Enter dry-bulb temperature plus one other known value — wet-bulb, relative humidity, or dew point — and get the full set of moist-air properties: RH, humidity ratio, dew point, enthalpy, specific volume, and wet-bulb temperature. Includes a live, computed psychrometric chart.
101.325 kPa = sea level. Roughly 89.9 kPa at 1,000 m / 3,300 ft, ~84.6 kPa at 1,500 m / 5,000 ft — lower pressure at altitude changes every result below.
Every curve is computed live from the same formulas used for the results above (saturation vapor pressure at each temperature across the chart) — it is not a static picture.
Check several air states at once — e.g. a full duct run or a day's logged readings — instead of one point at a time. One row per line: dry-bulb,second value, using the same unit and "second known value" type selected above (currently relative humidity).
Moist air has more going on than a thermometer shows. This calculator takes your dry-bulb temperature (the plain air-temperature reading) plus one other measurement and works out the rest of the air's state using the same psychrometric relationships found in ASHRAE Fundamentals and printed psychrometric charts.
Relative humidity (RH) is how close the air is to saturation at its current temperature — 50% RH means the air is holding half the water vapor it could hold before water starts condensing out. It's the number most people are used to, but it's temperature-dependent: the same air parcel reads a different RH at a different temperature, which is why RH alone doesn't tell you how much moisture is actually in the air.
Humidity ratio (W), in grams of water per kilogram of dry air, answers that question directly — it's the actual mass of moisture carried by the air, independent of temperature. HVAC engineers use humidity ratio (not RH) for load calculations, dehumidifier sizing, and duct/coil moisture balances, because it doesn't shift as air is heated or cooled.
Dew point (Tdp) is the temperature air would have to be cooled to (at constant pressure and moisture content) before condensation begins. It's why a cold glass of water sweats and why a cooling coil below the dew point pulls water out of an airstream — coil dehumidification design starts with the entering air's dew point.
Wet-bulb temperature (Twb) is what a thermometer reads when its bulb is kept wet and air moves past it — evaporative cooling drops the reading below dry-bulb, and the drier the air, the bigger that drop. It's the classic field measurement (see the FAQ on the sling psychrometer) and it's the number that determines evaporative cooler performance and cooling tower approach.
Enthalpy (h) is the total heat content of the air — sensible heat from temperature plus latent heat carried by its moisture, in kJ per kg of dry air. Coil selection and total cooling-load calculations use enthalpy difference (entering minus leaving air) because it captures both the temperature drop and the moisture removed.
Specific volume (v) is how many cubic meters one kilogram of dry air occupies at the given temperature, moisture, and pressure — it's what converts a mass airflow (kg/s) into a volumetric airflow (m³/s or CFM) for fan and duct sizing, and it's why the same fan moves less mass of air at altitude, where pressure — and therefore density — is lower.
The chart plots dry-bulb temperature (x-axis) against humidity ratio (y-axis). The heavy curve along the top is the saturation line (100% RH) — air can't exist above it at that pressure without condensing. The lighter dashed curves below it are constant-RH lines at 75%, 50%, and 25%. Your calculated point is marked in red: how close it sits to the saturation curve is a quick visual read on how humid the air actually is, and its height above the x-axis is the humidity ratio directly.
Informational engineering tool. Verify critical or code-driven designs against ASHRAE Fundamentals or a licensed engineer.
The free calculator handles one air-state point at a time. Pro adds a batch calculator:
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