Interactive psychrometric chart
A psychrometric chart drawn for your elevation, in °F or °C. Plot a point from the dry-bulb temperature and relative humidity, wet-bulb, dew point or humidity ratio, and read every property of that air.
Click the chart to plot a point, or type the values. Barometric pressure 101.325 kPa (14.696 psia). Enthalpy lines are labelled in BTU/lb; wet-bulb lines in °F.
Chart data as a table
Humidity ratio (gr/lb) at each dry-bulb temperature for 20 %, 40 %, 60 %, 80 % and 100 % relative humidity, at this elevation. A dash means the value is above the top of the chart.
| Dry-bulb (°F) | 20 % RH | 40 % RH | 60 % RH | 80 % RH | 100 % RH |
|---|---|---|---|---|---|
| 30 | 4.8 | 9.6 | 14.4 | 19.2 | 24.1 |
| 35 | 5.9 | 11.9 | 17.8 | 23.8 | 29.8 |
| 40 | 7.2 | 14.5 | 21.7 | 29.0 | 36.4 |
| 45 | 8.8 | 17.6 | 26.4 | 35.3 | 44.2 |
| 50 | 10.6 | 21.2 | 31.9 | 42.6 | 53.4 |
| 55 | 12.7 | 25.5 | 38.4 | 51.3 | 64.4 |
| 60 | 15.2 | 30.6 | 46.0 | 61.6 | 77.3 |
| 65 | 18.2 | 36.5 | 55.0 | 73.7 | 92.5 |
| 70 | 21.6 | 43.5 | 65.5 | 87.8 | 110.3 |
| 75 | 25.6 | 51.6 | 77.8 | 104.4 | 131.2 |
| 80 | 30.3 | 61.0 | 92.1 | 123.7 | 155.7 |
| 85 | 35.6 | 71.8 | 108.7 | 146.1 | 184.2 |
| 90 | 41.8 | 84.4 | 127.9 | 172.2 | — |
| 95 | 48.9 | 98.9 | 150.1 | 202.4 | — |
| 100 | 57.0 | 115.6 | 175.7 | — | — |
| 105 | 66.3 | 134.8 | 205.3 | — | — |
| 110 | 77.0 | 156.7 | — | — | — |
| 115 | 89.1 | 181.8 | — | — | — |
Properties of this air
- Dry-bulb
- 75.0 °F
- Wet-bulb
- 62.6 °F
- Dew point
- 55.1 °F
- Relative humidity
- 50.0 %
- Humidity ratio
- 64.6 gr/lb
- Enthalpy
- 28.11 BTU/lb
- Specific volume
- 13.679 ft³/lb
- Vapor pressure
- 1.483 kPa
How to read the chart
- Dry-bulb temperature runs along the bottom. Vertical lines are constant dry-bulb.
- Humidity ratio — the weight of water vapor per pound (or kilogram) of dry air — runs up the right side. Horizontal lines are constant moisture, so they are also constant dew point.
- The heavy curved line on the left is saturation (100 % relative humidity). Air cannot hold more moisture than this; to its left is fog.
- The blue curves are relative humidity, every 10 %.
- The dashed lines slope down to the right and are constant wet-bulb temperature, labelled where they meet the saturation curve. The dotted lines, if you turn them on, are constant enthalpy.
How it is made
Nothing on this chart is traced from a printed chart. Every line is calculated with the moist-air equations of the ASHRAE Handbook—Fundamentals as implemented in the open-source PsychroLib library: the Hyland–Wexler saturation pressure, the humidity ratio, the thermodynamic wet-bulb relation and the enthalpy equation. Barometric pressure comes from the standard atmosphere at your elevation. The point values are tested against PsychroLib, and the same functions run the site’s dew point and cooling capacity calculators.
The chart covers about 30 °F to 115 °F (−1 °C to 46 °C) and up to 210 grains per pound (30 g/kg). Points outside that window are still calculated, and the property list shows them.
Common uses
- Air across a cooling coil: plot the return and supply air to see how much the coil cools and dries the air. The Cooling Capacity Calculator turns the two states and the airflow into BTU/h.
- Condensation risk: any surface colder than the dew point of the room air will collect moisture. The Dew Point Calculator gives it directly.
- Mixing outdoor and return air: the mixed state lies on the straight line between the two points. The Air Mixing Calculator finds it.
Frequently asked questions
Why does the chart change with elevation?
At altitude there is less air, so the same amount of water vapor is a larger share of it. The saturation curve and every humidity line move up: at 5,000 ft, saturated air at 70 °F holds about 133 grains of moisture per pound of dry air, against about 110 at sea level. Charts printed for sea level give wrong humidity ratios and enthalpies at altitude.
Why are the enthalpy values different in °F and °C?
The two unit systems measure enthalpy from different starting points: 0 °F dry air for BTU/lb and 0 °C dry air for kJ/kg. Differences in enthalpy convert normally (1 BTU/lb = 2.326 kJ/kg), but the values themselves do not.
Is the wet-bulb here what my sling psychrometer reads?
The chart uses the thermodynamic wet-bulb temperature. A well-ventilated psychrometer with a clean, wet wick reads very close to it; a poorly ventilated one reads high.