Dew Point Calculator

A dew point calculator converts between the two common ways of describing moisture in the air. Relative humidity says how close the air is to saturation at its current temperature; the dew point says how much water vapor is actually there, expressed as a temperature. Give the calculator an air temperature and either one, and it returns the other, along with the heat index, the National Weather Service comfort level and the spread between temperature and dew point.

Two sister tools help with the inputs and the next steps. The Celsius to Fahrenheit Converter switches a reading between scales, and the BTU Calculator estimates the heating or cooling a room needs. Use this page when the question is about moisture: how muggy it feels, whether windows will fog, or how close the air is to dew, fog and cloud.

What dew point and relative humidity measure

The National Weather Service defines the dew point as the temperature the air needs to be cooled to, at constant pressure, to reach a relative humidity of 100%. At that point the air cannot hold more water as a gas, and further cooling forces it out as dew, fog or precipitation. Because the dew point depends only on how much water vapor is present, it stays nearly constant as an afternoon warms up, while relative humidity falls.

That is why relative humidity can mislead. The NWS compares 30°F with a 30°F dew point, which is 100% relative humidity, to 80°F with a 60°F dew point, which is about 50% (the formula below gives 50.6%). The warm day feels far more humid despite the lower percentage, because its air carries much more water. For judging how humid it feels outside, the dew point is the better number.

The dew point formula (Magnus)

Both directions of the calculation use the Magnus approximation for the saturation vapor pressure over liquid water. With temperatures in °C:

The constants a = 17.62 and b = 243.12°C are the ones the NOAA Weather Prediction Center uses in its relative humidity calculator. They come from the World Meteorological Organization's Guide to Meteorological Instruments and Methods of Observation (WMO-No. 8, Annex 4.B), which gives the same equation for saturation over water and states its range as −45 to 60°C (−49 to 140°F); the calculator stays inside it by accepting −40 to 140°F. Fahrenheit readings are converted first with T(°C) = (T(°F) − 32) × 5/9, and the result is converted back. NOAA's heat index calculator still writes the vapor pressure in an older base-10 form, 6.11 × 10^(7.5T / (237.7 + T)); between 32°F and 110°F and at 10% relative humidity or more, that version gives dew points within 0.24°F of the constants used here.

Worked example: dew point at 80°F and 60% humidity

The spread between temperature and dew point is 15.1°F. In vapor pressure terms the air holds 20.93 hPa of water vapor out of a possible 34.88 hPa, which is the 60% you started with. A 64.9°F dew point sits in the NWS range that becomes sticky with muggy evenings, a tenth of a degree short of the oppressive level. The heat index at 80°F and 60% is 81.8°F, shown as 82°F, which falls in the NWS Caution category.

Worked example: humidity at 92°F with a 72°F dew point

A 72°F dew point is well into the oppressive range even though the humidity reads only 52%. The heat index is 99.8°F, shown as 100°F, in the NWS Extreme Caution category. A third check uses the National Weather Service's own example: at 96°F and 65% relative humidity the calculator returns a heat index of 121°F, the value the NWS gives, with a dew point of 82.3°F.

The same steps work directly in Celsius, without the conversion. Air at 30°C and 70% relative humidity gives γ = ln(0.70) + 17.62 × 30 / 273.12 and a dew point of 23.9°C (75.1°F), well into the oppressive range. Because the heat index is defined in Fahrenheit, the calculator converts 30°C to 86°F for that step: the result is 95°F, or 35°C, in the Extreme Caution category.

Dew point chart for common conditions

Dew points in °F, computed with the formula above and rounded to the nearest degree. Find the air temperature on the left and the relative humidity across the top.

Rounding matters near the comfort boundaries: 80°F at 60% is 64.9°F, which the chart shows as 65. At 100% relative humidity the dew point always equals the air temperature, so that column is left out.

