SizingKitMoist air as a state point
Psychrometric calculator
Free and without a signup: give a dry bulb and either a relative humidity or a wet bulb, and this returns the complete state point — wet bulb, dew point, humidity ratio, enthalpy, specific volume and moist-air density — then places it on a psychrometric chart generated at your own barometric pressure instead of at sea level. Add a leaving condition and an airflow and it draws the process line and splits it into sensible and latent BTU/h from the enthalpy difference. Every equation is ASHRAE Handbook—Fundamentals Chapter 1, cited beside the result.
- 100% free
- No signup
- ASHRAE Ch. 1 equations
- Chart drawn at your elevation
- 10 properties per point
The air you measured
A dry bulb and one moisture reading fix the state completely. Everything else on this page — the wet bulb, the enthalpy, the point on the chart — follows from those two and the barometric pressure.
Default 0 ft — Sea level — the elevation ASHRAE Psychrometric Chart No. 1 and every paper chart in a service van are drawn at. Barometric pressure falls with it: 14.696 psia at sea level, 14.17 at 1,000 ft, 12.23 at 5,000 ft. The wet bulb, the specific volume and both airflow factors move with the pressure, so entering the job's real elevation changes the answer, not just the label.
Filled from the elevation by ASHRAE Handbook—Fundamentals Ch. 1 Eq. (3), the standard atmosphere — not today's weather. Type a barometer reading over it and yours wins; clear the field and it goes back to 14.696 psia.
Where the air goes next — optional
Give a leaving condition and the process line is drawn between the two points and split into sensible and latent. Add an airflow and the split turns into BTU/h.
WET BULB
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ENTHALPY
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| Property | Imperial | Metric |
|---|---|---|
| Dry bulb | — | — |
| Relative humidityOne quantity, no second unit — it is already a ratio. | — | — |
| Wet bulb | — | — |
| Wet-bulb depressionDry bulb minus wet bulb — a difference, converted as one. | — | — |
| Dew point | — | — |
| Dew-point spreadHow far this air can cool before the first drop appears. | — | — |
| Humidity ratio WPer pound of DRY air, which is why it can exceed the mixture's own water content. | — | — |
| Enthalpy hPer pound of dry air, referenced to 0 °F dry air and liquid water at 32 °F. | — | — |
| Specific volume vAlso per pound of dry air. Its reciprocal is the dry-air mass in a cubic foot. | — | — |
| Moist-air densityThe mixture, air plus its water. Compare it with the 0.075 lb/ft³ the factors assume. | — | — |
The chart, drawn at 14.696 psia
0 ft elevation. Every curve below moves with that pressure, which is the one thing a printed chart cannot do.
ASHRAE Handbook—Fundamentals Ch. 1, Eqs. (5) and (6) — the Hyland and Wexler (1983) formulation for the saturation pressure of water vapor over ice and over liquid water, in IP units. The wet bulb is solved from the adiabatic-saturation energy balance itself rather than from a curve fit, so it is the thermodynamic wet bulb; a sling psychrometer reads the psychrometric wet bulb, which differs by a few hundredths of a degree for air and water at these conditions. Enthalpy and specific volume are ASHRAE Handbook—Fundamentals Ch. 1 Eqs. (32) and (28); the 0.240 and the 1061 in the enthalpy equation are that equation’s own coefficients, and Varies little over HVAC temperatures — under 0.5% from 0 to 200 °F — but the air in a duct is moist, and the specific heat of moist air is nearer 0.244 BTU/(lb·°F). Using the moist value is what turns the classic 1.08 into ASHRAE's current 1.10.
The chart above is already yours: it is redrawn for 14.696 psia, which at sea level is 14.696 psia. Enter a dry bulb and one moisture reading to put a point on it.
How to fix a state point and read the process off it
Two readings and an elevation are the whole input. The third step is what a paper chart cannot do for you.
Enter the pair you actually measured
Dry bulb, then either the relative humidity from a hygrometer or the wet bulb from a sling. The wet-bulb pair is the more trustworthy of the two near a coil, because a capacitive humidity sensor loses accuracy above about 90% while a wick does not.
