Audited —·Last updated 27 Jul 2026·7 citations·Tier 1·0 uses

Dew Point Calculator

Free dew point calculator using the Magnus-Tetens approximation. Enter temperature and humidity to get dew point in °F and °C plus a comfort rating.

Dew Point Calculator

Current dry-bulb air temperature. The Magnus-Tetens approximation used here is valid roughly from −40 °C to +60 °C (−40 °F to 140 °F).
Temperature unit
Relative humidity as a percentage from 1 to 100. This is the ratio of the actual water-vapour pressure in the air to the saturation vapour pressure at the same temperature.
%
Dew point
56.9437
Dew point temperature in degrees Fahrenheit — the temperature to which the air would have to cool, at constant pressure and water-vapour content, for water vapour to condense. The lower this is relative to the air temperature, the drier the air.
Dew point (°C)
13.8576 °C
Comfort level
1

Background.

This dew point calculator uses the Magnus-Tetens approximation to turn any temperature and relative humidity pair into the dew point temperature — the temperature to which the air would have to cool, at constant pressure and constant water-vapour content, for water vapour to start condensing out as dew, fog, frost, or cloud. Enter the current air temperature in either Celsius or Fahrenheit, enter the relative humidity from a thermometer-hygrometer or your phone's weather app, and the tool returns the dew point in °F (the headline number meteorologists actually quote on US forecasts) and °C plus a NOAA-style comfort rating from 0 (dry) through 4 (miserable).

The reason dew point is worth calculating, and the reason airports, pilots, agronomists, HVAC engineers, and brewery floor managers all watch it rather than relative humidity, is that dew point is an absolute measure of how much water vapour is actually in the air, while relative humidity is a ratio that changes whenever the temperature changes even though no water has been added or removed. A summer afternoon at 90 °F with 50% relative humidity holds vastly more water than a winter morning at 30 °F with 90% relative humidity — the humid summer afternoon has a dew point near 69 °F (oppressive); the freezing winter morning has a dew point near 28 °F (bone dry).

The Magnus-Tetens approximation that powers this calculator was first published in two parts: August Magnus derived the underlying saturation-vapour-pressure curve in 1844 in Annalen der Physik, and Tetens in 1930 published the compact two-coefficient form (a, b) that is still used today. The coefficients we use — a = 17.625, b = 243.04 °C — come from the 1996 refinement by Alduchov and Eskridge in the Journal of Applied Meteorology, which tightened the curve fit so the formula stays within about 0.4% of the exact Goff-Gratch saturation vapour pressure across the meteorologically interesting range of −40 °C to +50 °C. That is plenty accurate for forecasting, HVAC design, agriculture, and home comfort decisions; it is not the right tool for laboratory hygrometry below −40 °C, where you would want the ice-phase Magnus coefficients or the full Wexler formulation.

Once you know the dew point, you know three useful things at once. First, you know how the air feels: a dew point above 65 °F is the threshold at which most people start to perceive humidity as a discomfort independent of temperature, and above 70 °F outdoor work becomes physiologically taxing because sweat no longer evaporates efficiently. Second, you know when condensation will happen: any surface in the air that is at or below the dew point will get wet — that is why a cold drink sweats, why your bathroom mirror fogs, why dew forms on grass overnight as the ground radiates heat to space, and why fog appears when the air temperature drops to meet the dew point. Third, you know whether you will see dew or frost in the morning: if the dew point is above 0 °C the deposit is liquid (dew); if it is below 0 °C and the surface temperature drops below it, the deposit is solid (frost) because the water vapour transitions directly from gas to solid by deposition.

The rest of this page walks through the Magnus-Tetens derivation, the NOAA dew-point comfort scale used by US TV meteorologists, why pilots and airports report dew point and temperature as a pair rather than relative humidity, how dew point relates to wet-bulb temperature and the heat index, and a worked example at 25 °C / 50% RH that produces Td ≈ 13.86 °C.

What is dew point calculator?

