Vapor Pressure Unit Conversion: Calculator and Formula

Convert vapor pressure between common units by entering a pressure value and choosing its starting and target units. The calculator returns the equivalent pressure; vapor pressure itself does not change when its unit changes.

Formula

Convert through pascals, the SI unit of pressure: P_target = P_input × F_from ÷ F_to. Here, P_input is the entered pressure, P_target is the equivalent pressure in the selected target unit, and each F is the number of pascals represented by one unit. The calculator uses: 1 Pa = 1 Pa; 1 kPa = 1,000 Pa; 1 bar = 100,000 Pa; 1 atm = 101,325 Pa; 1 mmHg ≈ 133.322368 Pa; 1 torr ≈ 133.322368 Pa; and 1 psi ≈ 6,894.757293 Pa.

Worked Example

With the defaults, convert 101.325 kPa to pascals:

1. Starting-unit factor: 1 kPa = 1,000 Pa.
2. Target-unit factor: 1 Pa = 1 Pa.
3. Calculation: 101.325 × 1,000 ÷ 1 = 101,325 Pa.
4. Rounded to the calculator’s four decimal places: 101,325.0000 Pa.

The same pressure is 1 standard atmosphere. Small differences among conversions may appear if a reference uses rounded constants; the calculator uses the factors listed above.

What Vapor Pressure Means

Vapor pressure is the pressure exerted by a vapor in equilibrium with its liquid or solid at a specified temperature. In a closed space, molecules leave the condensed phase and enter the gas phase while others return. At equilibrium, those opposing rates balance. The equilibrium pressure depends strongly on the substance and temperature: for a given pure substance, vapor pressure generally rises as temperature rises.

That makes vapor pressure different from a substance’s total pressure in a mixture. For example, air above a liquid can exert its own pressure, while the liquid’s vapor contributes a partial pressure. A vapor-pressure conversion only changes the unit used to express that pressure; it does not calculate temperature effects, total mixture pressure, or a new equilibrium state.

Common Units and Their Uses

Pressure units appear in different scientific fields, instruments, and reference tables. These are useful approximate relationships:

• Pascal (Pa) and kilopascal (kPa): SI units; 1 kPa equals 1,000 Pa. Many engineering and scientific sources use kPa.
• Bar: 1 bar equals 100,000 Pa, or 100 kPa. It is common in technical contexts, but is not the same as an atmosphere.
• Standard atmosphere (atm): exactly 101,325 Pa by definition. It is a reference pressure, not a claim about the pressure at every location.
• Millimeters of mercury (mmHg) and torr: closely equivalent pressure units. Their conventional conversion is approximately 133.322368 Pa per unit.
• Pounds per square inch (psi): a customary pressure unit, often seen in US equipment specifications. One psi is approximately 6,894.757293 Pa.

For orientation, 1 atm is about 101.325 kPa, 1.01325 bar, 760 mmHg, 760 torr, or 14.6959 psi. These rounded equivalents are helpful for checking a result, but retain more significant digits when the source data and application justify them.

How to Convert a Value Reliably

First identify the unit attached to the vapor-pressure number. Then select that unit as the input unit, choose the desired output unit, and enter the value. The conversion factor method works in either direction: to convert from a larger unit to a smaller one, the numerical value usually increases; converting to a larger unit usually makes the number smaller.

For manual conversion, multiply the entered value by the source unit’s pascal factor, then divide by the target unit’s pascal factor. For instance, converting 2.5 bar to kPa gives 2.5 × 100,000 ÷ 1,000 = 250 kPa. Converting 30 mmHg to kPa gives approximately 30 × 133.322368 ÷ 1,000 = 3.9997 kPa.

Keep units attached to values during calculations. A bare number such as “20” is ambiguous: it could mean 20 Pa, 20 kPa, or 20 mmHg, with very different physical magnitudes. Also distinguish an absolute pressure from a gauge pressure. Vapor pressure is ordinarily an absolute thermodynamic pressure; a gauge reading measures pressure relative to surrounding atmosphere and cannot be compared directly without accounting for that reference.

Accuracy, Rounding, and Limits

The calculator reports four decimal places, but that display precision does not make the input or conversion constants exact to four decimal places in every practical measurement. A measured pressure may have uncertainty, and a vapor-pressure table may round values or specify a temperature range. Do not report more meaningful precision than the source data support.

Temperature is essential when looking up or predicting vapor pressure, but it is not needed to convert an already-known pressure between units. If a question asks for vapor pressure at a particular temperature, a unit converter alone cannot provide the answer: the substance, temperature, and a valid measurement or vapor-pressure relation are needed. Likewise, converting units cannot turn a partial pressure into total pressure or establish whether a sample is at equilibrium.

Frequently Asked Questions

How many kPa are in 1 atm?
One standard atmosphere is exactly 101.325 kPa, because 1 atm is defined as 101,325 Pa and 1 kPa is 1,000 Pa.

Is mmHg the same as torr?
They are treated as equivalent for most practical conversions: each is approximately 133.322368 Pa. Some high-precision contexts distinguish their definitions, so follow the convention specified by the data source.

How do I convert mmHg to kPa?
Multiply the mmHg value by approximately 0.133322368. For example, 760 mmHg is about 101.325 kPa.

Does vapor pressure change when I convert units?
No. Only its numerical representation changes. The physical pressure remains the same, provided the original value and unit are interpreted correctly.

Can I calculate vapor pressure from temperature with this converter?
No. This tool converts a pressure already known. Estimating vapor pressure from temperature requires substance-specific data or an appropriate equation, and the temperature scale and valid range must be respected.

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in p | Vapor pressure | kPa | 101.325 | 0
sel from | Convert from | Pa=1, kPa=1000, bar=100000, atm=101325, mmHg=133.322368, torr=133.322368, psi=6894.757293
sel to | Convert to | Pa=1, kPa=1000, bar=100000, atm=101325, mmHg=133.322368, torr=133.322368, psi=6894.757293
out result | Converted vapor pressure | selected unit | p * from / to | 4