Psychrometric Chart.

Drag a point across the chart and read every property of the air at once. Set your altitude, run a heating or cooling process, then test yourself.

Mode

Units

SI reads °C, g/kg and kJ/kg. IP reads °F, grains per pound and Btu/lb.

Psychrometric chart — sea level, 101.3 kPa

Layers

Saturation (100% RH) Relative humidity Wet-bulb Enthalpy Specific volume

Dry-bulb

25.0 °C

Rel. humidity

50.0 %

Hum. ratio

9.88 g/kg

Enthalpy

50.3 kJ/kg

Wet-bulb

17.9 °C

Dew point

13.9 °C

Click anywhere on the chart to move the active point, or drag it. With the chart focused, the arrow keys nudge it: left and right change dry-bulb, up and down change humidity ratio, and Shift moves ten steps at a time.

Scenario

State points

Atmospheric pressure

Altitude 0 m

Barometric pressure 101.3 kPa

Properties of point A

I know

Dry-bulb °C Wet-bulb °C Dew point °C Rel. humidity % Hum. ratio g/kg Enthalpy kJ/kg Spec. volume m³/kg Density kg/m³

Type into the two outlined fields. Everything else follows, including the chart.

HVAC process

Pick a process to draw the line the air travels along, and read what it costs.

Design modules

Outdoor air mixes with return air, the mixture crosses the coil, and the supply air carries the room loads away. The chart shows the whole path, and extends the coil line to the apparatus dew point it is aiming at.

Outdoor °C Outdoor RH % Outdoor flow kg/s Room °C Room RH % Return flow kg/s Room sensible kW Room latent kW

Mixed air

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Supply air

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Apparatus dew point

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Bypass factor

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RSHF

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GSHF

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Coil latent

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Coil duty

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The equations, with your numbers in them

Everything the chart draws comes from ASHRAE Handbook of Fundamentals, Chapter 1. These are the five relations behind the active point, evaluated at the state it is in right now.

Saturation pressure — Hyland–Wexler, split at 0 °C: ln(p_ws) = C₈/T + C₉ + C₁₀T + C₁₁T² + C₁₂T³ + C₁₃ ln(T)—
Humidity ratio: W = 0.621945 · p_w / (p − p_w)—
Relative humidity: φ = p_w / p_ws—
Specific enthalpy: h = 1.006 t + W(2501 + 1.86 t)—
Specific volume: v = R_da T(1 + 1.607858 W) / p—
Thermodynamic wet-bulb — solved, not fitted: W = ((2501 − 2.326 t*)W_s* − 1.006(t − t*)) / (2501 + 1.86 t − 4.186 t*)—

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The link carries the point, the pressure and the units, so it opens exactly what you are looking at.

How to read a psychrometric chart

Find your air on the chart: dry-bulb temperature along the bottom, and slide up the vertical until you meet the relative-humidity curve you want. Or click the chart, or type any two properties into the grid.
Read the humidity ratio W straight off the right-hand axis — that is the mass of water each kilogram of dry air is carrying, and it does not change when you simply heat or cool the air.
Follow the sloping wet-bulb line down-left to the saturation curve for the wet-bulb temperature, and the horizontal line left to the saturation curve for the dew point.
Set your altitude before you size anything. Humidity ratio depends on barometric pressure, so a sea-level chart reads the wrong W anywhere above about 750 m.
Pick a process — heating, cooling, humidification, mixing — to draw the line the air travels along, and read the sensible, latent and total load it costs.

What a psychrometric chart is for

Almost every question about air comes down to two things at once: how hot it is, and how much water it is carrying. Heat a room and the relative humidity falls without a drop leaving. Cool a window below the dew point and water appears on it. Blow air through a wet pad and it gets colder and wetter together. None of those are separate facts — they are all one relationship, and a psychrometric chart is that relationship drawn out so it can be read rather than recalculated.

The chart is a map with dry-bulb temperature across the bottom and humidity ratio up the right-hand side. Fix the barometric pressure, and a single dot on that map carries seven properties at once: dry-bulb, wet-bulb, dew point, relative humidity, humidity ratio, enthalpy and specific volume. Every curve family on the chart is one more way of reading the same dot. That is why two numbers are enough to place it, and why an engineer who can read one of these can answer in seconds a question that otherwise needs a page of algebra.

The one thing a chart cannot do is adapt to where you are standing. Humidity ratio depends explicitly on barometric pressure, so a chart drawn for sea level is simply the wrong chart in Denver, Mexico City or Bengaluru. This one redraws for whatever altitude you set, which is the main reason it exists as software rather than as a printed sheet.

The processes an air-conditioning system draws on it

Every piece of HVAC equipment is a line on this chart, and the shape of the line is what the machine does.

Sensible heating and cooling are horizontal. Energy goes in or out, the water does not change, and relative humidity moves purely because the ceiling moved. Cooling this way can only run as far as the dew point.
Cooling with dehumidification bends down toward the coil. Past the dew point water starts condensing onto the fins, and the load splits into a sensible part and a latent part. Extending that line to the saturation curve gives the apparatus dew point — the effective surface temperature of the coil.
Steam humidification is nearly vertical. The steam brings its own energy, so the dry-bulb barely moves and the cost is almost all latent.
Evaporative humidification runs down a wet-bulb line. Here the heat to evaporate the water is taken out of the air itself, so it cools as it wets — and it stops dead at the wet-bulb temperature.
Mixing two airstreams lands on the straight line between them, at the point their mass flows set. Mix cold saturated air with warm humid air and that line can cross the saturation curve, which is exactly why you can see your breath.

