, 25 C air at 60% relative humidity has a dew point of about 16.7 C (62.1 F), which sits in the "sticky" comfort range.
What is the Dew Point Calculator?
The dew point is the temperature to which air must be cooled, at constant pressure and moisture, for it to reach 100% relative humidity. Below that temperature the water vapor can no longer stay as gas, so it condenses into dew, fog, or droplets on a cold glass. Because the dew point depends only on how much water is actually in the air, it is a far more honest measure of humidity than relative humidity alone. Two days can both read 60% relative humidity yet feel completely different if their temperatures differ, and this dew point calculator makes that difference obvious.
Under the hood the tool uses the Magnus formula, the standard approximation meteorologists rely on for everyday temperatures. It takes the air temperature in Celsius and the relative humidity, computes an intermediate term called gamma, and then solves for the dew point. The constants a = 17.625 and b = 243.04 are the widely used Alduchov and Eskridge coefficients, which keep the error under about 0.1 C across the range you meet in normal weather. If you type the temperature in Fahrenheit, the tool converts it to Celsius first with Tc = (F - 32) x 5 / 9, then converts the answer back so you see both scales.
Relative humidity and dew point move together but are not the same thing. Relative humidity is a percentage that compares the current vapor to the maximum the air could hold at that temperature, so it rises as the air cools even when no moisture is added. The dew point, by contrast, is an absolute temperature that barely changes as the day warms and cools, unless moisture is actually added or removed. That is why forecasters quote the dew point when they want to describe how muggy it will feel: a dew point above 21 C (70 F) feels oppressive to most people, while a dew point below 10 C (50 F) feels crisp and dry regardless of the thermometer reading.
Knowing the dew point has real practical value. It tells you when condensation will form on windows, pipes, walls, and camera lenses, which matters for preventing mold, rust, and fogging. It sets the base of cumulus clouds, since rising air cools until it hits the dew point and clouds begin. It guides comfort and health decisions, from setting a dehumidifier target to judging heat stress during exercise, because a high dew point limits how well sweat can evaporate. This calculator turns a temperature and a humidity reading into that single, actionable number in one step.
One useful rule of thumb: the closer the dew point is to the current air temperature, the higher the relative humidity, and when they are equal the air is fully saturated at 100%. If the air temperature is 25 C and the dew point comes out at 16.7 C, the gap of roughly 8 degrees corresponds to the 60% relative humidity you entered. Watching that gap shrink through the evening is a good early warning that fog or dew is on the way.
When to use it
- Checking whether condensation will form on cold windows, pipes, or walls so you can prevent mold and rust.
- Judging how humid and muggy the day will actually feel, since dew point predicts comfort better than relative humidity.
- Setting a target for a dehumidifier or HVAC system to keep an indoor space comfortable and dry.
- Estimating the cloud base for aviation, gliding, or weather study, since clouds form where rising air cools to the dew point.
- Deciding when photography or optics gear will fog up when moved between warm and cold environments.
- Monitoring heat stress during sports and outdoor work, where a high dew point makes it harder for sweat to cool the body.
How to use the Dew Point Calculator
- Enter the current air temperature in the temperature box.
- Choose Celsius or Fahrenheit from the unit selector (the default is Celsius).
- Enter the relative humidity as a percentage between 1 and 100.
- Read the dew point instantly in both Celsius and Fahrenheit, with a comfort label.
- Adjust either input to see the dew point update live as conditions change.
- Use the Copy summary button to save or share the full result.
Formula & method
Worked examples
A warm room at 25 C with 60% relative humidity (the default example).
- Temperature is already in Celsius, so Tc = 25.
- gamma = ln(60 / 100) + (17.625 x 25) / (243.04 + 25) = ln(0.6) + 440.625 / 268.04
- gamma = -0.5108 + 1.6439 = 1.1331
- Td = (243.04 x 1.1331) / (17.625 - 1.1331) = 275.39 / 16.4919
- Td = 16.7 C, then 16.7 x 9 / 5 + 32 = 62.1 F
Result: Dew point is about 16.7 C (62.1 F), a "sticky" feel.
A hot, humid summer day at 30 C with 80% relative humidity.
- Temperature is in Celsius, so Tc = 30.
- gamma = ln(0.80) + (17.625 x 30) / (243.04 + 30) = -0.2231 + 528.75 / 273.04
- gamma = -0.2231 + 1.9366 = 1.7135
- Td = (243.04 x 1.7135) / (17.625 - 1.7135) = 416.45 / 15.9115
- Td = 26.2 C, then 26.2 x 9 / 5 + 32 = 79.1 F
Result: Dew point is about 26.2 C (79.1 F), an "oppressive" feel.
A cool spring evening entered in Fahrenheit: 68 F with 50% relative humidity.
- Convert to Celsius: Tc = (68 - 32) x 5 / 9 = 20.
- gamma = ln(0.50) + (17.625 x 20) / (243.04 + 20) = -0.6931 + 352.5 / 263.04
- gamma = -0.6931 + 1.3401 = 0.6470
- Td = (243.04 x 0.6470) / (17.625 - 0.6470) = 157.25 / 16.978
- Td = 9.3 C, then 9.3 x 9 / 5 + 32 = 48.7 F
Result: Dew point is about 9.3 C (48.7 F), a "dry" and comfortable feel.
