Dew Point Calculator
Dew point from temperature and relative humidity using the Magnus formula with WMO coefficients, plus absolute humidity, vapour pressure and a comfort reading.
Dew Point Calculator: with the default inputs, dew point is 55.1 °F.
Relative humidity is relative to the current temperature — the same air is 'drier' when you warm it.
Cool the air to this and it starts to condense.
- In both scales
- 55 °F (13 °C)
- How it feels
- Very comfortable
- Absolute humidity
- 10.79Grams of water vapour per cubic metre of air.
- Vapour pressure
- 14.78Partial pressure of water vapour, in hectopascals (millibars).
- Saturation vapour pressure
- 29.57What the vapour pressure would be at 100% humidity, in hPa.
- Condensation on the surface?
- Yes — a surface at 50 °F (10 °C) is below the 55 °F dew point, so water will condense on it.
Assumptions
- Magnus formula with WMO / Sonntag coefficients (a = 6.112 hPa, b = 17.62, c = 243.12 °C) over a plane water surface, valid −45 °C to +60 °C.
- Below freezing the true frost point differs slightly, because saturation pressure over ice is lower than over supercooled water.
- Absolute humidity uses AH = 216.7 · e / T(K), the ideal-gas form for water vapour.
- Assumes standard sea-level conditions; the pressure dependence of the dew point is negligible for ordinary altitudes.
| Dew point (°F) | Dew point (°C) | How it feels |
|---|---|---|
| Under 50 | Under 10 | Dry and pleasant; static and chapped lips indoors |
| 50–55 | 10–13 | Very comfortable |
| 55–60 | 13–16 | Comfortable |
| 60–65 | 16–18 | Getting sticky — most people notice it |
| 65–70 | 18–21 | Somewhat uncomfortable, humid |
| 70–75 | 21–24 | Very humid, quite uncomfortable |
| 75–80 | 24–26 | Extremely uncomfortable, oppressive |
| Over 80 | Over 26 | Severely oppressive; dangerous with exertion |
Dew point, not relative humidity, is what forecasters use to describe mugginess — 90% humidity at 45 °F is crisp, 60% at 90 °F is miserable.
How this is worked out
The formula
Magnus formula with WMO / Sonntag coefficients (b = 17.62, c = 243.12 °C): γ = ln(RH/100) + b·T / (c + T) Td = c·γ / (b − γ) Saturation vapour pressure: e_s = 6.112 · exp(b·T / (c + T)) hPa Actual vapour pressure: e = e_s × RH/100 Absolute humidity: AH = 216.7 · e / (T + 273.15) g/m³
Open How it’s calculated above to see this worked through with your own numbers.
What you enter
- Air temperature
- A number.in °F, °C, K · from -60 to 140 · defaults to 75
- Relative humidity
- Relative humidity is relative to the current temperature — the same air is 'drier' when you warm it.a percentage · from 0.1 to 100 · defaults to 50
- Surface temperature(under More options)
- A window, a wall, a cold drink or a duct. If it is below the dew point, water condenses on it.in °F, °C, K · from -60 to 140 · defaults to 50
What you get back
- Dew pointmain answer
- Cool the air to this and it starts to condense.
- In both scales
- How it feels
- Absolute humidity
- Grams of water vapour per cubic metre of air.
- Vapour pressure
- Partial pressure of water vapour, in hectopascals (millibars).
- Saturation vapour pressure
- What the vapour pressure would be at 100% humidity, in hPa.
- Condensation on the surface?
What this assumes
- Magnus formula with WMO / Sonntag coefficients (a = 6.112 hPa, b = 17.62, c = 243.12 °C) over a plane water surface, valid −45 °C to +60 °C.
- Below freezing the true frost point differs slightly, because saturation pressure over ice is lower than over supercooled water.
- Absolute humidity uses AH = 216.7 · e / T(K), the ideal-gas form for water vapour.
- Assumes standard sea-level conditions; the pressure dependence of the dew point is negligible for ordinary altitudes.
About this calculator
The dew point is the temperature you would have to cool the air to before water starts condensing out of it. Unlike relative humidity, it is an absolute measure — it tells you how much water is actually in the air, not how close that air is to its own capacity at the current temperature.