Comfort levels by dew point

The National Weather Service office in La Crosse, Wisconsin publishes a simple guide for the summer months: a dew point of 55°F or lower feels dry and comfortable, between 55 and 65°F the air becomes sticky with muggy evenings, and at 65°F or higher there is lots of moisture in the air and it becomes oppressive. The table translates those dew points into relative humidity at three air temperatures.

The same 60°F dew point reads as 71% humidity on a 70°F morning and 37% on a 90°F afternoon, yet the air carries the same moisture both times. A 75°F dew point cannot occur in 70°F air, because the dew point can never exceed the air temperature.

Heat index: the NWS formula and a computed chart

The heat index is the National Weather Service's measure of how hot it feels when humidity is combined with the air temperature. NOAA's Weather Prediction Center documents the method. A simple formula, consistent with Steadman's original results, is computed first:

If that value is 80°F or higher, the full Rothfusz regression replaces it, with T in °F and RH in percent:

Two adjustments follow. When humidity is below 13% and the temperature is between 80 and 112°F, the result is lowered by ((13 − RH) / 4) × √((17 − |T − 95|) / 17); at 100°F and 10% that gives 94.1°F. When humidity is above 85% and the temperature is between 80 and 87°F, it is raised by ((RH − 85) / 10) × ((87 − T) / 5); at 84°F and 95% that gives 100.9°F. The NWS notes that the regression carries an error of about ±1.3°F and that heat index values were devised for shady, light-wind conditions, so full sunshine can raise them by up to 15°F. The NWS heat categories are:

Applying the method to a grid of conditions gives the chart below, in °F and rounded to whole degrees. Cells marked 125+ are in Extreme Danger; NOAA states that the regression is not valid for extreme combinations beyond the data it was built on, so the chart shows the category there instead of a number.

How to use this calculator

Choose Find dew point to enter an air temperature and a relative humidity, or Find humidity to enter an air temperature and a dew point. Pick Fahrenheit or Celsius; switching converts the numbers you have already typed. The headline shows the value you asked for with its comfort level, and the three tiles show the other moisture value, the heat index and the spread. Below them, one card explains the comfort level and the heat index category, and another walks through the Magnus calculation with your own numbers. The example buttons load the cases discussed on this page.

Who uses dew point, and for what

Homeowners and renters use it to understand condensation. Moisture condenses on any surface colder than the dew point, so indoor air at 70°F and 40% relative humidity, with a dew point of 44.6°F, will fog a window pane colder than that. At 30% the dew point drops to 37.1°F; at 50% it rises to 50.5°F. The EPA recommends keeping indoor humidity below 60%, ideally between 30 and 50%, and notes that condensation can be a sign of high humidity. The same logic explains sweating pipes and damp basement walls in summer: air at 85°F and 50% has a dew point of 64.3°F, so a cold-water pipe at 55°F collects droplets even though the room does not feel especially humid.

Pilots watch the spread. The FAA's Pilot's Handbook of Aeronautical Knowledge explains that rising unsaturated air cools about 5.4°F per 1,000 feet while its dew point falls about 1°F, so the two converge at roughly 4.4°F per 1,000 feet. Dividing the spread by 4.4 and multiplying by 1,000 estimates the base of convective clouds: the handbook's example of 85°F with a 71°F dew point gives about 3,180 feet above ground. When temperature and dew point are close together, fog and low clouds become likely. Travelers planning the trip itself can estimate the time en route with the Flight Time Calculator.

Outdoor workers, coaches and event planners use the heat index category to schedule breaks and shade, keeping in mind that the NWS describes wet bulb globe temperature, which adds wind and sunshine, as a particularly effective heat stress indicator for outdoor workers and athletes. Gardeners and photographers use the dew point to anticipate dew and frost: on clear, calm nights surfaces cool below the dew point, and below freezing the moisture arrives as frost.

• Compare days by dew point, not by relative humidity; the percentage swings with the temperature.
• Measure indoors away from windows, heaters and kitchens, which distort the reading.
• Read the heat index as a shade value and add caution for work in full sun.
• A spread that keeps shrinking through the evening points to dew or fog by morning.