Dew point is the temperature, at constant pressure, to which a parcel of air must be cooled in order for its water vapour to reach saturation — at which point further cooling causes the water to condense into liquid droplets (dew, fog, or cloud) or, below 0 °C, deposit directly as solid ice crystals (frost). Unlike relative humidity (which expresses water-vapour content as a fraction of the maximum the air could hold at the current temperature, and therefore changes whenever the temperature changes), dew point is a near-conservative measure of the absolute moisture content of the air. If you take a parcel of air, heat it up, and measure no change in actual water vapour, the dew point stays the same even though the relative humidity drops. That property is why dew point is the variable of choice for aviation METAR reports, HVAC psychrometric calculations, agriculture forecasting, brewing, and anyone reasoning about condensation. The NOAA dew-point comfort scale uses the following thresholds in degrees Fahrenheit: below 50 °F the air feels dry; 50–59 °F is comfortable for most outdoor activity; 60–64 °F starts to feel a bit muggy or sticky; 65–69 °F is oppressive and noticeably uncomfortable; at or above 70 °F the air is miserable, and at 75 °F and above sweat-cooling becomes ineffective and outdoor work carries real heat-stress risk. The dew point can never exceed the dry-bulb air temperature; when the two values converge, relative humidity is 100% and the air is saturated. The dew point depression — the gap between air temperature and dew point — is what meteorologists use to estimate cloud base height (the lifted condensation level rises about 125 metres for every 1 °C of depression) and to predict fog formation overnight. The Magnus-Tetens approximation captures this physics with a two-coefficient fit to the saturation-vapour-pressure curve, accurate to better than half a percent across normal meteorological conditions.

How to use this calculator.

  1. Enter the current air temperature into the first field. Use a thermometer reading, your car or thermostat display, or the temperature listed in your local forecast — do not use the 'feels like' or heat-index value.
  2. Select whether you entered Celsius or Fahrenheit. Fahrenheit inputs are converted internally to Celsius (using Tc = (Tf − 32) × 5/9) before the Magnus-Tetens math runs, so the output dew point is calculated identically either way.
  3. Enter the relative humidity from 1 to 100. Most consumer hygrometers and phone weather apps report this directly. Values below 1% (cold-rolled dry air, certain laboratory conditions) and above 100% (supersaturation, which is briefly possible in clouds but cannot be measured by ordinary instruments) are out of range for this calculator.
  4. Read the dew point in °F (the primary output, matching NOAA and US TV-forecast convention) and the same value in °C below it. The comfort level output translates the °F value onto the NOAA scale: 0 dry, 1 comfortable, 2 sticky, 3 oppressive, 4 miserable.
  5. To check whether condensation will form on a specific surface — a cold drink can, a window, a chilled mirror — compare that surface's temperature to the dew point. If the surface is at or below the dew point, water will condense on it. If you are designing an HVAC system or a cold-storage room, this is how you avoid sweating ducts and wet floors.
  6. To estimate the cloud base height in metres, multiply the dew point depression (T − Td in °C) by about 125. So if the temperature is 25 °C and the dew point is 13.86 °C, the depression is 11.14 °C and the lifted condensation level is roughly 1,400 m above ground.

The formula.

Td = b γ ⁄ (a − γ), γ = ln(RH ⁄ 100) + a T ⁄ (b + T)
Td · dew point (°C)T · air temperature (°C)RH · relative humidity (%)γ · intermediate terma · 17.625b · 243.04 °C

The Magnus-Tetens approximation expresses the dew point temperature Td (in °C) as a function of the dry-bulb air temperature T (also in °C) and the relative humidity RH (in percent, from 1 to 100):

γ(T, RH) = ln(RH / 100) + (a · T) / (b + T) Td = (b · γ) / (a − γ)

The two coefficients a and b come from the Alduchov-Eskridge (1996) refinement of the classical Magnus form:

