Psychrometric Calculator

What this tool actually does

For engineering and educational use only. Results are theoretical properties of ideal moist air, computed for the state you entered.

Computes the psychrometric state of moist air from two independent properties plus barometric pressure, using the ASHRAE Handbook of Fundamentals formulation as implemented in PsychroLib, and plots the point on a psychrometric chart. Humidity ratio, dew point, wet bulb, enthalpy, specific volume, degree of saturation and vapour pressure all fall out of that one state.

A state point is not a process, a load, or a piece of equipment. Nothing here sizes a coil, a dehumidifier, a dryer or an air handler.

Where the numbers go wrong in practice

• Barometric pressure is an input, not a constant. It must be the absolute station pressure at your altitude. Weather services publish sea-level-corrected pressure, and using that at 1500 m shifts humidity ratio and enthalpy by a margin that matters. Denver is not at 101.325 kPa.
• Relative humidity sensors drift. ±2% RH is a good instrument and ±5% is common in the field. Near saturation a small RH error becomes a large dew point error, which is exactly the region where condensation questions get asked.
• Wet bulb needs aspiration. An unaspirated or dirty wick reads high, and a wet bulb taken with a sling in still air is not the thermodynamic wet bulb the equations assume.
• Two inputs that nearly coincide give a badly conditioned answer. Solving from a dry bulb and a wet bulb a fraction of a degree apart amplifies measurement error into the derived properties.
• Below freezing the saturation curve branches. Saturation over ice and over supercooled water are different, and reported frost point and dew point are not interchangeable.

Not modelled at all

• Real gas behaviour at elevated pressure, and any gas mixture other than dry air plus water vapour.
• Contaminants, aerosols, entrained droplets, fog and carryover past a coil.
• Coil bypass factor, apparatus dew point, fan heat, duct gains and any part of an air handling process.
• Transient behaviour, moisture buffering in materials, and vapour transport through an envelope.
• Mould, corrosion and condensation risk in a real assembly, which depend on surface temperatures and time, not on a single air state.

Never use this for

• Equipment selection, capacity or load calculations for design or purchase, including anything submitted under ASHRAE Standard 90.1 or a local energy code.
• Building envelope condensation, mould or dew point risk assessment in place of a hygrothermal analysis to ASHRAE Standard 160 or equivalent.
• Thermal comfort compliance under ASHRAE Standard 55, or ventilation compliance under ASHRAE Standard 62.1.
• Cleanroom, pharmaceutical, laboratory or data centre qualification, validation or commissioning records.
• Drying, curing, storage or process control where product quality, food safety or material integrity depends on the humidity.
• Any test report, guarantee, certification or regulatory submission.

What it is checked against

The wrapper is pinned to PsychroLib reference cases in the test suite, in both SI and IP units, and the chart is regression tested. That proves the implementation reproduces the published formulation. It proves nothing about your barometric pressure, your instruments, or whether the state you entered describes the air in the room.

Get a professional involved for

System design and equipment sizing, envelope and condensation analysis, any regulated or validated environment, and any case where humidity control protects people, product or a building. A licensed mechanical engineer, and for envelope questions a building science professional, working from measured site conditions rather than a single calculated point.