Relative humidity calculator
85 °F with a 65 °F dew point. Enter the temperature and dew point from a weather report or a sensor, and the relative humidity calculator gives the RH with the formula behind it.
Notice: For general informational purposes only. Heat index values assume shade and light wind and do not account for exertion or individual health. Follow National Weather Service heat advisories, and seek medical attention for symptoms of heat-related illness.
Relative humidity
- Dew point As entered. The temperature this air would have to be cooled to, at constant pressure, for its water vapor to start condensing.
- 65.0 °F (18.3 °C)
- Relative humidity U = 100 x ew(td) / ew(t) = 100 x 21.03 hPa / 41.01 hPa (WMO-No. 8, eq. 4.B.11). The share of the vapor the air could hold at 85.0 °F that it actually holds.
- 51.3%
- Dew point reading In the words of the National Weather Service office at La Crosse, whose "general comfort levels using dew point" for the summer months are: 55 °F or less "dry and comfortable"; between 55 and 65 °F "becoming "sticky" with muggy evenings"; 65 °F or more "lots of moisture in the air, becoming oppressive". It is a summer scale.
- lots of moisture in the air, becoming oppressive
- Wet-bulb temperature The reading of a thermometer wrapped in a wet wick with air drawn past it: evaporation cools it until the heat taken from the air balances the heat carried off by the water. Solved from WMO's psychrometer equation e' = f ew(tw) - 0.000653 x (1 + 0.000944 tw) x p x (t - tw) at 1,013 hPa (eq. 4.B.8). It always sits between the dew point and the air temperature.
- 71.3 °F (21.8 °C)
- Heat index NWS's Rothfusz regression, used because the simple formula came to 80 °F or more. NWS: "The Rothfusz regression is not valid for extreme temperature and relative humidity conditions beyond the range of data considered by Steadman."
- 86.8 °F (30.4 °C)
- NWS heat index class NWS Amarillo's classification, effect on the body: "Fatigue possible with prolonged exposure and/or physical activity." The same page gives its values for shady locations: "If you are exposed to direct sunlight, the heat index value can be increased by up to 15°F."
- Caution
- Vapor pressure The partial pressure of the water vapor in the air. Saturation at 85.0 °F would be 41.2 hPa: ew(t) = 6.112 x exp(17.62 t / (243.12 + t)) hPa (eq. 4.B.1) times the moist-air enhancement factor f = 1.0047.
- 21.13 hPa (0.6238 inHg)
- Absolute humidity Grams of water vapor in each cubic meter of air: e' / (Rv x T), with Rv = R / (0.62198 x M_air) = 461.5 J/(kg K). It changes with temperature even when the dew point does not, because warm air takes up more room.
- 15.1 g/m3
- Humidity ratio Mass of water vapor per mass of dry air, r = 0.62198 x e' / (p - e'), at 1,013 hPa. Grains per pound is the same figure times 7, since a pound is 7,000 grains. This is the number on the vertical axis of a psychrometric chart, and the one HVAC load calculations use.
- 13.2 g/kg (92.7 gr/lb)
- Air pressure used The 1976 U.S. Standard Atmosphere at sea level. A real day's pressure moves a few percent with the weather, which shifts the wet bulb and the humidity ratio slightly and the dew point not at all.
- 1,013.3 hPa (29.92 inHg)
| Relative humidity | Dew point | Wet bulb | Heat index | Humidity ratio |
|---|---|---|---|---|
| 10% | 22.3 °F | 55.5 °F | 81.4 °F | 2.54 g/kg |
| 20% | 39.4 °F | 59.8 °F | 82.0 °F | 5.10 g/kg |
| 30% | 50.1 °F | 63.8 °F | 82.9 °F | 7.68 g/kg |
| 40% | 58.0 °F | 67.5 °F | 84.3 °F | 10.3 g/kg |
| 50% | 64.3 °F | 70.9 °F | 86.5 °F | 12.9 g/kg |
| 60% | 69.6 °F | 74.1 °F | 89.3 °F | 15.6 g/kg |
| 70% | 74.1 °F | 77.1 °F | 92.7 °F | 18.2 g/kg |
| 80% | 78.1 °F | 79.9 °F | 96.8 °F | 20.9 g/kg |
| 90% | 81.7 °F | 82.5 °F | 101.8 °F | 23.6 g/kg |
| 100% | 85.0 °F | 85.0 °F | 107.6 °F | 26.4 g/kg |
- Saturation, dew point, frost point, relative humidity and the psychrometer equation are WMO's Annex 4.B formulas (WMO-No. 8, Volume I, Chapter 4), valid for -45 to 60 °C over water and -65 to 0 °C over ice. WMO calls them sufficiently accurate for all normal meteorological applications.
- The heat index is the National Weather Service's, computed in °F as its Weather Prediction Center describes. It is for shade; it is not a measure of what any particular person will feel.
- Relative humidity here is with respect to liquid water at every temperature, as WMO prescribes for reporting.
- Not modeled: supersaturation, salt or other solutes, and the heat a real wet-bulb thermometer gains from sunlight or loses through poor ventilation.
