Relative Humidity Calculator
RH from dry-bulb & dew point or dry/wet-bulb — approximate psychrometrics. Click any i for detail.
Temperatures
Input mode
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Mode
Dew point is simplest; wet-bulb uses a psychrometric approximation. |
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Temperature unit
i
Unit
Enter all temperatures in the same unit. |
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Dry-bulb temperature
i
Dry
Air temperature from a normal thermometer. |
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Dew point
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Dew
Temperature at which moisture condenses. |
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Wet-bulb temperature
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Wet
From a sling psychrometer or wet-bulb sensor. |
How to use this calculator
- Choose dew-point or wet-bulb mode.
- Pick °F or °C and enter matching temperatures.
- Read approximate RH.
- Calibrated sensors beat napkin psychrometrics for critical jobs.
Results explained
Relative humidity compares actual vapor pressure to saturation vapor pressure at the dry-bulb temperature. The Magnus formula estimates saturation vapor pressure from temperature; dew-point mode is a direct ratio of those pressures. Wet-bulb mode uses a simple August–Roche–Magnus-style psychrometric estimate at ~sea-level pressure.
Quick reference: RH
RH ≈ 100 × e(dew) / e(dry) (Magnus), or from dry/wet-bulb psychrometry.
| Item | Detail |
|---|---|
| Comfort indoor | Often ~30–50% RH |
| Dew = dry | RH → 100% |
| Wet-bulb | Always ≤ dry-bulb |
| Pressure | High altitude needs better formulas |
Approximate — instruments and barometric pressure matter.
How the estimate is built
Magnus e(T); RH_dew = 100·e(Td)/e(Tdb); wet-bulb uses e_w − psychrometric term.
Example scenario
75°F dry-bulb, 55°F dew point → roughly 50% RH (order-of-magnitude check).
FAQ
Why not 100% when it rains?
Outdoor RH can be high without being saturated at your thermometer’s height and temperature.
Does this replace a hygrometer?
No — use it to cross-check or learn; buy a sensor for continuous monitoring.
Wet-bulb for HVAC?
Useful for evaporative cooling discussions; still approximate here.
Can wet-bulb exceed dry-bulb?
Not in normal air — check your readings if it does.