Psychrometric Calculator

Q: How do you calculate sensible cooling load from airflow and temperature?

At sea level use: Sensible BTU/hr = 1.08 × CFM × ΔT (°F) In SI: kW = 1.21 × L/s × ΔT (°C) ÷ 1000 Example: 1,000 CFM cooled from 75 °F to 55 °F gives 1.08 × 1,000 × 20 = 21,600 BTU/hr. The 1.08 factor comes from air density and specific heat, so it drops at high altitude.

Q: How much water does an air conditioner coil pull out of the air?

Use the humidity ratio drop across the coil: Latent BTU/hr = 0.68 × CFM × ΔW (grains/lb) For pints of water per hour, use CFM × ΔW ÷ 1,540 roughly. A 1,000 CFM system dropping air from 65 to 50 gr/lb pulls about 10 pints an hour on a humid day. That water drains off the coil pan.

Introduction

This psychrometric calculator finds the full properties of moist air. Type in the dry bulb temperature and one other value you know: wet bulb, relative humidity, or dew point. From those two, it solves the rest.

You get humidity ratio, relative humidity, wet bulb, dew point, enthalpy, specific volume, and air density. It also shows vapor pressure and saturation pressure. Pick SI or IP units, and every number switches right away.

Air pressure changes with height, so the tool sets barometric pressure from your altitude. You can also type your own pressure if you have a real reading from the site.

Add as many state points as you need. This helps you compare air before and after a coil, mix two air streams, or check supply and return air. A chart lines up all your points side by side.

Each point also comes with a step-by-step solution. You can see every formula and number used, based on ASHRAE equations. That makes it useful for HVAC load work, duct and coil sizing, checks in the field, and for students learning psychrometrics.

How to use our Psychrometric Calculator

Enter your unit system, your site's altitude or air pressure, and two known air properties for each state point. The calculator gives you humidity ratio, relative humidity, wet bulb, dew point, enthalpy, specific volume, and density, plus a full step-by-step solution.

Unit system: Pick SI (Metric) or IP (Imperial). All labels, inputs, and results switch right away.

Altitude above sea level: Type how high your site sits in meters or feet. Use 0 for sea level. The tool uses this to work out barometric pressure.

Barometric pressure: This fills in on its own from altitude. Type your own value here if you have a measured reading. Click "Re-derive from altitude" to go back to the auto value.

Dry bulb temperature: Enter the air temperature from a normal thermometer. This is needed for every state point.

Secondary input: Choose the one other property you know (wet bulb temperature, relative humidity, or dew point temperature), then type its value. Wet bulb and dew point must not be higher than the dry bulb temperature.

Add state point: Click this to compare more air conditions, like supply air and return air, side by side in the chart.

Calculate All: Click to solve every state point at once. Use "Show steps for this point" or the "Show for" dropdown to see the math for one point.

Reset All: Click to clear your work and start over with the default values.

What Is Psychrometrics?

Psychrometrics is the study of air and the water vapor mixed in it. Air is never fully dry. It always holds some moisture, and that moisture changes how the air feels, how heavy it is, and how much heat it carries. HVAC designers use psychrometric math to size air conditioners, heaters, coils, and dehumidifiers so a building stays comfortable and dry.

The Air Properties You Need to Know

Dry bulb temperature: the plain air temperature you read on a normal thermometer.
Wet bulb temperature: the reading from a thermometer with a wet sock on the tip. It shows how much cooling you can get from evaporating water.
Dew point: the temperature where air gets full of moisture and water starts to form on cold surfaces, like a cold can of soda "sweating."
Relative humidity (RH): how full the air is with water vapor, shown as a percent of the most it could hold at that temperature.
Humidity ratio: the real weight of water vapor in each pound (or kilogram) of dry air.
Enthalpy: the total heat in the air, both the heat you feel and the hidden heat stored in the moisture.
Specific volume and density: how much space the air takes up and how heavy it is. Warm, wet air is lighter than cool, dry air.

Why Only Two Inputs Are Needed

Once you know the air pressure, any two air properties lock in all the rest. So if you know the dry bulb temperature plus one more value, such as wet bulb, dew point, or relative humidity, every other property can be found. That is the idea behind a psychrometric chart, just done with equations instead of lines on paper.

