Relative Humidity Calculator
What this calculator does
This calculator estimates relative humidity when you already know the air temperature and dew point. It is useful because dew point and dry-bulb temperature together define the moisture state of the air well enough to solve for RH.
People often know dew point from weather data or field observations but still need relative humidity for reporting, equipment checks, or comfort comparisons. This page bridges that gap by converting both temperatures into vapor pressure terms and then computing the ratio between actual vapor pressure and saturation vapor pressure.
Inputs explained
- Air temperature: Enter the current dry-bulb air temperature.
- Dew point temperature: Enter the dew point associated with the same air sample.
- Internal units: The page evaluates the saturation relationships in Celsius and then reports the RH percentage.
How it works / method
The engine applies the same compact vapor-pressure-style approximation to both the dew point and the air temperature. Actual vapor pressure comes from the dew point, while saturation vapor pressure comes from the air temperature. Relative humidity is then calculated as 100 times the ratio of those two values.
Formula used
e = 6.105 x exp(17.27Td / (237.7 + Td)); es = 6.105 x exp(17.27T / (237.7 + T)); RH = 100 x (e / es)
T is air temperature in C and Td is dew point in C. Because this uses an approximation for vapor pressure, the result is a practical estimate rather than a high-precision psychrometric inversion.
Practical note: Relative humidity is temperature-dependent. The same dew point can correspond to a very different RH value when the air temperature changes, which is why RH alone does not fully describe moisture content.
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Vapor Pressure: --
Step-by-step example
Suppose the air temperature is 25 C and the dew point is 15 C. The air contains a fixed moisture amount, but RH depends on how close 25 C is to saturation.
- Enter 25 for air temperature.
- Enter 15 for dew point.
- The calculator estimates actual vapor pressure from the dew point and saturation vapor pressure from the air temperature.
- The resulting RH is a little above 50 percent.
- If the same dew point occurred at a cooler air temperature, RH would be higher because the air would be closer to saturation.
Use cases
- Converting forecast or observed dew point into relative humidity for reports and quick comparisons.
- Explaining why RH can change through the day even when moisture content changes very little.
- Supporting classroom psychrometric exercises and humidity-state interpretation.
- Checking indoor or outdoor moisture conditions when dew point is the primary available measurement.
Assumptions and limitations
- The result depends on the approximation used for vapor pressure and on the accuracy of both temperatures.
- Relative humidity can be a useful display value but is not always the best standalone indicator of moisture comfort.
- The page does not explicitly model barometric pressure or specialty psychrometric edge cases.
- Rounding both temperatures too aggressively can introduce visible RH differences in near-saturation situations.
If you want a more stable description of moisture content across changing temperatures, compare this page with the dew point and vapor pressure calculators.
Frequently Asked Questions
Use the Magnus-form ratio: RH = 100 × exp[(17.625 × Td)/(243.04 + Td)] / exp[(17.625 × T)/(243.04 + T)], with both temperatures in °C. At 28°C with an 18°C dew point, RH ≈ 56%. The formula compares the saturation vapour pressure at the dew point — which equals the actual vapour pressure — against the saturation vapour pressure at the current temperature. The ratio, scaled to a percent, is RH. Two inputs only; no humidity sensor needed.
RH = 100 × es(Td)/es(T), where es is saturation vapour pressure as a function of temperature. Using the Magnus form, es(T) = 6.112 exp(17.625 T/(243.04 + T)) in hPa. So plug Td and T separately, take the ratio, multiply by 100. Both temperatures must be in Celsius. Below freezing, the constants change slightly because saturation over ice differs from over water — most calculators flag a switch at 0°C and use a separate ice-form formula.
You need the ambient air temperature too — dew point alone is not enough. With both, RH = 100 × es(Td)/es(T) using the Magnus saturation pressure formula. Walk through it: at 32°C with a 22°C dew point, es(22) ≈ 26.4 hPa, es(32) ≈ 47.6 hPa, RH ≈ 55%. Same dew point but at 26°C: es(26) ≈ 33.6 hPa, RH ≈ 79%. The dew point did not change, but as the room cools, the relative humidity climbs.
The air is fully saturated — it is holding all the water vapour it can at that temperature. Any further cooling, or any extra moisture, forces vapour to condense out as dew, fog, or cloud droplets. It does not mean "the air is 100% water." Most of the air is still nitrogen and oxygen; saturation just tells you about the vapour content relative to capacity. At 100% RH the dew point equals the air temperature. That is the defining condition.
Closeness to saturation matters but is not the only requirement. Once the dew-point depression — air temperature minus dew point — drops below about 2°C, fog or low cloud is likely. For rain you also need lifting (a front, convection, terrain) to produce cloud at altitude, condensation around aerosol nuclei, and droplets large enough to fall. So a small spread is necessary but not sufficient. Dry air aloft, even with saturated surface air, will leave you with fog and no rain.
Plug both temperatures in Celsius into the Magnus ratio: RH = 100 × exp(17.625 × Td/(243.04 + Td)) / exp(17.625 × T/(243.04 + T)). The constants 17.625 and 243.04 are tuned for °C, so do not convert to kelvin — that breaks the empirical fit. At 30°C with a 25°C dew point, RH ≈ 75%. The formula is valid roughly from −40°C to +50°C with errors below 0.4% in the meteorological range. Use it freely for everyday calculations.
Indoor comfort guidelines from ASHRAE and most public-health bodies suggest 30–60% RH. Below 30% you get dry skin, irritated airways, static shocks, and faster influenza spread. Above 60% you get mould, dust mites, condensation on cool surfaces, and a muggy feel. The sweet spot is around 40–50%. In tropical climates, hitting 50% indoors in summer needs aggressive dehumidification; in dry winters, humidifiers do the opposite. Pair RH with dew point for a fuller comfort picture, especially for sensitive populations.
Relative humidity falls — assuming no water is added or removed. Warmer air can hold more water vapour, so the same absolute moisture amount becomes a smaller fraction of capacity. Example: a sealed room at 20°C and 60% RH has a dew point of about 12°C. Heat the room to 25°C and the dew point stays at 12°C, but RH drops to about 44%. This is why winter heating dries indoor air dramatically and why humidifiers become useful in cold climates.
Sources & references
- NOAA National Weather Service - Dew point glossary
- NOAA National Weather Service - Relative humidity glossary
- NOAA National Weather Service - Dry bulb, wet bulb, and dew point definitions
- NOAA National Weather Service - Vapor pressure glossary
- American Meteorological Society - Stull wet-bulb approximation
- NIST Chemistry WebBook - Water data
- Schema.org - FAQPage and WebApplication vocabulary