a = 17.625 b = 243.04 °C

The physical reasoning behind the formula is the Clausius-Clapeyron relation, which says that the saturation vapour pressure of water grows approximately exponentially with temperature. Magnus (1844) fit this exponential with the compact form e_s(T) = 6.1094 · exp(a·T/(b+T)) hPa, where e_s is the saturation vapour pressure at temperature T. The relative humidity is by definition RH/100 = e/e_s, where e is the actual vapour pressure in the air. The dew point is the temperature at which the saturation vapour pressure equals that actual vapour pressure — that is, e_s(Td) = e = (RH/100) · e_s(T). Taking logs and rearranging yields the closed-form expression above. The intermediate quantity γ is the natural log of (RH/100) plus the dimensionless term a·T/(b+T); the dew point is then b·γ / (a − γ). When RH = 100%, ln(RH/100) = 0 and γ collapses to a·T/(b+T), and the inversion correctly returns Td = T as expected. The formula is most accurate from about −40 °C to +50 °C in air over a liquid water surface. Below 0 °C, the air may be saturated with respect to ice rather than liquid water, and a separate ice-phase Magnus pair (a = 22.587, b = 273.86 °C) gives a more accurate frost point; but for ordinary weather and indoor air applications, the liquid-phase coefficients used here are standard practice, including in NOAA NWS reference tooling. The maximum error across the −40 °C to +50 °C range is below 0.4% of the exact Goff-Gratch saturation curve, well within the practical accuracy of any consumer hygrometer.

A worked example.

Example

A textbook indoor-air example: the room is at 25 °C (77 °F) with 50% relative humidity — the kind of conditions a well-tuned air conditioner would maintain in an office on a summer day. Enter temperature = 25, temperatureUnit = C, humidityPercent = 50. The calculator first builds γ = ln(50/100) + 17.625 × 25 / (243.04 + 25). The first term ln(0.5) ≈ −0.6931. The second term 17.625 × 25 / 268.04 ≈ 1.6437. So γ ≈ −0.6931 + 1.6437 ≈ 0.9506. The dew point in Celsius is then Td = 243.04 × 0.9506 / (17.625 − 0.9506) = 231.04 / 16.6744 ≈ 13.86 °C. Converting to Fahrenheit: 13.86 × 9/5 + 32 ≈ 56.94 °F. That falls into the NOAA 'comfortable' band (50–59 °F), so the comfort level output is 1. Interpreting these numbers: the air would have to cool by about 11 °C (around 20 °F) before any water would condense out, which is why a cold drink at refrigerator temperature (~4 °C) will quickly sweat in this room — its surface is well below 13.86 °C. The dew point depression of 11.14 °C also implies a lifted condensation level (if this air parcel were lifted upward in the atmosphere) of about 1,400 metres above ground, which is a typical fair-weather cumulus cloud base. If you held the actual moisture content fixed and warmed the room to 30 °C, the relative humidity would drop to about 38% while the dew point stayed at 13.86 °C — that invariance is exactly why meteorologists prefer dew point to relative humidity when they want to know whether the air is actually wet or just cool.

temperature UnitC
temperature25
humidity Percent50

Frequently asked questions.