Why Altitude and Pressure Matter

Air pressure drops as you go higher above sea level. Thinner air holds less mass per cubic foot, so coils, fans, and airflow all behave differently in a mountain town than at the beach. Using the right barometric pressure keeps humidity, enthalpy, and density results honest. If you have a real reading from a barometer or weather station, use it instead of the altitude estimate.

How HVAC Pros Use This

Contractors and engineers compare air at different points in a system, such as outdoor air, return air, mixed air, and supply air off the coil. The change in enthalpy between two points tells you the total cooling or heating load. The change in humidity ratio tells you how much water the coil pulls out of the air. The dew point warns you where condensation and mold could show up. These numbers guide equipment sizing, duct design, and comfort checks in homes, offices, schools, and clean spaces.

Common Comfort Targets

Most indoor spaces feel best near 68 to 78 °F (20 to 26 °C) with relative humidity between 30% and 60%. Air that is too dry causes static and dry skin. Air that is too wet feels sticky and can grow mold. The calculations here use the standard ASHRAE equations for saturation pressure and moist air, the same base used by professional psychrometric charts.

Formulas used

Barometric pressure from altitude

p = 101325\left(1 - 2.25577\times10^{-5}\,z\right)^{5.2559}

Saturation vapor pressure (ASHRAE Hyland-Wexler, t \ge 0 ^\circ C)

\ln p_{ws} = \frac{-5.8002206\times10^{3}}{T} + 1.3914993 - 4.8640239\times10^{-2}T + 4.1764768\times10^{-5}T^2 - 1.4452093\times10^{-8}T^3 + 6.5459673\ln T

Humidity ratio from partial vapor pressure

W = \frac{0.621945\,p_w}{p - p_w}

Relative humidity

\varphi = \frac{p_w}{p_{ws}}\times 100\ \%

Humidity ratio from wet bulb temperature (t_{wb} \ge 0 ^\circ C)

W = \frac{(2501 - 2.326\,t_{wb})W_{s,wb} - 1.006\,(t_{db} - t_{wb})}{2501 + 1.86\,t_{db} - 4.186\,t_{wb}}

Specific enthalpy of moist air

h = 1.006\,t_{db} + W\left(2501 + 1.86\,t_{db}\right)

Specific volume of moist air

v = \frac{0.287042\,(t_{db} + 273.15)\left(1 + 1.607858\,W\right)}{p/1000}

Moist air density

\rho = \frac{1 + W}{v}

Frequently asked questions

What is the difference between dry bulb and wet bulb temperature?

Dry bulb is the plain air temperature from a normal thermometer. Wet bulb is the temperature you get when a wet wick is put on the thermometer tip and air blows over it.

Water evaporating off the wick cools the bulb. Dry air makes lots of water evaporate, so the wet bulb reads much lower. In humid air, little evaporates, so the two readings are close.

Can wet bulb temperature be higher than dry bulb temperature?

No. Wet bulb is always equal to or lower than dry bulb.

They are only the same when the air is at 100% relative humidity, which means the air is fully saturated. If your wet bulb reading is higher, the wick is dirty, the air is not moving over it, or the thermometer needs checking.

How do you find dew point from temperature and relative humidity?

The exact way is to find the saturation vapor pressure at the air temperature, multiply it by the relative humidity, then find the temperature where that vapor pressure would be saturated.

A quick field estimate in Celsius when RH is above 50%:

Dew point ≈ Temperature − (100 − RH) ÷ 5

So 25 °C air at 60% RH gives about 25 − 8 = 17 °C dew point.

What does 100% relative humidity mean?

It means the air holds all the water vapor it can at that temperature. The air is saturated.

At that point dry bulb, wet bulb, and dew point are all the same number. Any extra cooling makes water condense out as fog, dew, or water on a coil.

Why does relative humidity go up at night when no water is added?

Because cool air can hold less water vapor than warm air.

The amount of water in the air (the humidity ratio) stays the same, but the air's capacity drops as it cools. So the same moisture fills a bigger share of the capacity, and relative humidity climbs. If the air cools to the dew point, RH hits 100% and dew forms.

What is humidity ratio and how is it different from relative humidity?

Humidity ratio is the real weight of water vapor per unit weight of dry air. It is measured in kg/kg or grains per pound (gr/lb).

Relative humidity is only a percent of capacity, and that capacity changes with temperature. Heating air raises its temperature and drops its RH, but the humidity ratio does not change. That is why engineers track humidity ratio when sizing coils and dehumidifiers.

What are grains of moisture per pound of air?

