Vapor Pressure of Water Calculator (kPa, mmHg, psi and more)
Get fast, accurate vapor pressure of water or ice at any temperature. Convert between kPa, Pa, bar, atm, psi, Torr, and mmHg. Formulas: Buck, Magnus, Tetens, Antoine.
Enter a temperature and this Vapor Pressure of Water Calculator returns the saturation vapor pressure of water or ice in your preferred units. It works across common formula families used in meteorology, HVAC, food science, and lab work. You pick the units. It handles °C, °F, or K for temperature and converts pressure to kPa, Pa, bar, atm, psi, Torr, or mmHg without fuss.
How the calculator works
Vapor pressure tells you how strongly water wants to exist as vapor at a given temperature. At saturation the air holds as much water vapor as it can at that temperature. That pressure depends on the phase at the interface. Temperatures below 0 °C use ice constants. Temperatures at or above 0 °C use liquid water constants.
The calculator reads your temperature, converts it to Celsius internally, and then evaluates several well-known empirical equations. It then converts the result from kilopascals to the unit you choose. You will see values from these models:
- Magnus — simple and popular over water and ice.
- Tetens — a classic meteorology choice with a slightly different constant set.
- Buck — widely cited for higher accuracy across typical weather ranges.
- Antoine — works for liquid water from roughly 1–374 °C and outputs from a log relation in mmHg.
The tool displays all available formulas side by side. If a formula sits out of its valid range you will see a neutral placeholder instead of a number.
Quick start: three simple steps
- Type the temperature and pick the temperature unit.
- Pick the pressure unit for each result row if you want to compare in psi, kPa, bar, atm, Torr, or mmHg.
- Read the values. For most room-temperature work the Buck and Magnus rows will agree closely.
Formulas used (Magnus, Tetens, Buck, Antoine)
You do not need to memorize these equations to use the calculator. Still it helps to see the structure and the valid ranges. In the relations below T is temperature in °C and the outputs are shown in kilopascals unless stated otherwise.
Magnus formula
Over liquid water:
es(T) = 0.61094 × exp( 17.625 × T / (T + 243.04) ) [kPa]
Over ice:
es(T) = 0.61094 × exp( 22.587 × T / (T + 273.86) ) [kPa]
The Magnus form appears in many met toolkits because it is compact and behaves well near room temperature. Constants vary across sources by a few hundredths which shifts the result slightly. See an overview in Magnus formula.
Tetens formula
Over liquid water:
es(T) = 0.61078 × exp( 17.2694 × T / (T + 237.29) ) [kPa]
Over ice:
es(T) = 0.61078 × exp( 21.8746 × T / (T + 265.5) ) [kPa]
Tetens looks like Magnus at first glance. The constants differ and that leads to tiny differences in mid-range temperatures. Vapour pressure of water.
Buck equation
Over liquid water:
es(T) = 0.61121 × exp( (18.678 − T/234.5) × (T / (257.14 + T)) ) [kPa]
Over ice:
es(T) = 0.61115 × exp( (23.036 − T/333.7) × (T / (279.82 + T)) ) [kPa]
The Buck form fits laboratory data remarkably well across typical environmental ranges. It is a strong choice for weather work and humidity conversions. See a summary and coefficients in Buck equation and in instrumentation notes from Buck Research.
Antoine equation (liquid water only)
The Antoine relation is logarithmic and commonly published with pressure in mmHg:
log10(PmmHg) = A − B / (C + T)
Typical coefficients for water use T in °C. One set covers roughly 1–100 °C and another set extends toward the critical region. The calculator converts the mmHg result to your chosen pressure unit. See the coefficient tables in Antoine equation and curated data in the NIST Chemistry WebBook.
Which formula should you use?
Pick the equation that fits your temperature band and accuracy needs. The table below gives practical guidance.
| Formula | Phase handled | Typical valid range | Strengths | Notes |
|---|---|---|---|---|
| Magnus | Water & Ice | ~−40 to 50 °C | Simple, fast | Different constants exist across sources |
| Tetens | Water & Ice | ~−40 to 50 °C | Common in ag-meteorology | Very close to Magnus near room temp |
| Buck | Water & Ice | ~−40 to 50 °C | Excellent fit to lab data | Great general-purpose choice |
| Antoine | Water only | ~1 to 374 °C | Chemical-engineering staple | Presented in mmHg by convention then converted |
Worked examples
Example 1 — Vapor pressure at 25 °C
You enter 25 for temperature and keep the unit as °C. The calculator switches to the liquid-water branch and evaluates each equation. Using the Buck form as an anchor:
es(25) = 0.61121 × exp( (18.678 − 25/234.5) × (25/(257.14 + 25)) ) ≈ 3.17 kPa
Magnus and Tetens give values within a few thousandths at 25 °C. Antoine returns a very similar number after conversion from mmHg. Converting 3.17 kPa to other units:
- ≈ 23.8 mmHg (Torr)
- ≈ 0.0313 atm
- ≈ 0.460 psi
Example 2 — Vapor pressure over ice at −10 °C
Because −10 °C sits below zero the calculator uses the ice constants. With the Buck ice relation:
es(−10) ≈ 0.61115 × exp( (23.036 − (−10)/333.7) × (−10 / (279.82 − 10)) ) ≈ 0.286 kPa
A value near 0.29 kPa aligns with published tables for saturation over ice. This lower pressure explains why very cold air holds so little water vapor.
