Dew Point Calculator
The dew point is the temperature air must cool to before moisture condenses, and this dew point calculator works it out the moment you type a temperature and relative humidity. It uses the Magnus formula the National Weather Service (NWS) relies on, so you'll see the dew point, a plain-language comfort rating, the absolute humidity, and the spread all at once. Switch between Fahrenheit and Celsius and the numbers convert for you.
- Magnus formula
- °F or °C
- Comfort rating
- Absolute humidity
- Live thermometer
Last updated July 24, 2026 Method: Magnus formula Reviewed by the Calcowa weather team
Enter a temperature and a humidity between 1 and 100%.
Comfortable
γ = (17.625 × T) / (243.04 + T) + ln(RH/100)
How do you calculate dew point?
The Magnus formula is the standard, and it's the same one the NWS uses. First find gamma: γ = (17.625 × T) / (243.04 + T) + ln(RH/100), where T is in Celsius and RH is a percentage. Then the dew point in Celsius is Td = 243.04 × γ / (17.625 - γ). For 25°C at 60% humidity, that's a dew point of about 16.7°C.
That formula's accurate to within 0.35°C across the range you'd ever see outdoors. If you've got Fahrenheit, the calculator converts to Celsius first, runs the formula, then converts the answer back, so you don't have to juggle units yourself.
Finding the dew point step by step
You only need two numbers: the air temperature and the relative humidity. Here's how the calculator gets to the answer:
- 1
Convert to CelsiusIf your temperature is in Fahrenheit, subtract 32 and multiply by 5/9.
- 2
Work out gammaPlug T and RH into gamma = (17.625 × T) / (243.04 + T) + ln(RH/100).
- 3
Apply the formulaDew point in Celsius is 243.04 × gamma / (17.625 - gamma).
- 4
Convert back if neededMultiply by 9/5 and add 32 to get the dew point in Fahrenheit.
- 5
Read the comfort levelCompare the dew point to the comfort chart to see how the air will feel.
A dew point calculation example
Say it's 77°F outside and the relative humidity is 60%. Here's the Magnus formula worked all the way through, the same math the calculator runs when you load this page.
- 1. Convert to Celsius. (77 - 32) × 5/9 = 25°C.
- 2. Work out gamma. γ = (17.625 × 25) / (243.04 + 25) + ln(60/100) = 1.6439 + (-0.5108) = 1.1331.
- 3. Apply the dew point formula. Td = 243.04 × 1.1331 / (17.625 - 1.1331) = 275.4 / 16.492 = 16.7°C.
- 4. Convert back to Fahrenheit. 16.7 × 9/5 + 32 = 62.1°F.
So 77°F air at 60% humidity has a dew point near 62°F. The spread is 77 - 62.1 = 14.9°F, and a 62°F dew point sits in the noticeable-humidity range, so the air would feel a touch sticky.
Dew point chart by temperature and humidity
Want a quick read without typing? This chart shows the dew point for common temperature and humidity pairs, worked out with the same Magnus formula so the numbers line up with the calculator. Notice how the dew point climbs with both a warmer temperature and a higher humidity.
| Air temperature | 40% humidity | 60% humidity | 80% humidity |
|---|---|---|---|
| 70°F (21°C) | 44.6°F | 55.5°F | 63.5°F |
| 80°F (27°C) | 53.5°F | 64.9°F | 73.3°F |
| 90°F (32°C) | 62.5°F | 74.3°F | 83.0°F |
Dew points are rounded to one decimal. For an exact figure at any temperature and humidity, use the calculator at the top.
What's a comfortable dew point?
Most people don't notice humidity much when the dew point's under 55°F (13°C); the air feels dry and pleasant. Once it climbs past 60°F you'll start to feel it, and at 65°F it's genuinely muggy. Above 70°F the air is so saturated that sweat can't evaporate well, which is when heat turns dangerous. Here's how the ranges feel, based on NWS guidance.
| Dew point (°F) | Dew point (°C) | How it feels |
|---|---|---|
| Below 50°F | Below 10°C | Dry and comfortable |
| 50 to 55°F | 10 to 13°C | Comfortable |
| 55 to 60°F | 13 to 16°C | Slightly humid, still pleasant |
| 60 to 65°F | 16 to 18°C | Noticeable humidity, a little sticky |
| 65 to 70°F | 18 to 21°C | Muggy and uncomfortable |
| 70 to 75°F | 21 to 24°C | Oppressive, heavy air |
| Above 75°F | Above 24°C | Extremely humid, potentially dangerous |
What does a specific dew point mean?