Set the site elevation before you read anything
The barometric pressure field fills itself from the elevation using the standard atmosphere and takes a barometer reading over the top of it. Change the elevation and watch the wet bulb move while the dew point stays where it was — that difference is exactly what a sea-level chart hides.
Give the leaving condition, then the airflow
A leaving dry bulb and relative humidity draw the process line and report the change per pound of dry air. Adding the airflow turns that into total, sensible and latent BTU/h, a sensible heat ratio, and the pounds of condensate an hour the drain has to carry.
Technical specifications
| Saturation pressure | ASHRAE Handbook—Fundamentals Ch. 1, Eqs. (5) and (6) — the Hyland and Wexler (1983) formulation, over ice below 32 °F and over liquid water above it. Valid −148 to 392 °F; it returns 0.36328 psia at 70 °F against a published 0.3632 and 0.088649 psia at the triple point against 0.08865. |
|---|---|
| Wet bulb | Solved from the adiabatic-saturation energy balance itself rather than from a fitted polynomial, so both the above-freezing and below-freezing branches come out of one equation. It is the thermodynamic wet bulb; a sling psychrometer reads the psychrometric wet bulb, a few hundredths of a degree away for air and water. |
| Barometric pressure | ASHRAE Ch. 1 Eq. (3), the standard atmosphere: 14.696 psia at sea level, 14.173 psia at 1,000 ft, 12.228 psia at 5,000 ft. Editable — a real barometer reading replaces it, and the whole chart is redrawn. |
| Chart span | 32 to 120 °F dry bulb against 0 to 200 gr/lb humidity ratio, the sheet of ASHRAE Psychrometric Chart No. 1. At sea level the saturation line leaves the top edge at 87.5 °F, which is why the paper chart appears to stop there too. |
| Properties returned | Ten per state point — dry bulb, relative humidity, wet bulb, wet-bulb depression, dew point, dew-point spread, humidity ratio, enthalpy, specific volume and moist-air density — each printed in imperial and metric on the same row. |
| Worked reference point | 75 °F at 50% relative humidity, sea level: 62.5 °F wet bulb, 55.1 °F dew point, 64.6 gr/lb, 28.11 BTU per lb of dry air, 13.679 ft³/lb. Take the same air to 5,000 ft and the wet bulb drops to 61.8 °F while the dew point does not move at all. |
| Load method | Total heat is the enthalpy difference times the dry-air mass flow, 60 × CFM ÷ specific volume; sensible is the enthalpy change at constant humidity ratio. The customary 1.08, 0.68 and 4.5 are printed beside it corrected to the air on screen — 1.05, 0.665 and 4.39 for 75 °F room air at sea level, 0.873, 0.552 and 3.64 at 5,000 ft. |
| How the chart is produced | Inline SVG generated from the equations when you load the page: no chart library, no tile server, no image request. It prints on one sheet with the property table above it. |
Frequently asked questions
Why is the wet bulb here lower than the chart taped up in the van?
Because that chart is drawn at sea level and your job probably is not. Barometric pressure sets how much water a pound of dry air can carry, so it moves the saturation line and every relative-humidity curve with it: 75 °F at 50% has a 62.5 °F wet bulb at sea level and a 61.8 °F wet bulb in Denver. The dew point of that same air does not budge, which is the giveaway that the difference is real and not a rounding error.
Should I enter dry bulb with relative humidity, or dry bulb with wet bulb?
Enter whichever pair you actually measured, and prefer the wet bulb near a coil. A capacitive humidity sensor drifts and loses accuracy above roughly 90% relative humidity, which is exactly where leaving air off a wet coil sits; a wetted wick in moving air has no such ceiling. In a conditioned space away from a coil the hygrometer is the easier reading and the two pairs agree.
Enthalpy per pound of what — the air, or the air plus its water?