Dew point (C) for common temperature and relative humidity combinations
| Air temp | 40% RH | 60% RH | 80% RH |
|---|---|---|---|
| 20 C | 6.0 | 12.0 | 16.4 |
| 25 C | 10.5 | 16.7 | 21.3 |
| 30 C | 14.9 | 21.4 | 26.2 |
Dew point comfort scale (how the air feels)
| Dew point | Label | How it feels |
|---|---|---|
| Below 10 C (50 F) | Dry | Crisp and comfortable, low humidity |
| 10 to 16 C (50 to 61 F) | Comfortable | Pleasant for most people |
| 16 to 18 C (61 to 64 F) | Sticky | Getting humid, noticeable moisture |
| 18 to 21 C (64 to 70 F) | Uncomfortable | Muggy, sweat evaporates slowly |
| Above 21 C (70 F) | Oppressive | Heavy, oppressive, hard to cool down |
Common mistakes to avoid
- Confusing dew point with relative humidity. Relative humidity is a percentage tied to the current temperature, while the dew point is an absolute temperature. A 60% reading feels very different at 15 C than at 35 C, but the dew point captures the true moisture directly.
- Forgetting to convert Fahrenheit before applying the formula. The Magnus formula uses Celsius. If your thermometer reads Fahrenheit, convert with Tc = (F - 32) x 5 / 9 first. This calculator does that automatically when you pick the Fahrenheit unit.
- Entering humidity above 100 or at 0 percent. Relative humidity is only physically meaningful from just above 0 up to 100 percent. At 0% the formula has no answer, so this tool asks for a value between 1 and 100.
- Expecting the dew point to exceed the air temperature. The dew point can equal the air temperature at 100% humidity but never rises above it. If you ever compute a higher dew point, check that the temperature and humidity inputs were not swapped.
- Ignoring pressure for extreme altitudes. The Magnus formula assumes near sea level pressure. It stays accurate for everyday weather, but at very high altitude or in specialized industrial settings a pressure corrected method is more precise.
Glossary
- Dew point
- The temperature at which air becomes saturated and water vapor begins to condense into dew, fog, or droplets.
- Relative humidity
- The amount of water vapor in the air as a percentage of the maximum the air can hold at its current temperature.
- Magnus formula
- A standard approximation that computes dew point from temperature and relative humidity using the constants a = 17.625 and b = 243.04.
- Saturation
- The state where air holds the maximum possible water vapor, corresponding to 100% relative humidity.
- Gamma term
- The intermediate value in the Magnus formula, gamma = ln(RH / 100) + (a x Tc) / (b + Tc), used to solve for the dew point.
- Condensation
- The process of water vapor turning back into liquid water once air is cooled below its dew point.
- Absolute humidity
- The actual mass of water vapor per volume of air, unlike relative humidity which is a ratio.
- Comfort level
- A plain-language rating (dry, comfortable, sticky, uncomfortable, oppressive) based on the dew point rather than the thermometer.
Frequently asked questions
What is a dew point calculator?
A dew point calculator finds the temperature at which air becomes saturated from the current air temperature and relative humidity. It uses the Magnus formula and returns the dew point in both Celsius and Fahrenheit.
How do you calculate dew point from relative humidity?
Use the Magnus formula: gamma = ln(RH / 100) + (17.625 x Tc) / (243.04 + Tc), then dew point Td = (243.04 x gamma) / (17.625 - gamma). Convert Fahrenheit to Celsius first if needed. This tool does the full calculation instantly.
What is a comfortable dew point?
A dew point below about 16 C (61 F) feels comfortable to most people. Between 16 and 18 C it turns sticky, from 18 to 21 C it feels uncomfortable, and above 21 C (70 F) the air feels oppressive.
Is dew point better than humidity for comfort?
Yes. Dew point is an absolute measure of moisture, so it describes how muggy the air feels far more reliably than relative humidity, which changes with temperature even when the moisture does not.
Can the dew point be higher than the air temperature?
No. The dew point can equal the air temperature when relative humidity reaches 100%, but it never rises above it. A higher value means the temperature and humidity inputs were probably swapped.
What dew point causes condensation on windows?
Condensation forms on any surface colder than the dew point. If the indoor dew point is 12 C and a window pane drops below 12 C, moisture will collect on the glass, which is a common cause of winter window fog.
How do I get dew point in Fahrenheit?
This dew point calculator shows both scales at once. It computes the dew point in Celsius, then converts with Td in Fahrenheit = Td x 9 / 5 + 32, so you never have to convert by hand.
How accurate is the Magnus formula?
The Magnus formula with the Alduchov and Eskridge constants is accurate to within about 0.1 C for the temperature and humidity range of everyday weather, which is more than enough for comfort and condensation decisions.
Why does relative humidity rise at night if the moisture is the same?
As air cools toward its dew point, it can hold less vapor, so the same moisture becomes a higher percentage. That is why relative humidity climbs at night while the dew point stays roughly constant.
What is the difference between dew point and relative humidity?
Relative humidity is a temperature dependent percentage, while dew point is a fixed temperature that reflects the actual water content of the air. Dew point vs humidity matters because only the dew point tells you how the air truly feels.