Example 3 — Using Fahrenheit
Maybe your sensor reports in °F. Enter 77 °F and the tool converts to 25 °C internally. You get the same 3.17 kPa result since the physics do not care about the input scale.
Unit conversion cheat sheet
Here are quick factors for converting from kilopascals to the most common pressure units:
- Pa = kPa × 1000
- hPa = kPa × 10
- bar = kPa ÷ 100
- atm = kPa ÷ 101.325
- psi = kPa ÷ 6.894757
- Torr or mmHg = kPa ÷ 0.133322368
- inHg = kPa ÷ 3.386389
The calculator applies these conversions automatically. You pick units for each row and it formats the value with sensible precision.
Reference table: vapor pressure vs temperature
Use this compact table for quick checks. Values are typical saturation vapor pressures over liquid water unless the temperature is below zero where the ice relation applies. Numbers are approximate to keep the table readable.
| Temperature (°C) | Vapor pressure (kPa) | Vapor pressure (mmHg) |
|---|---|---|
| −20 | 0.103 | 0.77 |
| −10 | 0.286 | 2.15 |
| 0 | 0.611 | 4.58 |
| 10 | 1.228 | 9.21 |
| 20 | 2.338 | 17.5 |
| 25 | 3.169 | 23.8 |
| 30 | 4.243 | 31.8 |
| 40 | 7.385 | 55.4 |
| 50 | 12.35 | 92.6 |
| 60 | 19.95 | 149.7 |
| 70 | 31.17 | 233.8 |
| 80 | 47.38 | 355.5 |
| 90 | 70.10 | 525.9 |
| 100 | 101.33 | 760.0 |
If you work above 100 °C under pressure you should prefer Antoine with the appropriate coefficient set from a trusted data source such as NIST.
Where this matters in the real world
- HVAC and building science — Vapor pressure drives moisture migration through assemblies. It connects to dew point and condensation risk.
- Meteorology — Weather stations compute humidity from temperature and vapor pressure. Forecast tools need saturation values for cloud formation models.
- Food processing — Drying curves depend on saturation vapor pressure near product surfaces.
- Brewing and distilling — Boiling behavior shifts with ambient pressure and temperature. Knowing saturation pressure helps with process control.
- Environmental engineering — Open-water evaporation uses vapor pressure deficits between water and air.
- Laboratory work — Calibrating hygrometers or generating controlled humidity requires an accurate saturation reference.
Frequently asked questions
What is vapor pressure in plain language?
Think of it as the push from water molecules that have enough energy to escape into the air. Warmer water pushes harder. Colder water pushes less.
Why do results differ across formulas?
Each formula fits a different set of measurements. They aim for low error over a certain temperature band. The constants are not identical which leads to small differences. Buck and Magnus tend to agree within a few tenths of a percent near room temperature.
How do I get relative humidity from vapor pressure?
Relative humidity is the ratio of actual vapor pressure to the saturation vapor pressure at the same temperature. If you know dew point you can get actual vapor pressure with the same equations then divide by the saturation value.
What happens at the boiling point?
At the boiling point the saturation vapor pressure equals the ambient pressure. At sea level that means 100 °C where saturation equals 101.325 kPa.
Is the Antoine equation better?
It is excellent for liquid water over a very wide range. It is also more sensitive to coefficient choice. If your temperature sits within normal weather ranges the Buck relation offers accurate and stable results without coefficient switching.
Can I use this calculator for seawater?
No. Dissolved salts lower the saturation vapor pressure. You would need activity corrections based on salinity. This tool targets pure water and pure ice.
Troubleshooting & tips
- Negative temperatures — The tool switches to the ice branch automatically below 0 °C.
- Out-of-range warnings — If a model does not apply at your temperature the calculator disables its number. Switch to Buck or Antoine as needed.
- Unit sanity check — kPa near 3.17 at 25 °C is a good test value. If you see 3170 you likely chose Pa by mistake.
- High-temperature work — Use Antoine coefficients from a trusted table then convert units. NIST is your friend for this scenario.
How to read the outputs
You will see small differences between models. At 20–30 °C they usually sit within a few hundredths of a kPa. At colder temperatures ice constants matter and the offset grows. Your application decides which model to consider the source of truth.
If you prefer round numbers pick kPa. If you work in legacy instrumentation choose mmHg or Torr. Engineers in gas-handling use bar and atm. The calculator prints values with precision that matches each unit so the number stays easy to read on mobile and desktop.
Why this calculator is handy
- It converts temperature from °C, °F, or K without a second step.
- It computes with four respected formula families so you can cross-check.
- It presents pressure in the unit you already use at work.
- It explains ranges and caveats inline which saves you a detour to datasheets.
A tiny mental model
Picture a crowd at the door of a theater. Warm the room and the crowd gets pushier. More people make it through the door which raises the pressure on the other side. Cool the room and the line calms down. That picture mirrors how vapor pressure responds to temperature.
Key takeaways
- Vapor pressure rises exponentially with temperature.
- Buck, Magnus, and Tetens produce very similar results in everyday ranges.
- Antoine reaches into high temperatures and needs the right coefficient set.
- Always check phase: below 0 °C use the ice constants.
Use this Vapor Pressure of Water Calculator whenever you need a quick, trustworthy saturation value. It keeps the math behind the scenes while giving you control over units and formulas. That balance speeds up lab work, design checks, and field decisions.
