Updated October 2026
ASHRAE's printed psychrometric charts are the industry reference most HVAC professionals learned on — so it's a fair question whether a browser-based calculator actually reproduces the same thing, or something subtly different. Here's the honest comparison.
ASHRAE's official charts are built from the Hyland-Wexler formulation (1983), a more elaborate equation set designed for high accuracy across an unusually wide temperature range, including conditions well below freezing. This calculator uses the simpler Tetens approximation (see the Tetens formula page), which tracks Hyland-Wexler to within roughly 0.1–0.2% across the normal 0–50°C HVAC range — smaller than the precision most people can read off a printed chart by eye in the first place. Below freezing, the two diverge more, since this calculator's formula assumes saturation over liquid water rather than switching to an ice-phase equation.
Printed ASHRAE charts are drawn for one specific barometric pressure — Chart No. 1 for sea level/normal temperatures, separate charts exist for different altitude bands (and other vendors publish charts specifically calibrated to a city's typical elevation, like Denver). If your site's elevation doesn't match the chart you're holding, you either need a different printed chart or you're reading numbers with a built-in altitude error. This calculator takes barometric pressure as a direct numeric input (default 101.325 kPa, sea level) and recomputes every formula — RH, humidity ratio, dew point, enthalpy, specific volume, and wet-bulb — for exactly that pressure, with no need to own a different chart for a different elevation. See the altitude page for how much that pressure input actually changes the results.
A physical chart lets you draw and visually trace an entire multi-step process — mixing, then coil, then reheat — as connected line segments across the full chart in one view, which is genuinely useful for system-level design work. This tool's live chart marks a single calculated point rather than letting you draw and chain multiple process lines across it; for full multi-stage process design, a printed chart or dedicated HVAC design software is still the more natural tool.
For single-point property lookups — "what's the dew point of 24°C/55% RH air" — this calculator and an ASHRAE chart should agree to within the chart's own reading precision, at whatever pressure you actually enter. For full system process design involving multiple sequential equipment stages, a printed chart (or dedicated design software) remains the better tool for visualizing the whole path at once.