Dew Point Calculator
Find the dew point from air temperature and relative humidity, or solve for humidity or temperature, with comfort level, vapor pressure and the full Magnus formula steps.
Air pressure (optional)
Surfaces at or below this temperature collect dew. Becoming sticky, with muggy evenings (NWS summer dew point guidance).
NWS summer comfort: 55 °F or lower dry, 55 to 65 °F becoming sticky, 65 °F or higher oppressive. Bar spans 30 to 85 °F.
Surface temperature limits
Any surface at or below the dew point, such as a cold pipe, a window in winter or a chilled glass, collects water. Coating data sheets ask for a surface at least 5 °F (3 °C) above the dew point.
EPA indoor guidance: keep relative humidity below 60 percent, ideally 30 to 50 percent. At 60.0% this air is at or above the 60 percent limit for indoor air.
Outdoors, dew or fog forms overnight only if the ground or air cools to the dew point.
Your calculation
The same steps the calculator runs, with your numbers.
- Temperature in Celsius: (77 − 32) × 5 ÷ 9 = 25.00 °C.
- γ = ln(60 ÷ 100) + 17.625 × 25.00 ÷ (243.04 + 25.00) = −0.5108 + 1.6439 = 1.1331.
- Dew point = 243.04 × 1.1331 ÷ (17.625 − 1.1331) = 16.70 °C = 62.06 °F.
- Saturation vapor pressure: 6.1094 × exp(17.625 × 25.00 ÷ (25.00 + 243.04)) = 31.62 hPa.
- Vapor pressure: 60 ÷ 100 × 31.62 = 18.97 hPa.
- Absolute humidity: 216.7 × 18.97 ÷ (25.00 + 273.15) = 13.79 g/m³.
- Mixing ratio: 621.97 × 18.97 ÷ (1013.25 − 18.97) = 11.87 g/kg.
Dew point chart
Dew point in °F for air temperatures of 50 to 100 °F. The cell nearest your conditions is highlighted.
| Temp (°F) | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% | 100% |
|---|---|---|---|---|---|---|---|---|---|
| 50 | 10 | 20 | 27 | 32 | 37 | 41 | 44 | 47 | 50 |
| 55 | 15 | 24 | 31 | 37 | 41 | 45 | 49 | 52 | 55 |
| 60 | 19 | 28 | 36 | 41 | 46 | 50 | 54 | 57 | 60 |
| 65 | 23 | 33 | 40 | 46 | 51 | 55 | 59 | 62 | 65 |
| 70 | 27 | 37 | 45 | 51 | 55 | 60 | 64 | 67 | 70 |
| 75 | 31 | 41 | 49 | 55 | 60 | 65 | 68 | 72 | 75 |
| 80 | 35 | 46 | 54 | 60 | 65 | 69 | 73 | 77 | 80 |
| 85 | 39 | 50 | 58 | 64 | 70 | 74 | 78 | 82 | 85 |
| 90 | 44 | 54 | 62 | 69 | 74 | 79 | 83 | 87 | 90 |
| 95 | 48 | 59 | 67 | 73 | 79 | 84 | 88 | 92 | 95 |
| 100 | 52 | 63 | 71 | 78 | 84 | 88 | 93 | 97 | 100 |
Magnus formula over liquid water with Alduchov and Eskridge (1996) coefficients. Results are estimates for weather, comfort and building use, not a substitute for a calibrated hygrometer.
What Is Dew Point?
The dew point is the temperature air must cool to, at constant pressure, before it reaches 100 percent relative humidity. Cool it any further and water vapor condenses into dew, fog or cloud. This dew point calculator finds that temperature from the air temperature and relative humidity, and it also works backward to find relative humidity from the dew point, or the air temperature from the dew point and humidity.
Because the dew point depends only on how much water vapor the air holds, it stays the same when a sunny afternoon warms the air or a clear night cools it, as long as no moisture is added or removed. Relative humidity swings with every temperature change. That is why forecasters use the dew point to describe how humid it feels, and why HVAC technicians, painters, growers and pilots use it to predict condensation, fog and frost.
Along with the main answer, the calculator reports vapor pressure, saturation vapor pressure, absolute humidity in grams per cubic meter, the mixing ratio in grams per kilogram and grains per pound, the dew point depression and, below freezing, the frost point. Every result shows the formula with your own numbers.
Dew Point Formula (Magnus Equation)
Saturation vapor pressure, the most water vapor air can hold, rises almost exponentially with temperature. The calculator uses the Magnus form of that curve with the coefficients of Alduchov and Eskridge (1996), the set Lawrence (2005) recommends for converting between relative humidity and dew point. Temperatures are in degrees Celsius and pressures in hectopascals (1 hPa equals 1 millibar).
