The psychrometric calculator above solves the complete state of moist air. Give it a dry-bulb temperature, the barometric pressure and any one further property, and it returns the other properties that a psychrometric chart would give you: humidity ratio, enthalpy, wet-bulb temperature, dew point, specific volume, relative humidity and vapour pressure. It is the numerical equivalent of putting a pin in the chart and reading every axis.
Arb Digital builds free tools that show their working and name their sources. The saturation vapour pressure here comes from the formulation published in the ASHRAE Handbook — Fundamentals, and the psychrometric relationships are the standard ones from the same chapter. Nothing here is a curve fit invented for convenience, and where a quantity must be found by iteration, the page says so rather than pretending the answer is closed form.
What This Psychrometric Calculator Does
Moist air is a mixture of dry air and water vapour, and at a fixed pressure its thermodynamic state is fixed by any two independent properties. That is the whole basis of the psychrometric chart, and it is why this tool asks for a dry-bulb temperature plus one other measurement rather than for everything at once. Once those two are known, every remaining property follows.
The four second-property options cover the four ways people normally have a humidity measurement in hand: a capacitive sensor gives relative humidity, a sling psychrometer gives a wet-bulb, a chilled-mirror instrument gives a dew point, and a mixing calculation gives a humidity ratio directly. All four converge to the same state point.
Pressure is a required input rather than a convenience. The humidity ratio depends on the ratio of vapour pressure to total pressure, so the same temperature and relative humidity produce a different moisture content in Denver than at sea level, and a different specific volume again. Any psychrometric tool that hides the pressure has silently assumed 101.325 kPa on your behalf. The air pressure at altitude calculator will give you the station pressure to enter if you only know the elevation.
How to Use It
- Enter the dry-bulb temperature. This is the plain air temperature from a shaded, unwetted sensor. Choose the unit first, because the same selector governs the wet-bulb and dew point fields and the results.
- Enter the station pressure. Not the sea-level-corrected barometric pressure from a forecast. At 1,600 m the true station pressure is roughly 83 kPa, and using 101.325 there will overstate the specific volume by about a fifth.
- Choose which second property you have. Use the one you measured rather than one you derived, so that the measurement error enters the calculation once instead of twice.
- Enter its value. Relative humidity as a percentage, wet-bulb and dew point in the selected temperature unit, humidity ratio in grams of water per kilogram of dry air.
- Read the whole state, not just the headline. The enthalpy answers energy questions, the humidity ratio answers moisture-mass questions, and the dew point answers condensation questions. They are different questions, and the chart carries all three at once for a reason.
The Formulas: How the Moist Air State Is Calculated
The chain begins with saturation vapour pressure over liquid water, pws, as a function of absolute temperature. The formulation published in the psychrometrics chapter of the ASHRAE Handbook — Fundamentals expresses its natural logarithm as a sum of an inverse-temperature term, a constant, three polynomial terms and a logarithmic term, with a separate coefficient set used below the triple point where the saturated phase is ice rather than water. This calculator uses both sets and switches at 0 °C.
From there the relationships are algebraic. Relative humidity is the ratio of the actual vapour partial pressure to the saturation value at the same dry-bulb temperature, φ = pw/pws. The humidity ratio follows from the molecular mass ratio of water to dry air: W = 0.621945 pw/(p − pw). Specific enthalpy in kilojoules per kilogram of dry air is h = 1.006t + W(2501 + 1.86t), with t in degrees Celsius. Specific volume is v = 0.287042(t + 273.15)(1 + 1.607858W)/p, with pressure in kilopascals.
Two properties do not come out of a formula and must be found by iteration. The dew point is the temperature at which the saturation pressure equals the actual vapour pressure, which means inverting the saturation function, and this page does that by bisection to a tolerance far below any measurement you could make. The wet-bulb temperature satisfies an energy balance that has the wet-bulb value on both sides, so it is also bisected, over the interval between the dew point and the dry-bulb temperature, where the true value always lies.
