Absolute pressure P
Absolute pressure only, P > 0. Add ambient pressure to a gauge reading first.
Thermal engineering · Thermodynamics
Ideal gas law calculator
Find one unknown in PV = nRT from the other three quantities. Use absolute pressure and select your temperature unit: Celsius and Fahrenheit are converted to kelvins before calculation.
Created v1.0
By Thibaut Grzelak, Mechanical Analysis Engineer
02
Results
Gas volume V
Volume occupied by the gas, V > 0.
Amount of substance n
mol
Amount in moles, n > 0; not a mass in kilograms.
Absolute temperature T
Choose K or °C (also °F). The engine always uses T > 0 K.
Molar gas constant
J/(mol·K)
R = 8.31446261815324 J/(mol·K).
Relation and numerical substitution
PV = nRT—
Equilibrium ideal gas with negligible molecular volume and intermolecular interactions. P, V, n and T are strictly positive.
01Formulas and symbols
Formulas used
P
P = n × R × T / VAbsolute pressure.V
V = n × R × T / PVolume of the ideal gas.n
n = P × V / (R × T)Amount of substance in moles.T
T = P × V / (n × R)Absolute temperature in kelvins.R
R = 8.31446261815324 J/(mol·K)Exact SI molar gas constant, the product of Avogadro and Boltzmann constants.02Assumptions and limits
Scope of validity
- Equilibrium ideal gas with negligible molecular volume and intermolecular interactions. P, V, n and T are strictly positive.
03Validation example
Reference numerical case
- Given n = 1 mol, T = 300 K and P = 100 kPa = 100000 Pa: V = nRT/P = 1 × 8.31446261815324 × 300 / 100000 = 0.024943387854… m³ = 24.943… L (≈ 25 L).
04References
Technical references
- Chemistry, §9.2 The Ideal Gas Law — OpenStax
- CODATA molar gas constant — NIST
FAQ
Can I enter gauge pressure?
Convert it first: absolute pressure equals gauge pressure plus local ambient pressure. The calculator never adds atmospheric pressure implicitly.
Why is −273.15 °C rejected?
It is 0 K. This gas state model requires a strictly positive absolute temperature. A negative Celsius value can still be valid if it exceeds absolute zero.
Is one mole always 22.4 litres?
No. Molar volume depends on temperature and absolute pressure. State both conditions when comparing volumes.
05How PV = nRT behaves
At fixed amount and temperature, doubling absolute pressure halves volume. At fixed pressure and amount, volume is proportional to kelvin temperature. R uses coherent Pa, m³, mol and K units.
06Real gas limits
The approximation is often less appropriate at high density, high pressure, low temperature or near condensation. No gas-specific compressibility factor is applied, so this result alone cannot quantify real-gas error.
07Common mistakes
Do not substitute Celsius directly into PV = nRT or mix litre-atmosphere constants with pascals and cubic metres. Use moles, not mass; conversion from mass requires molar mass.