Sea level, 101.325 kPa standard atmosphere

Psychrometric chart reading: 24.0°C dry-bulb 50% relative humidity 12.9°C dew point

Tap the chart or type two readings

Read the Chart

Every line is drawn from the equations for this altitude and carries its value where it runs: relative humidity curves, dashed wet-bulb lines, enthalpy hairlines read on the scale past the saturation curve, dotted lines of specific volume. The carmine pencil is your air, and each dashed construction runs from the point to the number it gives.

Hatched: the ASHRAE 55 comfort zones for 0.5 clo, summer clothing, and 1.0 clo, winter clothing, at 1.1 met and still air. They are computed from the PMV model, not traced from the standard's figure, and they are a model of most people, not a promise to anyone.

Set the Air

The Reading

Dry-bulbyour reading: 24.0°C
Relative humidityeq. 12 and 22: pw ÷ pws: 50.0%
Humidity ratioeq. 20: 0.621945 pw ÷ (p − pw): 9.30g/kg dry air
Dew pointeq. 6 solved for t: 12.9°C
Wet-bulbeq. 33 and 35 solved for t*: 17.1°C
Enthalpyeq. 30: 1.006 t + W (2501 + 1.86 t): 47.8kJ/kg dry air
Specific volumeeq. 26: R T (1 + 1.607858 W) ÷ p: 0.8544m³/kg dry air
Densityeq. 11: (1 + W) ÷ v: 1.1813kg/m³
Vapour pressureeq. 20 solved for pw: 1.493kPa
Saturation pressureeq. 6, Hyland and Wexler: 2.985kPa
Degree of saturationW ÷ Ws, 2009 edition eq. 12: 0.493

Per kilogram of dry air. Equation numbers are those of the ASHRAE Handbook—Fundamentals (2017), chapter 1.

Draw a Process

Pick a change and read what it costs

Process

No process drawn yet. Pick one above, and the leaving air and its load appear here and on the chart.

How It Is Computed

Published equations, checked against published numbers

Every line on the chart and every number in the reading comes from the moist-air equations of the ASHRAE Handbook—Fundamentals (2017), chapter 1, computed in this page for the pressure of the altitude you set. Nothing is read from a printed chart or a table. The equation numbers beside each figure are the Handbook's.

Pressure From Altitude

The standard atmosphere, eq. 3: p = 101.325 (1 − 2.25577 × 10⁻⁵ Z)^5.2559 kPa, with Z in metres. A chart printed for sea level is wrong in Denver: at 1,600 m the air holds about a fifth more water per kilogram before it saturates, so every line here moves when the altitude does. Weather moves the real pressure a few percent around this figure; the page uses the standard atmosphere, not a barometer.

Saturation

The saturation vapour pressure is Hyland and Wexler's fit, eq. 6 over liquid water and eq. 5 over ice below the triple point, 0.01 °C:

ln p_ws = C₈/T + C₉ + C₁₀T + C₁₁T² + C₁₂T³ + C₁₃ ln Teq. 6, T in kelvin, p_ws in Pa

The humidity ratio at saturation follows from eq. 20 with p_w = p_ws. That is the curve every other line ends on. Below freezing the dew point becomes a frost point, read over ice, and the reading says so.

The Point

W = 0.621945 p_w ÷ (p − p_w)eq. 20, humidity ratio

φ = p_w ÷ p_wseq. 12 and 22, relative humidity

h = 1.006 t + W (2501 + 1.86 t)eq. 30, enthalpy in kJ/kg, t in °C

v = 0.287042 (t + 273.15) (1 + 1.607858 W) ÷ peq. 26, specific volume in m³/kg, p in kPa

The wet-bulb is the thermodynamic wet-bulb, eq. 33 above freezing and eq. 35 on an iced wick, solved for t* by bisection between the dew point and the dry-bulb. The dew point is eq. 5 or 6 inverted by Newton's method on ln p.

SI and IP enthalpy are not conversions of each other. ASHRAE zeroes SI enthalpy at 0 °C and IP enthalpy at 0 °F, so 47.8 kJ/kg and 28.2 Btu/lb are both right for the same 24 °C air at 50 %. The page keeps the two equation sets apart, as the Handbook does.

Relative humidity is the least useful number here for a wall that sweats. The dew point says whether water will condense on a surface; 50 % says nothing until you know the temperature.

