Rex says
Everyday utility math — the kind you'd otherwise pull up four browser tabs for. I keep it to one clean answer.
Try a scenario
Click to load — tweak from there.Inputs
Result
Relative humidity
53.4%
Dew point
66.2
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One click, a permanent link with your numbers baked in.
How to use this
- 1Enter dry bulb temperature (°F).
- 2Enter wet bulb temperature (°F).
- 3Enter station pressure (hPa).
- 4Read your relative humidity on the right — it updates as you type.
- 5Hit Share to keep the scenario or send it to someone.
About this calculator
A sling psychrometer measures two temperatures: dry bulb (regular air temperature) and wet bulb (a thermometer wrapped in a wet wick, cooled by evaporation). The gap between them tells you humidity — a big gap means dry air evaporating water fast, no gap means saturated air. This calculator uses a standard psychrometric approximation (a form of the Sprung equation) to back out relative humidity and dew point from those two readings, which is how humidity was measured for over a century before capacitive humidity sensors became cheap. It's still the standard field method for calibrating electronic hygrometers and is used in industrial and agricultural settings where a simple, no-battery instrument is preferred.
Worked example
Using the values the calculator loads with:
Inputs
- Dry bulb temperature: 85 °F
- Wet bulb temperature: 72 °F
- Station pressure: 1013 hPa
Results
- Relative humidity: 53.4%
- Dew point: 66.2
What each field means
Inputs
- Dry bulb temperature (°F)
- The dry bulb temperature used in the calculation, measured in °F. Starts at 85 °F so you have a working example on load. Accepted range: 0–130 °F.
- Wet bulb temperature (°F)
- The wet bulb temperature used in the calculation, measured in °F. Starts at 72 °F so you have a working example on load. Accepted range: 0–130 °F.
- Station pressure (hPa)
- The station pressure used in the calculation, measured in hPa. Starts at 1013 hPa so you have a working example on load. Accepted range: 800–1050 hPa.
Results
- Relative humidity
- Returned as a percentage and shown as the headline result. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
- Dew point
- Returned as a whole number. It recalculates instantly whenever you change an input, so you can compare scenarios without reloading.
FAQ
Why is wet bulb always lower than or equal to dry bulb?
Evaporation from the wet wick pulls heat from the thermometer, cooling it below ambient air temperature. In perfectly saturated air (100% RH) no net evaporation occurs, so the two readings converge and equal each other.
Does elevation matter for this calculation?
Yes — station pressure appears directly in the psychrometric equation because evaporation rate depends on ambient air pressure. At high elevation (lower pressure) the same wet bulb depression corresponds to slightly higher humidity than at sea level.
Is wet bulb temperature the same as wet bulb globe temperature (WBGT)?
No. Plain wet bulb temperature measures evaporative cooling only. WBGT, used for heat stress guidelines in sports and military settings, combines wet bulb, dry bulb, and a black globe thermometer to also account for radiant solar heat.
How accurate is a sling psychrometer compared to digital sensors?
A well-maintained sling psychrometer with fresh distilled water on the wick is accurate to about ±2-3% RH, comparable to mid-range digital hygrometers, and it doesn't drift with age the way capacitive sensors can.
Accuracy and limitations
- Estimates assume standard, average conditions — local rules, pricing, and materials vary.
- Results are rounded for readability; add a buffer before ordering, booking, or committing.
- Double-check anything with a real cost attached against a local quote.
Related tools
Cite this calculator
Writing about this topic? Grab a citation — every link helps keep these tools free.
RevenueLab. (2026). Relative Humidity from Wet Bulb Calculator. Retrieved from https://www.revenuelab.fyi/toolbox/relative-humidity-wet-bulb
<p>Source: <a href="https://www.revenuelab.fyi/toolbox/relative-humidity-wet-bulb" target="_blank" rel="noopener">Relative Humidity from Wet Bulb Calculator — RevenueLab</a> (2026).</p>
Source: [Relative Humidity from Wet Bulb Calculator — RevenueLab](https://www.revenuelab.fyi/toolbox/relative-humidity-wet-bulb) (2026).