Key terms

• Dew point (Td): the temperature at which the air would reach 100% relative humidity if cooled at constant pressure.
• Relative humidity (RH): actual vapor pressure divided by saturation vapor pressure at the air temperature, in percent.
• Vapor pressure (e): the part of the air pressure exerted by water vapor, in hectopascals (hPa).
• Saturation vapor pressure (es): the vapor pressure at which air of a given temperature is saturated.
• Spread: air temperature minus dew point; zero means saturated air.
• Heat index: the NWS apparent temperature that combines air temperature and relative humidity, valid for shade and light wind.
• Frost point: the saturation temperature over ice, slightly higher than the dew point over water below freezing.
• Magnus formula: the exponential approximation for saturation vapor pressure used here, with a = 17.62 and b = 243.12°C.

What changes the result

The inputs matter far more than the formula. A hygrometer that reads 3 points high (63% instead of 60% at 80°F) raises the dew point from 64.9°F to 66.3°F, and a thermometer that reads 1°F high (81°F at 60%) raises it to 65.8°F. Either error moves this example across the 65°F oppressive line. The choice of Magnus constants shifts results by a quarter of a degree at most in everyday conditions.

Rounding is the other common source of small differences. NOAA's own heat index calculator rounds the humidity to a whole percent before computing, so at 92°F it would use 52% where this calculator keeps 52.26%: 99.7°F instead of 99.8°F, both shown as 100°F. Air pressure is not an input, because the dew point is defined at the air's current pressure and the formula's pressure effect is negligible for everyday use.

Limitations

The calculator accepts air temperatures from −40 to 140°F (−40 to 60°C) and works with dew points down to −40°, inside the range the WMO gives for these constants; drier air than that gets a message instead of a number. Below freezing it reports the dew point over liquid water; the frost point over ice is slightly higher and is not computed. The heat index carries an error of about ±1.3°F, is not valid for extreme combinations, and ignores wind, sunshine, clothing and exertion. Like the chart above, the calculator shows a heat index of 125°F or more as 125+ with its Extreme Danger category instead of an exact number. Nothing here is a forecast: the results describe the air you enter, and conditions change hour by hour. The heat index category is an educational estimate, not a safety device, so follow NWS heat advisories and warnings for your area.

Related calculators

• To switch a temperature between scales, use the Celsius to Fahrenheit Converter.
• To size an air conditioner or heater for a room, use the BTU Calculator.
• For pressure, length and other unit conversions, use the Unit Converter.
• To estimate how long a flight takes between two airports, use the Flight Time Calculator.

Sources

• National Weather Service La Crosse, Dew Point vs Humidity: definition of the dew point and the summer comfort levels at 55°F and 65°F.
• NOAA Weather Prediction Center, Relative Humidity Calculator: Magnus constants 6.112 hPa, 17.62 and 243.12°C used in its calculation.
• World Meteorological Organization, Guide to Meteorological Instruments and Methods of Observation (WMO-No. 8, 2008), Annex 4.B, hosted by the National Weather Service: the same equation for saturation over water, valid from −45 to 60°C.
• NOAA Weather Prediction Center, The Heat Index Equation: simple formula, Rothfusz regression and both adjustments.
• NOAA Weather Prediction Center, Heat Index Calculator: reference implementation, including the base-10 vapor pressure form.
• National Weather Service Amarillo, Heat Index: heat index categories and the ±1.3°F error of the regression.
• National Weather Service, Heat Forecast Tools: 96°F and 65% gives a heat index of 121°F; full sunshine can add up to 15°F.
• FAA, Pilot's Handbook of Aeronautical Knowledge, Chapter 12: Weather Theory: 4.4°F per 1,000 feet convergence rule, dew and frost.
• EPA, A Brief Guide to Mold, Moisture and Your Home: indoor humidity below 60%, ideally 30 to 50%.