The variable γ is the natural log of the actual vapor pressure divided by 6.1094 hPa. The second line inverts the Magnus curve to find the temperature whose saturation vapor pressure equals that actual vapor pressure, which is the definition of the dew point.
Relative humidity and temperature from the dew point
The first expression divides the vapor pressure at the dew point by the saturation vapor pressure at the air temperature. The second solves the dew point equation for T, so all three modes of the calculator share one curve and agree with each other.
How to Calculate Dew Point Manually
- Convert to Celsius. Subtract 32 from a Fahrenheit temperature and multiply by 5 ÷ 9.
- Take the natural log of the humidity. Use RH as a fraction: ln(0.50) = −0.6931. Use ln, not log base 10.
- Add the temperature term. Compute 17.625 × T ÷ (243.04 + T) and add it to step 2. The sum is γ.
- Solve for the dew point. Divide 243.04 × γ by 17.625 − γ. The answer is in degrees Celsius.
- Convert back if needed. Multiply by 9 ÷ 5 and add 32 for Fahrenheit.
Worked example in Celsius for air at 30 °C and 50 percent relative humidity:
Quick rule of thumb
Lawrence (2005) points out a mental shortcut for moist air: above 50 percent relative humidity the dew point falls about 1 °C for every 5 percent drop in humidity, starting from the air temperature at 100 percent.
| Air temp | RH | Magnus | Rule of thumb | Difference |
|---|---|---|---|---|
| 25 °C | 60% | 16.70 °C | 17.0 °C | +0.30 °C |
| 15 °C | 80% | 11.58 °C | 11.0 °C | −0.58 °C |
| 0 °C | 80% | −3.04 °C | −4.0 °C | −0.96 °C |
| 30 °C | 50% | 18.45 °C | 20.0 °C | +1.55 °C |
The shortcut stays within 1 °C over most of 0 to 30 °C and 50 to 100 percent, as Lawrence reports, but drifts to 1.55 °C at 30 °C and 50 percent. Below 50 percent humidity the rule does not apply, so use the full formula.
What Is a Comfortable Dew Point? (Dew Point Comfort Chart)
The National Weather Service office in La Crosse, Wisconsin publishes general summer comfort levels based on dew point. The comfort scale in the results uses these bands.
| Dew point (°F) | Dew point (°C) | How it feels in summer |
|---|---|---|
| 55 °F or lower | 12.8 °C or lower | Dry and comfortable |
| 55 to 65 °F | 12.8 to 18.3 °C | Becoming sticky, with muggy evenings |
| 65 °F or higher | 18.3 °C or higher | Lots of moisture in the air, becoming oppressive |
Dew point vs humidity
Relative humidity alone can mislead. The NWS compares 30 °F air with a 30 °F dew point, which is at 100 percent humidity, to 80 °F air with a 60 °F dew point, which is near 50 percent. The warm day feels far more humid because it holds much more water vapor.
| Air temp | RH at a 60 °F dew point | RH at a 70 °F dew point |
|---|---|---|
| 70 °F | 70.6% | 100% |
| 80 °F | 50.5% | 71.6% |
| 90 °F | 36.7% | 52.0% |
A 60 °F dew point reads 70.6 percent humidity at 70 °F but only 36.7 percent at 90 °F, yet the moisture in the air is identical. Sweat evaporates more slowly into air that already holds more vapor, a link John Dalton measured in 1802, so the dew point is the better guide to how sticky the air feels.
Dew Point Chart in Celsius
Dew points in degrees Celsius from the same Magnus formula, rounded to 0.1 °C. The results panel adds a Fahrenheit chart from 50 to 100 °F, or a Celsius chart from 10 to 40 °C, with your conditions highlighted.
| Air temp | 30% | 40% | 50% | 60% | 70% | 80% | 90% |
|---|---|---|---|---|---|---|---|
| 0 °C | −15.5 | −12.0 | −9.2 | −6.8 | −4.8 | −3.0 | −1.4 |
| 5 °C | −11.2 | −7.5 | −4.6 | −2.1 | 0.0 | 1.8 | 3.5 |
| 10 °C | −6.8 | −3.0 | 0.0 | 2.6 | 4.8 | 6.7 | 8.4 |
| 15 °C | −2.4 | 1.5 | 4.7 | 7.3 | 9.6 | 11.6 | 13.4 |
| 20 °C | 1.9 | 6.0 | 9.3 | 12.0 | 14.4 | 16.4 | 18.3 |
| 25 °C | 6.2 | 10.5 | 13.9 | 16.7 | 19.1 | 21.3 | 23.2 |
| 30 °C | 10.5 | 14.9 | 18.4 | 21.4 | 23.9 | 26.2 | 28.2 |
| 35 °C | 14.8 | 19.4 | 23.0 | 26.1 | 28.7 | 31.0 | 33.1 |
Read across from the air temperature to the humidity column. A cellar at 15 °C and 70 percent has a dew point of 9.6 °C, so a cold water line at 8 °C running through it will drip.
