STP Temperature and Pressure Calculator

Standard temperature and pressure (STP) is an agreed reference state, not a law of nature. It exists so that gas volumes measured in different labs can be compared.

Current IUPAC definition:

$T = 273.15\ \text{K} = 0\ ^\circ\text{C}, \qquad P = 10^{5}\ \text{Pa} = 100\ \text{kPa} = 1\ \text{bar}$

Older definition, still the one used in most textbooks and on school exams:

$T = 273.15\ \text{K}, \qquad P = 1\ \text{atm} = 101.325\ \text{kPa} = 760\ \text{mmHg}$

The molar volume of an ideal gas follows from whichever you pick:

• 1 atm convention: $V_m = 22.414$ L/mol
• 1 bar convention: $V_m = 22.711$ L/mol

STP is not SATP. Standard ambient temperature and pressure is 298.15 K (25 °C) at 100 kPa, giving $V_m = 24.79$ L/mol.

The assumption people forget: the 22.4 L/mol shortcut is tied to 0 °C and 1 atm specifically. Quote the convention you are using, because the two answers differ by 1.3%.

Every STP calculation is the ideal gas law with the temperature and pressure already fixed:

$PV = nRT$

• $P$ — pressure, pascals (Pa) with the SI gas constant, or atmospheres (atm) with the L·atm form
• $V$ — volume, cubic metres (m³) or litres (L)
• $n$ — amount of substance, moles (mol)
• $T$ — absolute temperature, kelvin (K)
• $R$ — gas constant, 8.314 J/(mol·K) or 0.08206 L·atm/(mol·K)

Substituting the STP values for one mole gives the molar volume directly:

$V_m = \frac{RT}{P} = \frac{(0.08206)(273.15)}{1.00} = 22.414\ \text{L/mol}$

To move a sample measured elsewhere to STP, use the combined gas law:

$\frac{P_1 V_1}{T_1} = \frac{P_2 V_2}{T_2}$

When it applies: a fixed amount of gas, well above its condensation point and below a few atmospheres. Near liquefaction, real gases deviate and you need a compressibility factor $Z$ in $PV = ZnRT$.

• Using celsius in the gas law — every $T$ here is absolute. Add 273.15 first; a temperature of 0 in the denominator is the classic tell.
• Mixing $R$ with the wrong pressure unit — 0.08206 goes with atm and litres, 8.314 goes with pascals and cubic metres. Pick one system and stay in it.
• Quoting 22.4 L/mol under the bar convention — at 100 kPa the molar volume is 22.711 L/mol.
• Confusing STP with room temperature — STP is 0 °C, which is why gas volumes at STP look smaller than lab measurements.
• Applying the molar volume to a liquid or solid — it is a property of the ideal gas model only.
• Forgetting that $n$ must be constant in the combined gas law. If gas is added or consumed by reaction, go back to $PV = nRT$ on each side.