What is the difference between dew point and relative humidity?
Relative humidity is a ratio — it tells you how close the air is to being saturated at its current temperature, expressed as a percent. Because air can hold much more water vapour when warm than when cold, the same actual moisture content corresponds to very different relative humidities at different temperatures: 50% RH at 30 °C is much wetter air than 50% RH at 10 °C. Dew point, by contrast, is an absolute temperature that depends only on how much water vapour is actually in the air. If you take a parcel of air and just heat or cool it (without adding or removing water), the dew point stays the same while the relative humidity changes. That is why meteorologists, pilots, HVAC engineers, and agronomists prefer dew point — it directly tells you how much water is in the air, independent of what the thermometer reads.
What dew point feels comfortable?
The NOAA dew-point comfort scale (used by US TV meteorologists) sets the bands roughly like this in degrees Fahrenheit: below 50 °F the air feels noticeably dry; 50–59 °F is the comfort sweet spot for most people; 60–64 °F starts to feel sticky and gym-clothes-clammy; 65–69 °F is oppressive — you can feel it as soon as you walk outside; at or above 70 °F the air is miserable and you sweat without exerting yourself. In Celsius the same bands run roughly: < 10 °C dry, 10–15 °C comfortable, 16–18 °C sticky, 18–21 °C oppressive, > 21 °C miserable. These thresholds are why the air feels so different at 30 °C / 30% RH (Td about 11 °C, comfortable) versus 30 °C / 70% RH (Td about 24 °C, miserable) even though the temperature is identical.
How does fog form, and how is it related to dew point?
Fog forms when the air temperature drops to meet the dew point, so relative humidity reaches 100% and water vapour condenses out as suspended micro-droplets. The two most common ways this happens are radiation fog (clear nights, calm winds — the ground radiates heat to space, cools the air just above it down to the dew point, and a shallow ground-hugging fog forms before sunrise) and advection fog (warm moist air blowing over a cold surface, such as warm Gulf air over the cold California Current generating San Francisco's summer fog). You can predict overnight radiation fog by watching the dew point and the forecast minimum temperature: if the forecast min is at or below the dew point and winds are light, fog is likely. The calculator can help you size this up: if a summer evening is at 22 °C with 70% RH, the dew point is about 16.3 °C, so any place where the air or surface cools below 16.3 °C will see dew or fog.
What is the difference between dew and frost?
Dew is liquid water that condenses from the air onto a surface when the surface temperature drops to or below the dew point. Frost is solid ice that deposits directly from the air (skipping the liquid phase entirely) when the surface temperature drops below the frost point — the analogous temperature for saturation with respect to ice rather than liquid water. The practical distinction is whether the dew point is above or below 0 °C. If Td > 0 °C and the surface cools to or below Td but stays above 0 °C, you get dew. If Td < 0 °C, the appropriate quantity is the frost point Tf (slightly higher than Td below freezing because saturation vapour pressure over ice is lower than over supercooled water at the same temperature), and you get frost. This calculator returns the liquid-water dew point in all cases; below 0 °C, a frost-point calculation using the ice-phase Magnus coefficients (a = 22.587, b = 273.86) is more accurate by about 0.5 °C.
Why do airports and pilots report dew point instead of humidity?
Aviation METAR weather reports always include temperature and dew point as a pair (e.g. 'M02/M07' meaning −2 °C / −7 °C), not relative humidity. There are three reasons. First, the spread between temperature and dew point (the 'dew point depression') is a direct predictor of cloud base height: the lifted condensation level — and therefore the base of any convective cloud — sits about 125 metres above ground for every 1 °C of depression. Second, when temperature and dew point converge, fog and low ceilings are imminent — a critical visibility hazard for VFR pilots. Third, dew point is conservative under heating and cooling in the cockpit and around the aircraft, so the same value remains meaningful even as the static temperature of the airframe changes. Relative humidity, by contrast, would scramble with every altitude change and provide no useful operational signal.
What is the Magnus-Tetens formula and how accurate is it?
The Magnus-Tetens approximation is a two-coefficient curve fit to the saturation vapour pressure of water as a function of temperature, originally derived by Magnus in 1844 and put into its compact modern form by Tetens in 1930. With the Alduchov-Eskridge (1996) refinement of the coefficients (a = 17.625, b = 243.04 °C), the formula is accurate to within about 0.4% of the exact Goff-Gratch saturation curve across the temperature range −40 °C to +50 °C — well within the precision of any consumer hygrometer or aviation thermometer. Outside that range, accuracy degrades; for laboratory hygrometry below −40 °C or above +60 °C, the full Wexler (1976) or IAPWS formulations are preferred. For weather forecasting, HVAC design, agriculture, brewing, and home-comfort applications, the Magnus-Tetens form used here is the industry standard, and is exactly the same formula used by NOAA NWS reference tools.
Can the dew point be higher than the air temperature?
No — physically the dew point cannot exceed the dry-bulb air temperature. The dew point is defined as the temperature to which the air must cool to reach saturation; if the air is already saturated, the dew point equals the air temperature (relative humidity = 100%). If the dew point appeared to be higher, that would mean the air is supersaturated, which can only happen transiently in clouds and chambers and cannot be measured by ordinary hygrometers. If your sensor is reporting a dew point above the air temperature, it is a calibration error. This calculator caps relative humidity input at 100%, which guarantees Td ≤ T in every output.
What is the highest dew point ever recorded?
The highest reliably-measured dew point on Earth is around 35 °C (95 °F), recorded at Dhahran, Saudi Arabia in July 2003, and again similarly along the Persian Gulf coast during summer heat waves. To put that in perspective, a dew point of 35 °C means the air contains so much water vapour that human sweat evaporation is essentially zero, and ambient conditions cross into territory that approaches the human survivability limit (the so-called 'wet-bulb 35 °C' threshold studied by Sherwood and Huber in 2010). For comparison, the Mississippi delta and US Gulf coast can reach summer dew points of 27–28 °C (about 80–82 °F), which is already in the 'miserable' band of the NOAA scale. Anything above about 30 °C dew point is exceptional even in the world's most humid climates.
How is dew point related to wet-bulb temperature and the heat index?
Three different humidity-aware temperatures get confused in conversation. Dew point is the temperature to which the air would have to cool at constant moisture content to saturate. Wet-bulb temperature is the temperature a thermometer reads if its bulb is wrapped in a wet wick and ventilated — it is always between the dew point and the dry-bulb temperature, and it is what limits human survivability under heat stress because the human body cools by sweat evaporation, which is a wet-bulb process. Heat index (the US 'feels like' temperature) is an empirical function of temperature and relative humidity (Steadman 1979, NWS 1990) that estimates the perceived temperature for a moderately-active adult in shade. Heat index agrees with the body's actual heat-stress sensation around the 60–70 °F dew point range and starts to under-predict danger above 75 °F dew point, which is why public-health agencies in the Gulf and South Asia increasingly report wet-bulb temperature alongside heat index.
Does this calculator work below freezing?
Yes — the Magnus-Tetens formula is valid down to about −40 °C with the liquid-water coefficients (a = 17.625, b = 243.04) used here. Between 0 °C and −40 °C the formula returns the dew point with respect to supercooled liquid water, which is the convention used by most surface weather stations and aviation reports. If you specifically want the frost point — the temperature at which saturation is reached over an ice surface — that value is slightly higher (by 0 to ~0.5 °C in this range) and uses a different coefficient pair (a = 22.587, b = 273.86). For most practical purposes (dew on car windscreens, frost in vineyards, indoor condensation behind insulation) the liquid-phase result this calculator returns is the appropriate number; for high-accuracy upper-air or laboratory work below −40 °C, switch to a full Wexler or IAPWS formulation.