Work the defaults. At 25 °C, 50 per cent relative humidity and 101.325 kPa, the saturation pressure is 3.1698 kPa, so the vapour pressure is 1.5849 kPa. The humidity ratio is 0.621945 × 1.5849 ÷ 99.7401 = 0.009884, or 9.884 g/kg. The enthalpy is 1.006 × 25 + 0.009884 × (2501 + 46.5) = 50.33 kJ/kg of dry air. The specific volume is 0.8581 m³/kg. Iteration puts the dew point at 13.85 °C and the wet-bulb at 17.87 °C. Those figures agree with the published psychrometric tables to the last digit shown.
Why Enthalpy Is the Number That Sizes Equipment
Ask how much cooling a space needs and the temperature drop alone will not answer it, because removing moisture costs energy that never shows up as a temperature change. Enthalpy carries both. The sensible part, 1.006t, is the energy associated with temperature. The latent part, W(2501 + 1.86t), is dominated by the 2501 kJ/kg latent heat of vaporisation and is the energy locked in the water vapour.
The consequence is worth stating plainly. In the default state above, of 50.33 kJ/kg total, 25.15 comes from temperature and 25.18 from moisture. Half the energy content of ordinary comfortable air is water. Cool that air to 12 °C saturated and you have to remove the moisture as well as the heat, and the dehumidification is the larger half of the job in a humid climate. A calculation that tracks only dry-bulb temperature will undersize the plant by a wide margin, which is why the sensible and latent split is the first thing a load calculation separates.
This is also why enthalpy is expressed per kilogram of dry air rather than per kilogram of mixture. The dry air is conserved through a coil while the water is not, so the dry air mass flow is a stable denominator. Multiply an enthalpy change in kJ/kg by the dry air mass flow in kg/s and the result is a power in kilowatts directly. Use a mixture basis and the denominator shifts as moisture condenses, and the arithmetic stops being a simple subtraction.
Wet-Bulb, Dew Point and the Difference People Conflate
Both are temperatures, both indicate humidity, and they are not the same measurement in any sense. The dew point is the temperature at which the air, cooled at constant pressure and constant moisture content, becomes saturated. It depends only on how much water is present. Two air samples with the same humidity ratio have the same dew point whether they are at 5 °C or 40 °C.
The wet-bulb temperature is what a thermometer with a wet wick reads in a moving airstream. Evaporation from the wick cools it until the cooling balances the heat arriving from the air, so the reading depends on both the temperature and the moisture. It always lies between the dew point and the dry-bulb, and it collapses onto both at saturation. The United States National Weather Service publishes its own dewpoint and wet-bulb from relative humidity calculator, a useful independent check on the two iterated values above.
The practical distinction: the dew point tells you what surface temperature causes condensation, so it is the number for glazing, cold pipework, ductwork and building fabric. The wet-bulb tells you the lowest temperature evaporative cooling can reach, so it is the number for cooling towers, swamp coolers and any adiabatic process. Asking a dew point to size a cooling tower, or a wet-bulb to predict condensation, gets an answer that is wrong in a direction that is hard to notice. The site also has a dedicated dew point calculator using the simpler Magnus approximation, and a relative humidity calculator for going the other way from a single reading.
Where the Answers Stop Being Reliable
The saturation formulation is valid from roughly −100 °C to 200 °C, which covers anything an HVAC or meteorological problem produces, but the moist air relations carry a narrower assumption. They treat the mixture as two ideal gases with a small correction absorbed into the constants, which holds well at atmospheric pressure. At high pressure that treatment degrades and a real-gas enhancement factor is needed.
The tool also refuses to report a supersaturated state. If the inputs imply more water vapour than saturation allows — a dew point above the dry-bulb, a relative humidity over 100, a humidity ratio beyond saturation — it clamps the state to the saturation line and says so, because air above that line holds liquid droplets rather than vapour. That is a fog, and a fog is outside the chart.
One more caveat concerns the wet-bulb itself. The value computed here is the thermodynamic wet-bulb temperature, a defined state property. What a real sling psychrometer reads is the psychrometric wet-bulb, which is close but not identical, because it depends on air velocity over the wick, on radiation to the bulb, and on the wick staying clean and wet. For air-water mixtures near room temperature the two agree closely enough to be interchangeable, which is an accident of the properties of water rather than a general rule.
