Vapor Pressure Calculator
What this calculator does
This vapor pressure calculator estimates the partial pressure of water vapor in the air from air temperature and relative humidity. Vapor pressure is a direct moisture quantity that sits behind many other humidity measures, including wet-bulb temperature, humidex, and some apparent-temperature formulas.
Relative humidity is familiar, but it is not a direct measure of the amount of water vapor in the air. Vapor pressure is closer to a physical state variable because it describes the share of total pressure contributed by water vapor. That makes it valuable when you want a more explicit moisture quantity for weather interpretation or thermodynamic estimates.
Inputs explained
- Air temperature: Enter the current dry-bulb temperature.
- Relative humidity: Enter the moisture percentage relative to saturation.
- Output units: The page reports both hPa and kPa so you can move easily between weather and engineering conventions.
How it works / method
The engine first estimates saturation vapor pressure from temperature using a compact exponential approximation. It then scales that saturation pressure by relative humidity to estimate actual vapor pressure. The page presents the result in two common pressure units for easier reuse in related calculations.
Formula used
e = (RH / 100) x 6.105 x exp(17.27T / (237.7 + T))
T is air temperature in C and e is reported in hPa before the page also converts it to kPa. This is a practical saturation-vapor-pressure approximation rather than a full property-table lookup.
Practical note: Vapor pressure estimates depend on the chosen saturation model and the quality of the humidity input. For laboratory work or high-precision phase-equilibrium calculations, consult a pressure-aware reference source.
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Kilopascals: --
Step-by-step example
Suppose the air temperature is 28 C and the relative humidity is 70 percent. Vapor pressure shows the moisture term more directly than RH alone.
- Enter 28 for air temperature.
- Enter 70 for relative humidity.
- The page estimates the saturation vapor pressure at 28 C and then takes 70 percent of that value.
- The result is displayed in hPa and kPa so it can be reused in comfort or engineering estimates.
- If temperature rises without adding moisture, relative humidity may fall even while vapor pressure stays similar.
Use cases
- Supporting humidity-state interpretation behind dew point, wet-bulb, and apparent-temperature calculations.
- Comparing atmospheric moisture using a direct pressure term rather than only RH percentages.
- Checking moisture-related calculations in weather education, greenhouse monitoring, or HVAC interpretation.
- Providing a reusable moisture value for humidex or other thermal comfort estimates.
Assumptions and limitations
- The page estimates vapor pressure with a compact saturation formula and does not replace full property tables.
- At extremes of temperature or pressure, specialized formulations may be more appropriate than a simple online approximation.
- Relative humidity measurements often carry their own uncertainty, which feeds directly into the pressure result.
- The calculator assumes an ordinary air-water context and is not intended for full thermodynamic phase-equilibrium modeling.
For weather comfort interpretation, pair this tool with dew point, wet bulb, or humidex. For exact thermodynamic property work, consult NIST or another reference dataset.
Frequently Asked Questions
Actual vapour pressure equals RH divided by 100, multiplied by saturation vapour pressure at the current temperature: e = (RH/100) × es(T). Saturation pressure comes from Magnus: es(T) = 6.112 exp(17.625T/(243.04 + T)) in hPa. At 25°C and 60% RH, es ≈ 31.7 hPa, so e ≈ 19.0 hPa. RH alone has no absolute moisture meaning; you have to anchor it to temperature first. This step trips up new researchers more than any other in atmospheric calculations.
Magnus form is the everyday workhorse: es(T) = 6.112 × exp(17.625 × T / (243.04 + T)), with T in °C and es in hPa. At 0°C, es ≈ 6.11 hPa; at 30°C, ≈ 42.4 hPa; at 100°C, ≈ 1013 hPa, which is exactly atmospheric pressure — the definition of boiling. Tetens is similar with slightly different constants. For ice surfaces below 0°C, use the over-ice variant (constants 22.587, 273.86), because vapour pressure differs above ice and supercooled water.
At 25°C, water's saturation vapour pressure is about 3.17 kPa, equivalent to 31.7 hPa, 23.8 mmHg (torr), or 0.46 psi. This is the maximum partial pressure of water vapour the air can hold at that temperature — anything above forces condensation. Useful for psychrometrics, fume-hood design, and predicting humidity in an HVAC duct. The value comes straight from the Magnus or Antoine formula. Pure-water tables and online calculators all converge on roughly 3.169 kPa at 25.0°C.
Two common choices. Magnus is simple and accurate over normal weather: es(T) = 6.112 × exp(17.625T/(243.04 + T)). Antoine is the engineering standard with three substance-specific constants: log10(P) = A − B/(C + T). For water from 1°C to 100°C, A = 8.07131, B = 1730.63, C = 233.426, with P in mmHg and T in °C. Magnus for atmospheric work, Antoine for chemical engineering. For ultra-precise work like CIPM 2007 standards, more elaborate formulations like Wagner-Pruss replace both.
Two equivalent routes. From RH and temperature: e = (RH/100) × es(T). From dew point alone: e = es(Td), because actual vapour pressure equals saturation pressure evaluated at the dew point. So at a 17°C dew point, e ≈ 19.4 hPa regardless of the air temperature. This second route is cleaner and more direct — many meteorologists prefer it because dew point is conserved through normal atmospheric heating, while RH is not. Pick whichever inputs you have on hand.
Yes, exponentially. As temperature rises, water molecules at the surface gain kinetic energy and escape into the gas phase faster, so the equilibrium between liquid and vapour shifts toward higher vapour density. Magnus shows the exponential dependence: doubling temperature in Celsius does not double the vapour pressure — it multiplies it manyfold. From 0°C to 30°C, es goes from 6.1 to 42.4 hPa, almost a sevenfold rise. This steep curve is why a small temperature change drives big humidity changes.
Antoine is log10(P) = A − B/(C + T), where A, B, and C are substance-specific constants you look up in tables, and T is in °C (or K, depending on the source). For water in the range 1–100°C, A = 8.07131, B = 1730.63, C = 233.426 give P in mmHg. Always check the temperature range your constants are valid for — outside it, errors blow up. NIST publishes Antoine constants for thousands of compounds. It is the working horse of distillation column design.
Apply Magnus directly to the dew point: e = 6.112 × exp(17.625 × Td / (243.04 + Td)), with Td in °C and e in hPa. By definition, the actual vapour pressure equals the saturation vapour pressure at the dew point — that is what dew point means physically. At Td = 15°C, e ≈ 17.0 hPa. No air-temperature input needed. This is the cleanest one-step calculation in atmospheric humidity work, and it is why dew point is the moisture variable I prefer to record in the field.
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