Grains per pound (gr/lb) is the IP way to state humidity ratio. There are 7,000 grains in one pound of water.

Room air at 75 °F and 50% RH holds about 65 gr/lb. Supply air off a cooling coil at 55 °F and saturated holds about 64 gr/lb. Dry desert air can be under 20 gr/lb. To convert, divide grains by 7,000 to get lb water per lb dry air.

What is enthalpy of air and why does HVAC use it?

Enthalpy is the total heat in the air. It adds the heat you feel (sensible) and the hidden heat stored in the water vapor (latent).

Cooling loads depend on both, so pros compare enthalpy before and after a coil:

Total BTU/hr = 4.5 × CFM × (h_in − h_out)

Temperature alone misses the moisture load, which can be a third of the job in humid weather.

How do you calculate sensible cooling load from airflow and temperature?

At sea level use:

Sensible BTU/hr = 1.08 × CFM × ΔT (°F)

In SI: kW = 1.21 × L/s × ΔT (°C) ÷ 1000

Example: 1,000 CFM cooled from 75 °F to 55 °F gives 1.08 × 1,000 × 20 = 21,600 BTU/hr. The 1.08 factor comes from air density and specific heat, so it drops at high altitude.

How much water does an air conditioner coil pull out of the air?

Use the humidity ratio drop across the coil:

Latent BTU/hr = 0.68 × CFM × ΔW (grains/lb)

For pints of water per hour, use CFM × ΔW ÷ 1,540 roughly. A 1,000 CFM system dropping air from 65 to 50 gr/lb pulls about 10 pints an hour on a humid day. That water drains off the coil pan.

What is the standard density of air in HVAC?

Standard air is 0.075 lb/ft³ (1.2 kg/m³). That is dry air near 70 °F (20 °C) at sea level pressure.

Real air is lighter when it is hot, humid, or high above sea level. Fan ratings, duct charts, and the 1.08 and 4.5 factors all assume standard air, so they need correcting when conditions are far from it.

How does altitude affect air density and HVAC capacity?

Air pressure falls as you go up, so each cubic foot holds less mass.

At 5,000 ft the pressure is about 83% of sea level, so air is about 17% thinner. A fan moving the same CFM carries less heat, so cooling and heating capacity drops by about the same share. Multiply the 1.08 and 4.5 factors by the density ratio. Gas furnaces also need derating for altitude.

What indoor humidity level is healthy?

Aim for 30% to 60% relative humidity, with 40% to 50% being the sweet spot.

Below 30% you get static shocks, dry skin, and cracked wood. Above 60% the air feels sticky, dust mites thrive, and mold can grow on cool surfaces. In cold winters, lower indoor RH to about 30% or less to stop window condensation.

Why do windows, pipes, and ducts sweat?

Because their surface is colder than the dew point of the air around them.

Air touching the cold surface cools past the point where it can hold its moisture, so water forms. Two fixes: raise the surface temperature with insulation or better glass, or lower the room dew point with a dehumidifier or air conditioning.

Why does air conditioning dry out the air?

The cooling coil runs colder than the air's dew point, usually around 40 to 45 °F.

Air passing over it cools below its dew point, so water vapor turns to liquid on the fins and drips to the drain pan. The air leaves both cooler and drier. Oversized units short cycle and do not run long enough to remove much moisture, which leaves a cold, clammy room.

What wet bulb temperature is dangerous for people?

A wet bulb near 95 °F (35 °C) is the survival limit. At that point sweat cannot evaporate, so the body cannot cool itself, even in shade with water.

Danger starts much lower. Heavy work gets risky above about 80 °F (27 °C) wet bulb. This is why humid heat is far more dangerous than dry heat at the same temperature.

What is saturation vapor pressure?

It is the pressure water vapor pushes with when air is holding all it can at a given temperature.

It rises fast with temperature. At 32 °F (0 °C) it is about 611 Pa, at 68 °F (20 °C) about 2,339 Pa, and at 104 °F (40 °C) about 7,384 Pa. Relative humidity is just the actual vapor pressure divided by this value, times 100.

Why do you only need two properties to find all the others?

Once the barometric pressure is fixed, moist air has just two free variables. Pick any two and the rest are locked in.

So dry bulb plus wet bulb, or dry bulb plus relative humidity, or dry bulb plus dew point all define the same state point. That is exactly what a psychrometric chart shows: cross any two lines and read every other property where they meet.

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