Psychrometric Calculator is an independent HVAC engineering tool built to turn two measured air properties into the full standard set of psychrometric quantities — without a printed chart, a slide rule, or a spreadsheet.
You enter dry-bulb temperature and one other known value — wet-bulb temperature, relative humidity, or dew point — plus barometric pressure (default 101.325 kPa, sea level). From that, the tool computes vapor pressure using the Tetens saturation-vapor-pressure formula (es(T) = 0.61078 × exp(17.27T / (T+237.3)), T in °C, result in kPa), then derives relative humidity, humidity ratio (W = 0.622e / (P−e)), dew point (via the inverse Tetens relation), enthalpy (h = 1.006T + W(2501 + 1.86T)), and specific volume (v = 0.287(T+273.15)(1+1.6078W) / P). When wet-bulb temperature isn't the known input, it is recovered by solving the psychrometer equation (es(Twb) − 0.000662·P·(Tdb−Twb) = e) numerically by bisection — there's no closed-form solution for wet-bulb, so the calculator searches for it directly rather than approximating it with a shortcut formula. These are the same standard, published relationships behind ASHRAE Fundamentals psychrometric charts, not a proprietary or simplified method.
The psychrometric chart on the homepage is generated the same way, live: the saturation curve and the 25/75/50% relative-humidity curves are each computed point-by-point from the Tetens formula across the full temperature range, then rendered as an SVG — it updates with the pressure you enter and is not a static image.
HVAC technicians and engineers doing a field or design check, and students learning to read a psychrometric chart or verify a hand calculation. It's built for single-point property lookups — not a substitute for full psychrometric process modeling (mixing, multi-stage cooling/reheat, or system-level load calculations), which need dedicated design software.
This site is not affiliated with, endorsed by, or produced in partnership with ASHRAE or any HVAC equipment manufacturer. The formulas used are standard published engineering references — the Tetens approximation and the psychrometer (wet-bulb) equation — not a different or unofficial method.
Every calculation, including the chart, runs entirely in your browser using JavaScript. The temperatures, humidity, and pressure values you enter are never sent to or stored on any server. See the full privacy policy.
Questions, corrections, or feedback: support@psychrometric-calculator.com