How the Dew Point Calculator works
Relative humidity is one ratio of two saturation vapor pressures:
RH = 100 x ew(td) / ew(t) ew(t) = 6.112 x exp(17.62 x t / (243.12 + t)) hPa, t in °C
The top of the fraction is the vapor pressure the air actually has, which is by definition the saturation pressure at its dew point. The bottom is the most it could have at its present temperature. This is equation 4.B.11 of WMO-No. 8, the World Meteorological Organization's guide to instruments and methods of observation.
The worked example above: 85 °F (29.44 °C) with a dew point of 65 °F (18.33 °C). ew(18.33) = 21.03 hPa and ew(29.44) = 41.01 hPa, so RH = 100 x 21.03 / 41.01 = 51.3%.
The point of running it this way round is that the dew point is the stable number. Hold it at 65 °F and move only the thermometer:
at 70 °F the relative humidity is 84.2% at 85 °F it is 51.3% at 95 °F it is 37.5%
Same air, same water, three different humidities. This is the ordinary daily cycle: the moisture barely changes from dawn to afternoon, the temperature climbs, and the relative humidity falls. It can drop from near 100% at sunrise to half that by mid-afternoon. A morning forecast of "90% humidity" and an afternoon one of "45%" can describe the same air mass.
It also explains winter indoor air. Outside air at 23 °F and 70% relative humidity has a dew point of 14.7 °F. Heat it to 70 °F without adding water and its dew point stays at 14.7 °F, so its relative humidity drops to 11.8%. Nothing dried the air; the heater raised the ceiling. The reverse is why basements are damp in summer: outside air with a 65 °F dew point that leaks into a 68 °F basement is at about 90% relative humidity there, and anything colder than 65 °F gets wet.
Two details. Relative humidity is reported against liquid water at every temperature, including below freezing, so the same air reads differently against ice. WMO prescribes it because most hygrometers respond that way and most cold clouds are supercooled water. And the moist-air enhancement factor WMO uses for real air cancels from this ratio, so elevation has no effect: a dew point and temperature give the same relative humidity in Denver as in Miami.
A dew point higher than the air temperature would mean more than 100%, which the formula cannot describe. The page holds such an entry at the air temperature and says so.
Dew Point Calculator FAQ
How do you calculate relative humidity from dew point?
RH = 100 x ew(td) / ew(t), where ew(x) = 6.112 x exp(17.62 x / (243.12 + x)) hPa and both temperatures are in °C. At 85 °F (29.44 °C) with a 65 °F (18.33 °C) dew point: 100 x 21.03 / 41.01 = 51.3%.
Why is relative humidity highest in the morning?
Because the air is coolest then. The amount of water in the air (the dew point) usually changes little over a day, while the temperature rises after sunrise, so the same vapor becomes a smaller share of what the air could hold. Air with a 65 °F dew point is at 84% at 70 °F and 37.5% at 95 °F.
Why is indoor air so dry in winter?
Heating raises the temperature without adding water. Air at 23 °F and 70% relative humidity has a dew point of 14.7 °F; warmed to 70 °F it still has that dew point, which is 11.8% relative humidity. Only adding water raises the dew point.
Can relative humidity be over 100%?
Against liquid water, only briefly and slightly, which is how cloud droplets form. Against ice, yes and often: below freezing the air can hold more vapor than an ice surface would leave in equilibrium, which is how frost and snow grow. Reported relative humidity is against water, and this calculator does not model supersaturation.
More from the Dew Point Calculator
Sources
- WMO Guide to Instruments and Methods of Observation (WMO-No. 8), Volume I, Chapter 4 'Measurement of humidity', preliminary 2018 edition: Annex 4.B 'Formulae for the computation of measures of humidity' eqs. (4.B.1)-(4.B.11) and its range statement; Annex 4.A eqs. (4.A.5), (4.A.6) and (4.A.13) for the 0.621 98 mass ratio; section 4.1.2 for the units (hPa, °C) (2018)
- NOAA/NWS Weather Prediction Center, 'The Heat Index Equation' (Rothfusz regression from NWS Technical Attachment SR 90-23, 1990, its low- and high-humidity adjustments, and the simpler Steadman-consistent formula for heat indices below about 80 °F); the 40 °F pass-through is from the WPC heat index calculator's own script (https://www.wpc.ncep.noaa.gov/html/heatindex.shtml) (1990)
- NWS Amarillo, TX, 'Heat Index' page: the 'Classification / Heat Index / Effect on the body' table, and the statements that chart values are for shady locations and that direct sunlight can raise the heat index by up to 15 °F (2026)
- NWS La Crosse, WI, 'Dew Point vs Humidity': 'General comfort levels USING DEW POINT that can be expected during the summer months' (2026)
- U.S. Standard Atmosphere, 1976 (NOAA, NASA, USAF; NOAA-S/T 76-1562, NASA-TM-X-74335), Part 1: Table 2 'Adopted constants' (g0, R*, P0, r0, T0, gamma), Table 3 (O2 fractional volume), Table 4 'The defined reference levels and gradients of the linearly segmented temperature-height profile' (H_b, L_M,b), section 1.2.4 (M0 = 28.9644 kg/kmol), section 1.3.1 eqs. (33a)/(33b), eq. (18) for geopotential altitude, eq. (42) for density, eq. (50) for speed of sound, Table 10 (sea-level values); Part 4 Table I 'Geopotential Altitude, Metric Units' for the printed base temperatures and pressures (1976)