Condensation, Fog and Frost
Windows and pipes
Condensation forms on any surface whose temperature is at or below the dew point of the air touching it. Room air at 70 °F and 40 percent relative humidity has a dew point of 44.6 °F, so a window pane that cools below 44.6 °F fogs on the room side. At 30 percent the dew point drops to 37.1 °F, and at 60 percent it climbs to 55.5 °F, high enough for a 55 °F cold water pipe to sweat.
The EPA advises keeping indoor humidity below 60 percent, ideally between 30 and 50 percent, and drying condensation on windows, walls or pipes quickly because it can be a sign of high humidity. The badge in the results compares your humidity with that range.
Painting and coatings
Coating manufacturers set a minimum gap between surface temperature and dew point so that no thin film of water forms under the coating. The PPG technical data sheet for BR DTM High Build Epoxy Primer, for example, states that the surface temperature must be at least 5 °F (3 °C) above the dew point. The card under the result shows that minimum surface temperature. Always follow the data sheet for the product you use.
Fog, dew and the frost point
Overnight, the ground and the air near it cool toward the dew point. When they reach it, dew forms on grass and fog can form in the air. A small dew point depression in the evening, the gap between air temperature and dew point, signals that little cooling is needed.
Below freezing, water vapor deposits as ice, and ice saturates at a lower vapor pressure than liquid water, so the frost point is slightly warmer than the dew point. The calculator uses the Alduchov and Eskridge ice curve, ei(T) = 6.1121 × exp(22.587 × T ÷ (273.86 + T)). For 32 °F and 85 percent, the vapor pressure is 5.19 hPa, the dew point is 28.0 °F (−2.22 °C) and the frost point is 28.5 °F (−1.96 °C).
Vapor Pressure, Absolute Humidity and Mixing Ratio
Once the vapor pressure e is known, other humidity measures follow from the ideal gas law. Absolute humidity is the mass of water vapor in a cubic meter of air. Mixing ratio is the mass of water vapor per kilogram of dry air and depends on air pressure, which is why the calculator accepts a station pressure. HVAC work in the United States often quotes it in grains per pound, and 1 gram per kilogram equals 7 grains per pound.
At a station pressure of 840 hPa, typical of high altitude cities, the same air keeps the same absolute humidity but its mixing ratio rises to 14.37 g/kg, because the same vapor is mixed with less dry air. Dew point and relative humidity do not depend on pressure in this calculator.
How to Use the Calculator
- Solve for. Pick dew point, relative humidity or air temperature.
- Units. Choose °F or °C. Entered values convert when you switch.
- Air temperature. The dry bulb reading from a thermometer in shade.
- Relative humidity. A percent above 0 and up to 100.
- Dew point. At or below the air temperature.
- Air pressure. Optional. It changes only the mixing ratio.
Common Mistakes
- Fahrenheit in the formula. The Magnus coefficients expect °C.
- Log base 10. The formula uses the natural log, ln.
- Typing 0.6 for 60 percent. The field expects a percent.
- Surface for air temperature. A sunny wall can be far warmer than the air.
- Expecting the dew point to fall at night. It holds steady; RH rises instead.
- The shortcut below 50 percent. Use the full formula there.
Accuracy and Limitations
Alduchov and Eskridge fitted the coefficients to reference formulations of saturation vapor pressure and report a maximum relative error under 0.384 percent over water from −40 to 50 °C, and under 0.213 percent over ice from −80 to 0 °C. At 25 °C the vapor pressure curve rises about 6 percent per degree, so a 0.384 percent error shifts the dew point by only about 0.06 °C.
Other calculators use different constants. The NWS El Paso dew point calculator uses 6.112, 17.67 and 243.5, the values published by Bolton (1980). Across −40 to 50 °C the two sets differ by no more than 0.04 °C in dew point.
Sensor error matters more. At 25 °C, a reading of 60 ± 2 percent relative humidity gives a dew point between 16.16 and 17.22 °C, a spread of about 1 °C. The formula also treats relative humidity as relative to liquid water, the convention most weather instruments follow, and assumes the air is not supersaturated.
Frequently Asked Questions
How do you calculate dew point from temperature and humidity?
