Calculate dew point, frost point, psychrometric vapor pressure, and thermal comfort levels from air temperature and relative humidity using the WMO Magnus formula.
Quick Presets:
Dew Point — Magnus Formula
Current ambient air temperature.
Relative humidity percentage (1–100).
Applies to both inputs and result outputs.
Dew Point Comfort Scale
Dew point is a superior measure of perceived atmospheric humidity because it directly indicates moisture content independent of ambient air temperature.
Below 10°C (50°F)Dry
10–16°C (50–60°F)Comfortable
16–20°C (60–68°F)Sticky
20–24°C (68–75°F)Muggy
24°C+ (75°F+)Oppressive
Enter a temperature and humidity on the Dew Point tab to see your reading here.
Reverse — Required Humidity
Current or forecast air temperature.
The target dew point temperature.
Applies to both inputs and humidity result.
Result
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Enter values above to compute.
DryComfortableStickyMuggyOppressive
Vapor Pressure (e)—
VPD (Plant/Dry Rate)—
Absolute Humidity—
Temp-Dew Point Spread—
4 min read3 steps7 terms3 examples6 FAQsγ = ln(RH/100) + aT/(b+T); Td = bγ / (a − γ)
Relative humidity is the number most weather reports lead with, but it's a moving target — the same moisture content reads as a low percentage on a hot day and a high percentage on a cool one.
On the Dew Point tab, enter the current air temperature and relative humidity, and pick Celsius or Fahrenheit. The calculator updates instantly using the Magnus formula.
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Read the dew point and comfort level
The result card shows the dew point in your chosen unit, along with the spread between air temperature and dew point and a comfort label (Dry, Comfortable, Sticky, Muggy, or Oppressive). Switch to the Comfort tab to see the full classification scale with your reading highlighted.
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Work backward on the Reverse tab
If you want to know what relative humidity would produce a specific dew point at a given temperature — for example, planning for a 'comfortable' day — use the Reverse tab. Enter the temperature and your target dew point, and it solves for the required humidity.
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Reference
Formula & Methodology
2 formulas▸
Dew Point — Magnus Formula
γ = ln(RH/100) + aT/(b+T); Td = bγ / (a − γ)
T is air temperature in °C, RH is relative humidity as a percentage, and a = 17.625, b = 243.04°C are the Magnus-Tetens constants (Alduchov & Eskridge, 1996), valid over roughly −45°C to 60°C and 1–100% RH. Example: T = 25°C, RH = 60% → γ = ln(0.60) + 17.625×25/268.04 = −0.5108 + 1.6439 = 1.1330; Td = 243.04×1.1330 / (17.625−1.1330) = 16.7°C.
Reverse — Required Humidity for a Target Dew Point
γ = aTd/(b+Td); RH = 100 × e^(γ − aT/(b+T))
Given an air temperature T and a target dew point Td, this rearranges the Magnus formula to solve for the relative humidity that would produce it. Because relative humidity cannot exceed 100%, this only has a solution when Td ≤ T — a target dew point above the air temperature is physically impossible.
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General reference
Trust, Methodology & Sources
Written by Calculover Editorial Team · Updated 2026-07-31▸
Editorial accountability
Author: Calculover Editorial Team - Editor
Owner: Calculover Editorial Team - Editorial owner
Last reviewed: 2026-07-31
Last verified: 2026-07-31
Methodology
Assumption: Results are approximations and may differ slightly from professional weather-service or building-science software. Psychrometric values assume standard atmospheric pressure of 1013.25 hPa.
Limitations & guidance
This calculator estimates dew point and related psychrometric values from air temperature and relative humidity. It is intended for general weather, HVAC, greenhouse, and educational use. Dew points use the Alduchov & Eskridge (1996) Magnus formulation for liquid water, while sub-zero frost points apply the Buck (1981) ice saturation equations.
Professional guidance:
Primary sources
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Glossary
Key Terms Explained
7 terms▸
Dew Point ↗The temperature to which air must be cooled, at constant pressure and moisture content, for water vapor to condense into liquid (dew, fog, or clouds). Unlike relative humidity, the dew point doesn't change as the air warms or cools during the day, which makes it a more consistent measure of how much moisture is actually in the air.
Relative Humidity ↗The amount of water vapor currently in the air, expressed as a percentage of the maximum the air could hold at its current temperature. Because warm air can hold more moisture than cold air, the same amount of moisture produces a lower relative humidity reading on a hot day than on a cool one — which is why relative humidity alone is a poor comfort indicator.
Magnus Formula ↗Also called the Magnus-Tetens approximation — an empirical equation that estimates the dew point from air temperature and relative humidity using two fitted constants (a = 17.625, b = 243.04°C). It's accurate to within about 0.4°C over typical weather conditions and is the formula meteorologists and weather apps commonly use.
Saturation ↗The point at which air is holding all the water vapor it can at its current temperature (100% relative humidity). At saturation, the air temperature and dew point are equal, and any further cooling or moisture addition causes condensation.
Condensation ↗The process of water vapor turning into liquid water, which happens whenever air is cooled to its dew point (or moisture is added until saturation is reached). Dew on grass, fogged-up windows, and morning fog are all condensation occurring at or near the dew point.
Comfort (Dew Point Scale) ↗A practical classification of how muggy the air feels based on dew point alone: below 10°C feels dry, 10–16°C is generally comfortable, 16–20°C starts to feel sticky, 20–24°C is muggy, and above 24°C is oppressively humid — regardless of the air temperature.