That distinction is the whole reason to use it. Ninety percent relative humidity at 45 °F is a crisp autumn morning; sixty percent at 90 °F is oppressive. The dew points are 42 °F and 74 °F, and the dew points tell you immediately which one you would rather be standing in.
The formula
This uses the Magnus formula with the WMO coefficients (b = 17.62, c = 243.12 °C), which is accurate to better than 0.1 °C between −45 °C and +60 °C — well beyond any weather you will experience. Older references use Bolton's b = 17.67, c = 243.5, or the Alduchov–Eskridge b = 17.625, c = 243.04; all three agree to within a couple of hundredths of a degree at ordinary temperatures.
What it is good for
- Comfort. The scale in the table is the one broadcast meteorologists use. Below 55 °F is comfortable, above 65 °F is sticky, above 70 °F is oppressive, and above 75 °F is genuinely hard on the body.
- Condensation. Anything colder than the dew point will sweat: windows, uninsulated cold-water pipes, ductwork in a humid crawlspace, the inside of a wall where warm indoor air reaches cold sheathing. Enter a surface temperature under More options to check.
- Fog and frost. When the air temperature falls to the dew point, you get fog or dew. When the dew point is below freezing, you get frost instead, and the correct term is the frost point — the saturation pressure over ice differs slightly from over water.
- Comparing air masses. Dew point does not change when you simply heat or cool air; relative humidity does. That's why the dew point is what forecasters track.
Absolute humidity
The other output worth knowing is grams of water per cubic metre. Air at 20 °C and 50% RH holds about 8.6 g/m³. The same 8.6 g/m³ in a 5 °C room would be over 100% — which is why cold outdoor air brought inside and heated becomes desert-dry, and why humidifiers exist in winter.
Frequently asked questions
▸What is the dew point at 20 °C and 50% humidity?
9.3 °C (48.7 °F). The Magnus formula gives γ = ln(0.5) + 17.62 × 20 / 263.12 = 0.6463, and 243.12 × 0.6463 / (17.62 − 0.6463) = 9.26 °C.
▸What is the difference between dew point and relative humidity?
Dew point is an absolute measure of how much water is in the air; relative humidity is a percentage of what that air could hold at its current temperature. Heat a room and the relative humidity falls while the dew point stays exactly where it was.
▸What dew point is uncomfortable?
Above 60 °F (16 °C) most people notice stickiness, above 65 °F it's humid, above 70 °F it's oppressive, and above 75 °F sweat barely evaporates at all. Below 55 °F feels comfortable and below 50 °F feels dry.
▸Why do my windows fog up?
Because the glass is colder than the dew point of the indoor air. Either raise the glass temperature (storm window, double glazing, curtains open so air can reach it) or lower the indoor dew point with ventilation and by fixing the moisture source.
▸Can the dew point be higher than the air temperature?
No. At 100% relative humidity they're equal, and that's the maximum — the air is saturated, and any further cooling produces fog, dew or rain.
Put this calculator on your own site
A working dew point, free for any site, with no ads and no sign-up. It resizes to fit wherever you paste it and updates itself as this page improves.
Paste this anywhere. It works on any site, carries no ads, never expires, and always shows the current version.
Dew Point Calculator by CalculateItNow
The page's own title. The clearest description of what the link leads to.
The credit line sits outside the widget on purpose, so it is a real link on your page rather than one buried in a frame. Please keep it — it is what pays for CalculateItNow staying free and ad-free. The script only resizes the widget to fit its contents; drop it and the widget still works.
Browse every calculator widget·How to add it to WordPress, Squarespace or Wix
Related calculators
The questions people ask next to a dew point.
Heat index (apparent temperature) from temperature and humidity using the NWS Rothfusz regression with both adjustments, plus the NWS risk category.
Wind chill from air temperature and wind speed using the 2001 NWS formula, with frostbite times and an honest note about where the formula stops applying.
Convert Celsius, Fahrenheit and Kelvin exactly, with the formula worked step by step, a weather and cooking reference table and the quick mental shortcuts.
Solve PV = nRT for pressure, volume, moles or temperature with any units (atm, kPa, psi, L, °C, K) using R = 8.314462618 J/(mol·K), with every step shown.
Q = mcΔT solved for heat, mass, specific heat or final temperature, with a sourced table of specific heats (water 4186, copper 385 J/kg·K) and J, kcal or kWh output.