Convert the air temperature to degrees Celsius and use the Magnus formula with the Alduchov and Eskridge coefficients. First find γ = ln(RH ÷ 100) + 17.625 × T ÷ (243.04 + T), then the dew point is Td = 243.04 × γ ÷ (17.625 − γ). For 77 °F (25 °C) at 60 percent relative humidity, γ = −0.5108 + 1.6439 = 1.1331, so Td = 243.04 × 1.1331 ÷ 16.4919 = 16.70 °C, or 62.1 °F. These coefficients keep saturation vapor pressure within 0.384 percent of reference values from −40 to 50 °C, which is less than 0.1 °C of dew point error.
What is a comfortable dew point?
The National Weather Service office in La Crosse, Wisconsin lists these summer comfort levels: a dew point of 55 °F (12.8 °C) or lower feels dry and comfortable, 55 to 65 °F (12.8 to 18.3 °C) is becoming sticky with muggy evenings, and 65 °F (18.3 °C) or higher puts lots of moisture in the air and becomes oppressive. Dew point tracks comfort better than relative humidity because it measures the actual amount of water vapor. Sweat evaporates more slowly into air that already holds more vapor, so a high dew point day feels stickier at the same temperature.
What is the difference between dew point and relative humidity?
Relative humidity compares the water vapor present with the most the air could hold at its current temperature, so it rises and falls with temperature even when no moisture is added. Dew point is the temperature the air must cool to, at constant pressure, to reach 100 percent relative humidity, and it changes only when the moisture content changes. The NWS gives an example: 30 °F air with a 30 °F dew point is at 100 percent humidity, while 80 °F air with a 60 °F dew point is near 50 percent, yet the 80 °F day feels far more humid because its dew point is higher.
How do you calculate relative humidity from dew point?
Run the Magnus formula in reverse with both temperatures in degrees Celsius: RH = 100 × exp(17.625 × Td ÷ (243.04 + Td) − 17.625 × T ÷ (243.04 + T)). This is the vapor pressure at the dew point divided by the saturation vapor pressure at the air temperature. For 80 °F (26.67 °C) air with a 65 °F (18.33 °C) dew point, the two terms are 1.2363 and 1.7426, so RH = 100 × exp(−0.5064) = 60.3 percent. In vapor pressure terms that is 21.03 hPa ÷ 34.90 hPa.
What is the dew point rule of thumb?
For moist air above 50 percent relative humidity, Lawrence (2005) in the Bulletin of the American Meteorological Society describes a quick rule: the dew point falls about 1 °C for every 5 percent drop in relative humidity, starting from the air temperature at 100 percent. Written out, Td ≈ T − (100 − RH) ÷ 5 in degrees Celsius. At 25 °C and 60 percent it gives 17.0 °C, against 16.70 °C from the Magnus formula. Lawrence reports accuracy better than 1 °C over most of 0 to 30 °C and 50 to 100 percent RH; the largest gap in that range is 1.55 °C, at 30 °C and 50 percent.
Why do windows and cold water pipes sweat?
A surface collects condensation when it is at or below the dew point of the air touching it. Room air at 70 °F and 40 percent relative humidity has a dew point of 44.6 °F, so a window pane colder than that fogs on the room side while a 55 °F cold water pipe stays dry. At 60 percent the dew point rises to 55.5 °F and the same pipe starts to drip. The EPA advises keeping indoor humidity below 60 percent, ideally 30 to 50 percent, and drying condensation quickly because it can signal high humidity.
Sources & References
- Alduchov, O. A. and Eskridge, R. E. Improved Magnus Form Approximation of Saturation Vapor Pressure. Journal of Applied Meteorology 35(4), 1996 (coefficients 17.625, 243.04 °C and 6.1094 hPa). doi.org
- Alduchov, O. A. and Eskridge, R. E. Improved Magnus' Form Approximation of Saturation Vapor Pressure, technical report, November 1997 (error limits over water and ice, ice coefficients). UNT Digital Library
- Lawrence, M. G. 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), 2005 (recommended coefficients, rule of thumb and its accuracy, Dalton's measurements). doi.org
- National Weather Service, La Crosse, WI. Dew Point vs Humidity (definition, summer comfort levels and the 30 °F and 80 °F example). weather.gov
- NOAA NESDIS. What Is Humidity? (dew point is always at or below the air temperature). nesdis.noaa.gov
- National Weather Service, El Paso, TX. Relative Humidity and Wet Bulb from Dewpoint calculator (constants 6.112, 17.67 and 243.5). weather.gov
- U.S. Environmental Protection Agency. A Brief Guide to Mold, Moisture and Your Home (indoor humidity below 60 percent, ideally 30 to 50 percent). epa.gov
- PPG. BR DTM High Build Epoxy Primer technical data sheet (surface at least 5 °F, 3 °C, above the dew point). ppg data sheet
Results are estimates for informational use and do not replace calibrated instruments. Every number on this page was recomputed with the same formulas the calculator uses. Methodology reviewed: 2026.