Wet bulb temperature calculator

What wet bulb temperature is

Wrap a thermometer bulb in a wet wick and move air across it. Water evaporates, the wick loses heat, and the reading falls until it stops at the wet bulb temperature: the lowest temperature evaporation can reach in that air. In saturated air nothing evaporates and the wet bulb equals the dry bulb. The drier the air, the further the two readings separate, and that gap is the wet-bulb depression.

Dew point is a different number. It is the temperature at which the water already in the air would start to condense, and it sits at or below the wet bulb. A coil running colder than the dew point of the air crossing it is the one pulling water out of that air.

Sources: DOE Building America Solution Center, Evaporative Cooling Systems (read 2026-10-18)

How the calculator gets there

Wet bulb comes from Roland Stull's 2011 equation in the Journal of Applied Meteorology and Climatology: an empirical fit that gives wet bulb directly from air temperature and relative humidity, with no iteration. It is valid from 5–99% relative humidity and from −20 to 50 °C, except where low humidity and cold come together, and its errors run from −1 to +0.65 °C, with a mean absolute error of 0.28 °C. It is fitted at standard sea-level pressure, 101.325 kPa; at altitude the true wet bulb sits lower than the figure shown.

Dew point uses the National Weather Service's form: saturation vapour pressure from es = 6.11 × 10^(7.5T ÷ (237.3 + T)), scaled by relative humidity, then solved for the temperature at which that vapour pressure saturates the air.

Sources: Stull (2011), Wet-Bulb Temperature from Relative Humidity and Air Temperature, J. Appl. Meteor. Climatol. 50, section 2 (read 2026-10-18) · NOAA National Weather Service, Vapor Pressure (wxcalc) (read 2026-10-18) · NOAA National Weather Service, Wet-bulb and Dewpoint from Air Temperature and RH (wxcalc) (read 2026-10-18) · DOE Building America Solution Center, Evaporative Cooling Systems (read 2026-10-18) · DOE Building America Solution Center, Air Conditioning (read 2026-10-18)

Indoor wet bulb and the charge

The cooling capacity on an AHRI certificate is measured at 95 °F outdoors with 80 °F, 67 °F wet-bulb air entering the indoor coil. Change the indoor wet bulb and the coil sees a different latent load, so the refrigerant side reads differently too. That is why a fixed-orifice system, the kind normally charged by superheat, is checked against a target that depends on indoor wet bulb and outdoor dry bulb. Take the wet bulb at the return, close to the coil, and read the target from the manufacturer's charging chart for that unit.

The superheat and subcooling calculator takes the indoor wet bulb this page produces. If the system is running but not keeping up, work the AC not cooling decision list first: airflow faults look like charge faults on the gauges.

Sources: DOE Building America Solution Center, Air Conditioning (read 2026-10-18)

Outdoor wet bulb and evaporative coolers

An evaporative cooler cannot deliver air colder than the outdoor wet bulb. How close it gets is its effectiveness: the drop it achieves divided by the wet-bulb depression. Thick modern media runs 75–93% effective, and DOE's rule of thumb is that places whose design wet bulb is 70 °F or lower suit evaporative cooling.

Phoenix's 38 °F depression lets a 90% cooler deliver 73.8 °F air on the design day. Sarasota's 13 °F depression leaves the same cooler pushing humid air at 80.3 °F, which is a job for refrigerated cooling. To size a cooler once the climate passes, DOE uses air changes per hour: the air changes per hour calculator turns a regional ACH and the house volume into CFM.

Sources: DOE Building America Solution Center, Evaporative Cooling Systems (read 2026-10-18)

Design conditions

Cooling loads are run at an outdoor design dry bulb with a coincident wet bulb, and those figures are looked up by county rather than guessed. The wet bulb sets the latent part of the load: two towns with the same design dry bulb can need very different coils. The BTU calculator gives a first estimate and the Manual J load worksheet splits the load room by room. Indoors, the 80 °F, 50% default above has a dew point of 59.7 °F; a supply coil has to run below that to dry the house.

Sources: EPA ENERGY STAR, National HVAC Design Report v3.1/3.2/3.3 (Rev. 14, revised 2025-01-15), Item 3.3 and Footnote 18 — Design Temperature Limit Reference Guide (read 2026-09-04) · DOE Building America Solution Center, Air Conditioning (read 2026-10-18)