How This Differs From the Adjacent Air Tools
The boundary in one sentence: this page returns the entire moist air state from any two properties, while each of the adjacent tools returns one property from one specific input pair. The dew point calculator gives the dew point from temperature and relative humidity. The absolute humidity calculator gives the vapour mass per cubic metre of mixture, which is a volumetric measure and not the same thing as the humidity ratio per kilogram of dry air reported here.
The air density calculator gives moist air density in kg/m³ of mixture, the reciprocal cousin of the specific volume above but on a different basis, and the heat index calculator converts temperature and humidity into a perceived-temperature index rather than a thermodynamic property. For the underlying gas relationships, the ideal gas law calculator and the vapor pressure calculator handle the single-component cases.
Arb Digital builds free tools like this one because useful pages earn attention. If you want tools, calculators or content built for your own audience, we can help.
Browse All Free Tools Talk to Arb DigitalCommon Mistakes to Avoid
- Entering sea-level pressure at altitude — the forecast barometric figure is corrected to sea level, and the humidity ratio and specific volume both need the true station pressure.
- Treating enthalpy per kilogram of mixture — it is per kilogram of dry air, so the mass flow you multiply it by must be the dry air flow, not the total.
- Using the dew point where the wet-bulb belongs — evaporative cooling approaches the wet-bulb, never the dew point, and confusing them overstates what an evaporative stage can achieve.
- Deriving the second property before entering it — feed in what you actually measured, or the same measurement error is applied twice by two different routes.
- Reading a relative humidity as a moisture quantity — it is a ratio against a saturation value that itself moves with temperature, so 50 per cent at 35 °C is more than three times the water of 50 per cent at 10 °C.
Related Free Tools From Arb Digital
For single humidity properties, see the dew point calculator, the relative humidity calculator and the absolute humidity calculator. The air density calculator and the air pressure at altitude calculator cover the pressure and density side, the ideal gas law calculator and the vapor pressure calculator cover the underlying gas behaviour, and the heat index calculator covers perceived temperature. Convert units with the temperature converter, and browse the full free online tools hub for everything else.
Frequently Asked Questions
Because the humidity ratio depends on the ratio of vapour pressure to total pressure, and the specific volume depends on total pressure directly. The same temperature and relative humidity give a different moisture content and a noticeably different specific volume at altitude than at sea level.
Per kilogram of dry air, which is the standard psychrometric convention. The dry air mass through a duct is conserved while the water is not, so using dry air as the basis makes an enthalpy difference multiplied by mass flow a valid energy rate.
The dew point is the temperature at which air becomes saturated if cooled at constant moisture content, so it predicts condensation on surfaces. The wet-bulb is what a wetted thermometer reads in moving air, and it is the lower limit of evaporative cooling. The wet-bulb always sits between the dew point and the dry-bulb.
Neither can be written as a closed-form function of the inputs. The dew point requires inverting the saturation pressure equation, and the wet-bulb appears on both sides of its own energy balance, so both are solved here by bisection to a tolerance far tighter than any instrument could resolve.
The tool clamps the state to saturation and tells you, rather than returning an impossible answer. A dew point above the dry-bulb, a relative humidity over 100 per cent or an excessive humidity ratio all describe fog, which holds liquid droplets and is outside what the vapour-phase equations describe.
From the saturation pressure formulation published in the psychrometrics chapter of the ASHRAE Handbook — Fundamentals, with one coefficient set for saturation over liquid water and a separate set for saturation over ice below the triple point. This page switches between them at zero degrees Celsius.
Not reliably. The relationships treat moist air as a pair of ideal gases, which holds well at ordinary atmospheric pressures but degrades as pressure rises, where a real-gas enhancement factor is required. Treat results well above atmospheric pressure as indicative only.
Because relative humidity is a ratio against a saturation value that rises steeply with temperature. At 35 °C, 50 per cent relative humidity carries more than three times the water per kilogram of dry air that the same 50 per cent carries at 10 °C, and it is the moisture mass that drives both comfort and latent load.
This tool is provided for educational and estimating use only. It is not a substitute for a designed heating, cooling or ventilation calculation, and it does not address equipment selection, air quality, condensation risk in a specific construction or any code requirement. Building services design should be carried out by a qualified engineer against the standards applicable in your jurisdiction.