References& sources.

  1. [1]Alduchov, O. A. & Eskridge, R. E. (1996). Improved Magnus Form Approximation of Saturation Vapor Pressure. Journal of Applied Meteorology, 35(4), 601–609. The source of the modern coefficients a = 17.625 and b = 243.04 °C used in this calculator; refines the classical Magnus fit to within 0.4% of the exact Goff-Gratch curve over −40 °C to +50 °C.
  2. [2]Magnus, G. (1844). Versuche über die Spannkräfte des Wasserdampfes. Annalen der Physik und Chemie, 137(2), 225–247. The original 19th-century derivation of the saturation-vapour-pressure curve that underlies the modern Magnus-Tetens approximation.
  3. [3]Tetens, O. (1930). Über einige meteorologische Begriffe. Zeitschrift für Geophysik, 6, 297–309. Tetens' compact two-coefficient form of the Magnus equation that became the standard meteorological dew-point formula.
  4. [4]NOAA National Weather Service. Discussion: Dew Point vs. Humidity. NWS Forecast Office, La Crosse WI. Authoritative explainer for the NOAA dew-point comfort scale (dry / comfortable / sticky / oppressive / miserable) and the reasoning behind using dew point rather than relative humidity in public forecasts.
  5. [5]American Meteorological Society Glossary of Meteorology, entries 'dew point', 'frost point', 'saturation vapor pressure'. AMS, 2nd edition. The standard meteorological reference for definitions and conventions used in dew-point calculations.
  6. [6]Lawrence, M. G. (2005). The Relationship between Relative Humidity and the Dewpoint Temperature in Moist Air: A Simple Conversion and Applications. Bulletin of the American Meteorological Society, 86(2), 225–234. Clear derivation of the Magnus-Tetens inversion and an error analysis of common approximations.
  7. [7]Sherwood, S. C. & Huber, M. (2010). An Adaptability Limit to Climate Change Due to Heat Stress. Proceedings of the National Academy of Sciences, 107(21), 9552–9555. Establishes the wet-bulb 35 °C human-survivability limit, which is directly related to extreme dew points around 35 °C cited in the FAQs.
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