Forecasts often quote a single dew point number. Here's what the common readings feel like, based on the same NWS comfort guidance as the chart above.
- 55°F (13°C) dew point
- The comfort threshold. Below this the air feels dry and pleasant.
- 60°F (16°C) dew point
- You start to notice the humidity. A little sticky but manageable.
- 65°F (18°C) dew point
- Genuinely muggy and uncomfortable for most people.
- 70°F (21°C) dew point
- Oppressive. Sweat cannot evaporate well, so it feels heavy.
- 75°F+ (24°C+) dew point
- Extremely humid and potentially dangerous in the heat.
Why dew point beats relative humidity
Relative humidity changes with temperature, so a 90% reading at dawn can drop to 55% by afternoon even though the actual moisture in the air hasn't moved. Dew point doesn't do that. If the dew point's 65°F at 8 AM, it's still 65°F at 2 PM, and that's what makes the afternoon feel thick. That's why pilots, gardeners, and meteorologists reach for dew point over humidity.
The Heat Index Calculator pairs naturally with this tool: once you've got the dew point, you can see how combined heat and humidity affect the feels-like temperature. And the Temperature Converter helps when you're working with readings in different unit systems.
What is the dew point spread?
The spread is the gap between the air temperature and the dew point. A small spread (under 5°F or 3°C) means the air's nearly saturated, so fog, rain, or dew is close. A large spread means there's plenty of room before condensation kicks in. Pilots use it to estimate cloud base height, roughly 1,000 feet per 4.5°F of spread. A spread of zero is 100% humidity, which is what you get inside a cloud or in fog at sunrise.
Frequently asked questions
Is the dew point the same as the air temperature when it's foggy?
Close to it. In fog the air is saturated, so the dew point sits right at or within a degree of the air temperature. That tiny gap, called the spread, is what fog forecasting watches.
The dew point is the temperature the air has to cool to before it's saturated and moisture starts condensing out. When the air reaches its dew point, the relative humidity hits 100% and you'll see dew on grass, fog, or droplets on a cold glass. It's a better gauge of how humid the air feels than relative humidity on its own.
Relative humidity tells you how full the air is compared to its current capacity, which shifts as the temperature changes. Dew point is an absolute measure of the water vapor present. A dew point of 60°F feels muggy whether it's 70°F or 90°F out, so it's the number meteorologists trust for comfort.
Most people are fine when the dew point's below 55°F (13°C). Between 55°F and 60°F it starts to feel humid, and above 65°F (18°C) the air feels muggy and oppressive. Dew points above 70°F (21°C) are extremely humid and can make outdoor activity dangerous in the heat.
This calculator uses the Magnus formula: first work out gamma = (17.625 × T) / (243.04 + T) + ln(RH/100), then dew point = 243.04 × gamma / (17.625 - gamma), with T in Celsius. It's accurate within about 0.35°C for normal conditions and it's the approach the NWS relies on.
The Magnus formula estimates dew point from air temperature and relative humidity in two steps. First find gamma = (17.625 × T) / (243.04 + T) + ln(RH/100), where T is the temperature in Celsius and RH is the relative humidity as a percentage. Then the dew point in Celsius is Td = 243.04 × gamma / (17.625 - gamma). The 17.625 and 243.04 are the Magnus coefficients, and the formula stays accurate to about 0.35°C between -40°C and 50°C, which is why it's the version weather services use.
Absolute humidity is the mass of water vapor per cubic meter of air, in g/m³. Unlike relative humidity, it doesn't change when the temperature does, so it's a direct read of how much water is actually in the air. At 25°C and 60% RH it's about 13.8 g/m³.
No, it can't. The dew point can equal the air temperature, which is 100% relative humidity (think fog or a cloud), but it can never exceed it. When you see a dew point reading very close to the air temperature, the air's nearly saturated and fog or rain is likely.
You run the relationship backward. If you know the air temperature and the dew point, the relative humidity is the saturation vapor pressure at the dew point divided by the one at the air temperature, times 100. The rule of thumb is simpler: the closer the dew point sits to the air temperature, the higher the humidity, and when the two match it's 100%. This tool goes the common direction, temperature and humidity to dew point, but the physics works both ways.
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