Per pound of dry air, always. Every quantity on a psychrometric chart uses the dry air as its denominator, because the dry air is the part that does not change as the process runs: cool the air over a coil and the water leaves, so a per-pound-of-mixture figure would be measured against a mass that shrank. It is also why the total-heat factor is a clean 4.5 rather than 4.5 divided by one plus the humidity ratio.
Why does the saturation line run off the top of the chart around 87 °F?
Because the chart's humidity-ratio scale stops at 200 gr/lb and saturated air passes that at 87.5 °F at sea level. Nothing is wrong with the air above that line — saturated air at 120 °F carries about 568 gr/lb — the sheet simply does not extend that far, and the normal-temperature chart is scaled for the range buildings operate in. Properties are still reported exactly for a point off the sheet; only the marker is left off.
The sensible heat ratio came out above 1. What does that mean?
It means the process is cooling the air while adding moisture to it, so the latent term is negative and the sensible term is larger than the total. That is an evaporative cooler, a leaking humidifier upstream, or a mislabelled leaving reading. A ratio above 1 is not a bug in the arithmetic, it is the arithmetic telling you the two end points do not describe a coil.
Can I just use 1.08 and 0.68 instead of all this?
You can, and at sea level with room air you will be within a few percent. Those factors are 60 min/h multiplied by 0.075 lb/ft³ and by the specific heat, and 0.075 is dry air at about 69 °F — real 75 °F room air at 50% carries 0.0731 lb of dry air per cubic foot, so the honest factors are already 1.05 and 0.665 before altitude enters. At 5,000 ft they are 0.873 and 0.552, an 18% error if you use the printed ones, which is more than the tolerance on anything you would be sizing.
Does the dew point change with altitude?
No, and that is worth remembering because almost everything else on the chart does. Dew point is set by the partial pressure of the water vapor alone, and giving a dry bulb and a relative humidity fixes that pressure without reference to the barometer. The humidity ratio, the specific volume, the enthalpy and the wet bulb all move with elevation; the dew point and the vapor pressure do not.
About the chart, and what a state point is for
A psychrometric chart is one equation drawn nine ways. Fix the dry bulb and the vapor pressure and everything else is determined: the humidity ratio follows from the molar mass ratio of water to dry air, the enthalpy from h = 0.240·t + W·(1061 + 0.444·t), the specific volume from the ideal gas law, and the wet bulb from the energy balance of an adiabatic saturator. This page solves that balance directly instead of using the rearranged form printed in handbooks, which means the above-freezing and below-freezing branches cannot be transcribed wrongly and the crossover cannot land in the wrong place.
The thing most online psychrometric tools get wrong is not an equation, it is the pressure. They compute at one standard atmosphere and offer no way to say otherwise, which is fine in Houston and about 18% wrong in Denver — not in the properties themselves, but in every airflow decision that follows from them, because the mass of air a CFM represents falls with the barometer. Once you have the state point here, the airflow it implies goes to the CFM calculator, the equipment capacity to the AC tonnage calculator, and the room-by-room load it has to meet to the HVAC load calculator.
The sensible heat ratio is the number this page exists to produce and the one a nameplate will not give you. Equipment is selected against a total capacity and a sensible split at a stated entering condition, and if the coil cannot reach the entering dew point it delivers no latent capacity at all regardless of what the label says. Reference tables and arithmetic are what this page offers; the equipment selection, the Manual S check and the airflow that has to be measured rather than assumed belong to whoever signs the job off. If what you actually need is a moisture conversion or the temperature a surface has to stay above rather than a state point and a coil process, the humidity calculator is the shorter road.
Where the chart is drawn
Every number on this page is worked out by JavaScript running in the tab you are reading it in. Nothing you type — loads, lengths, nameplate ratings, the rates your utility charges you — is uploaded, logged or kept, which is also why the calculators carry on working in a mechanical room with no signal.
The chart is inline SVG built from the equations in your own browser, so nothing about the building — its elevation, its return conditions, the coil it runs — leaves the tab, and the page keeps working in a mechanical room on a phone with no bars.