The Processes

Each is a steady-flow balance per kilogram of dry air, from the Handbook's chapter on typical air-conditioning processes:

Heat or Cool. The humidity ratio holds. Load = ṁ (h₂ − h₁). The page refuses to cool past the dew point, because water would condense and that is a different process.
Cool and Dry. The leaving air is the coil surface, saturated at the apparatus dew point, mixed with the share that bypasses it: W₂ = W_s,adp + BF (W₁ − W_s,adp), and the same for h. The load counts the condensate leaving at the coil temperature. Apparatus dew point and bypass factor are Carrier's method.
Evaporative. The air moves down its own wet-bulb line: t₂ = t₁ − ε (t₁ − t*). No heat is added; water is.
Mix Two Streams. Humidity ratio and enthalpy mix by dry-air mass. Where the straight line between the streams crosses the saturation curve the mixture fogs, and the page finds the saturated temperature where the energy balances and reports the water that falls out.

The Comfort Zones

The hatched zones are ASHRAE Standard 55-2017's graphical method, computed rather than traced: their edges are where Fanger's predicted mean vote reaches −0.5 and +0.5, by the program of ISO 7730:2005 Annex D, at 1.1 met, 0.1 m/s and a mean radiant temperature equal to the air, under a humidity ratio of 0.012.

Receipts

The engine is checked against 146 published values, inside a suite of 62 tests. None misses its stated tolerance. The largest disagreement is 2.13 %, the enthalpy of saturated air at −5 °C against the Handbook's real-gas table, where PsychroLib's own suite allows 3 %: ASHRAE's chapter 1 equations treat moist air as an ideal-gas mixture, and its tables do not.

Checked against	Values	Worst disagreement	Allowed
Saturation vapour pressure, Handbook Table 3, SI and IP	16	0.020 %	0.03 %
Humidity ratio at saturation, Table 2	14	1.20 % at 85 °C	0.5 to 2 %
Enthalpy at saturation, Table 2	14	2.13 % at −5 °C	1 to 3 %
Dry air, Table 2	4	0.097 %	0.1 %
Standard atmosphere, Table 1	24	0.48 Pa; 0.19 °F at 30,000 ft	1 Pa; 0.2 °F
The Handbook's worked Example 1, SI and IP	14	dew point 0.43 °C	0.5 °C
PMV and PPD, ISO 7730 Table D.1	24	PMV 0.053	0.1
PMV and PPD, ASHRAE 55 validation points	24	PMV 0.031	0.1
Comfort-zone edges against those points	8	0.12 °C	0.25 °C
The same air in SI and in IP	4	0.0125 Btu/lb	0.05 Btu/lb

The Handbook values are quoted from PsychroLib's test suite, which names the table each comes from. pnpm test prints the full table, one line per value group.

Where It Holds

Dry-bulb from −100 to 200 °C (−148 to 392 °F), the range of the saturation equations. Outside it the page refuses the reading and names the range. The drawing shows −10 to 50 °C and up to 30 g/kg; a point off the drawing still reads in full.
Altitude from −500 to 11,000 m (−1,600 to 36,000 ft), the troposphere, where the standard atmosphere's lapse rate holds.
Moist air as an ideal-gas mixture: within 1.2 % of the Handbook's real-gas humidity ratio up to 85 °C.
Processes assume steady flow and a perfectly mixed leaving state. A real coil has a performance curve this page does not have, and the coil is modelled wet, never frosted.
The comfort zones are the simplified graphical method: one activity, still air, no draught, no warm floor, no sun through the window.

Sources

ASHRAE, ASHRAE Handbook—Fundamentals (2017), chapter 1, Psychrometrics. Its published equations are implemented and cited by number; none of its tables or figures is reproduced. © ASHRAE.
D. Meyer and D. Thevenard, “PsychroLib: a library of psychrometric functions to calculate thermodynamic properties of air,” Journal of Open Source Software 4 (33): 1137, 2019, doi:10.21105/joss.01137. Version 2.5.0 and its SI and IP test suites at commit 3066345, MIT licence.
F. Tartarini and others, validation-data-comfort-models v1.0.0, the ISO 7730 and ASHRAE 55 points the comfort tests use, MIT licence.
Carrier Air Conditioning Company, Handbook of Air Conditioning System Design, McGraw-Hill, 1965: apparatus dew point and bypass factor.

Sources retrieved 8 October 2026.