Because those shapes are so distinct, a process line is often easier to identify than to calculate. An evaporative cooler and a steam humidifier both add the same water; on a chart, nobody could confuse them.

Who this is for, and what it is not

It is built for the people who need a chart in front of them rather than a textbook: mechanical and building-services students working through refrigeration and air-conditioning problems, HVAC engineers sizing coils, cooling towers and evaporative systems, building-physics and energy modellers checking a state point, and anyone diagnosing condensation, mould or a window that keeps fogging. The Practice and Quiz modes exist because reading a chart is a skill that comes from doing it, not from being shown it once.

It is a teaching and checking tool. The relations are ASHRAE’s and the arithmetic is pinned by an automated test suite against published table values, but it is not a substitute for the Handbook of Fundamentals and it is not calibrated equipment. Cite Chapter 1 in a report; use this to understand the answer and to catch the times a number does not look right.

Frequently Asked Questions

How do you read a psychrometric chart?

Find the dry-bulb temperature along the bottom and go straight up until you meet the curve for your relative humidity. That point is your air, and everything else is read off it: straight across to the right-hand axis for the humidity ratio, back along that horizontal to the saturation curve for the dew point, and down the sloping line to saturation for the wet-bulb. The chart above draws all three of those construction lines for whichever point is active, so you can drag it and watch them move.

What is humidity ratio, and why not just use relative humidity?

Humidity ratio W is an absolute quantity — kilograms of water vapour carried per kilogram of dry air. Relative humidity is a ratio: the water present divided by the most that air could hold at that temperature. Heat a room without adding a drop and the humidity ratio does not move, but the relative humidity collapses, because the ceiling it is measured against has risen. That is why equipment is sized in W and comfort is discussed in RH, and why winter air indoors feels so dry: it was not dry outside, it was cold, and heating it made it dry.

What is the difference between wet-bulb temperature and dew point?

Dew point depends only on how much water is in the air — it is the temperature at which that air would start to condense, so it is what decides whether a window fogs or a cold pipe sweats. Wet-bulb depends on the moisture and the temperature together, because it is set by an evaporation balance: it is what a thermometer with a wet wick reads in moving air. The two are equal only on the saturation curve. Everywhere else the order is dew point ≤ wet-bulb ≤ dry-bulb, and a set of readings that breaks that order is wrong.

Why does altitude change a psychrometric chart?

Because humidity ratio depends on barometric pressure explicitly: W = 0.621945 · p_w / (p − p_w). Drop the pressure and the same partial pressure of water corresponds to more water per kilogram of air. Air at 25 °C carrying 9.88 g/kg is 50% relative humidity at sea level and about 30% at 4,000 m — the same air, the same water, a different chart. ASHRAE publishes separate charts above 750 m for this reason, and the tool above warns you past that height. Size a coil in Denver or Mexico City off a sea-level chart and it will be wrong.

How cold can an evaporative cooler make the air?

Down to the wet-bulb temperature and no further, however large you build it. Evaporation runs the air down a line of constant wet-bulb, and that line ends on the saturation curve — once the air is saturated nothing more will evaporate. The gap between dry-bulb and wet-bulb is the whole budget, which is why a desert cooler transforms 40 °C at 15% RH and does almost nothing on a humid coast. A two-stage cooler beats the ambient wet-bulb by cooling indirectly first, which lowers the wet-bulb without adding water, and only then running a direct stage against that lower line.

What is the sensible heat ratio, and what is the apparatus dew point?

Sensible heat ratio is the share of a cooling load that goes into changing the temperature rather than condensing water — sensible over total. A coil with SHR 0.75 spends a quarter of its duty pulling moisture out of the air. The apparatus dew point is where the coil process line, extended, meets the saturation curve: the effective surface temperature the fins would need if every molecule of air touched them. The gap between the air that actually leaves and that ideal is the bypass factor. The cooling-coil module above computes all three and draws the line it extends.

Why does the chart split its physics at 0 °C?

Saturation pressure over ice is a different function from saturation pressure over liquid water, and this tool uses the ASHRAE Hyland–Wexler forms for both. The split matters more than it looks: below freezing the latent heat in the wet-bulb balance jumps from about 2501 kJ/kg for evaporation to 2830 for sublimation, because turning ice straight into vapour also has to pay the heat of fusion. A tool that skips the split reads freezer rooms, defrost cycles and frost-protection problems wrong by a few percent, quietly.

Which two properties do I need to know?

Any two independent ones, plus the barometric pressure. That is the Gibbs phase rule applied to a single-phase mixture of dry air and water vapour, and it is the reason a psychrometric chart can be flat: two numbers place a dot, and the dot carries everything else. The grid above lets you pick which two you know — dry-bulb and relative humidity, wet-bulb and dew point, humidity ratio and enthalpy, and eleven other combinations. The one pairing it will not offer is dew point with humidity ratio, because those two say the same thing twice and fix no state at all.