Humidity ↗A general term for the amount of water vapor present in the air. It can be measured several ways — relative humidity (a percentage of the maximum possible), absolute humidity (mass of water vapor per volume of air), or dew point (a temperature) — each useful in different contexts.
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Scenarios
Real-World Examples
3 worked examples▸
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Everyday Weather Check
Typical warm afternoon
Air temperature 25°CRelative humidity 60%
The Magnus formula gives a dew point of about 16.7°C — just over the line into the Sticky band (16–20°C), right at the edge of Comfortable (10–16°C). The 8.3°C spread between air temperature and dew point means the air isn't holding an excessive amount of moisture for the temperature, so it should feel only mildly humid rather than muggy.
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Summer Storm Watcher
Muggy pre-storm conditions
Air temperature 30°CRelative humidity 75%
This works out to a dew point around 25°C, which lands in the Oppressive tier (above 24°C). This matches the classic mugginess people feel before a summer thunderstorm — the air is holding close to as much moisture as it can, so sweat evaporates poorly and the heat feels far worse than the air temperature alone suggests.
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Camping Trip Planner
Working backward to a target dew point
Air temperature 25°CTarget dew point 14°C
On the Reverse tab, solving for the humidity needed to hit a 14°C ('Comfortable') dew point at 25°C air temperature gives a relative humidity of about 50%. Anything wetter than that at the same temperature would push the dew point — and the mugginess — higher.
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Reference
Cite This Calculator
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Use either format to cite this calculator in a paper, report, or resource list.
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Deep Dive
Understanding Dew Point — Why It Beats Relative Humidity for Comfort
Relative humidity is the number most weather reports lead with, but it's a moving target — the same moisture content reads as a low percentage on a hot day and a high percentage on a cool one. Dew point measures the actual amount of moisture in the air directly, which is why meteorologists and HVAC engineers reach for it whenever they need to describe how muggy conditions really are.
What the Dew Point Actually Measures
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The dew point is the temperature at which air becomes saturated — unable to hold any more water vapor — given its current moisture content. Cool air down to its dew point and water starts condensing out as dew, fog, or cloud droplets. Because this only depends on how much moisture is actually present (not on the current air temperature), the dew point stays roughly constant through the day even as the temperature and relative humidity swing. That stability is exactly what makes it a better comfort indicator: a 60% relative humidity reading means something very different at 15°C than it does at 35°C, but a 20°C dew point feels the same regardless of the air temperature around it.
The Magnus Formula and Its Limits
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This calculator uses the Magnus-Tetens approximation, an empirical formula fitted to the physics of water vapor pressure over liquid water. It's simple, fast, and accurate to within about 0.4°C across the range of temperatures and humidities most people encounter day to day (roughly −45°C to 60°C). More rigorous formulas exist (based on the Clausius-Clapeyron relation with additional correction terms), but they add complexity without meaningfully changing the result for everyday weather, cooling, and comfort planning. The one edge case worth knowing: right at or below freezing, the formula technically describes the frost point rather than the dew point, since ice forms instead of liquid dew — the numeric difference is small enough that most practical uses don't need to distinguish them.
Reading the Comfort Scale
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The comfort tiers used here — Dry below 10°C, Comfortable from 10–16°C, Sticky from 16–20°C, Muggy from 20–24°C, and Oppressive above 24°C — are a simplified version of the ranges used by the U.S. National Weather Service. They're deliberately based on dew point rather than relative humidity, so the same tier means the same felt mugginess whether it's a 20°C spring day or a 35°C summer afternoon. The Reverse tab flips this around: instead of asking 'how muggy is it right now,' it answers 'how dry would the air need to be to feel comfortable at this temperature' — useful for evaluating a forecast or deciding whether a dehumidifier setting will actually get you into the comfortable range.
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Questions
Frequently Asked Questions
6 questions▸
What is the formula for dew point?+
This calculator uses the Magnus (Magnus-Tetens) approximation: γ = ln(RH/100) + aT/(b+T), then Td = bγ/(a−γ), with a = 17.625 and b = 243.04°C. It's accurate to within about 0.4°C for typical weather conditions.
What dew point counts as muggy?+
A dew point above 20°C (68°F) is generally considered muggy, and above 24°C (75°F) is oppressive. Below 16°C (60°F) most people find the air comfortable, and below 10°C (50°F) it starts to feel dry.
Why use dew point instead of relative humidity?+
Relative humidity is relative to the current air temperature, so the same percentage means something different on a hot day versus a cool one. Dew point measures the actual moisture content directly, so it stays consistent as the temperature changes throughout the day — making it a much more reliable comfort measure.
What does it mean when the temperature reaches the dew point?+
When air temperature drops to the dew point, the air becomes saturated (100% relative humidity) and water vapor starts condensing out as dew, fog, or clouds. This is exactly how morning fog and dew on grass form overnight as temperatures fall.
What units does this calculator use?+
You can switch between Celsius and Fahrenheit using the unit selector on each tab — it applies to both the temperature input and the dew point (or humidity) result.
Can I calculate humidity from a target dew point instead?+
Yes — use the Reverse tab. Enter the air temperature and the dew point you want to reach, and it solves for the relative humidity required. If your target dew point is higher than the air temperature, it isn't achievable, since that would require more than 100% relative humidity.
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