Gas Volume at STP / NTP Calculator

Understanding the gas volume at STP — concept and principle

The molar volume of a gas is the space one mole occupies at a stated temperature and pressure. Chemistry problems constantly convert between moles and litres of gas, but "standard conditions" are defined differently by different textbooks and industries. This calculator gives the volume of your amount of gas under three common reference conditions side by side, so you can match the one your course, data sheet or standard uses.

The same 1 mol takes slightly different volumes depending on which “standard” is used.

How it is calculated

IUPAC STP (0 °C, 100 kPa = 1 bar): V = n × 22.711 L/mol.
0 °C and 1 atm (101.325 kPa), the older convention: V = n × 22.414 L/mol.
NTP (20 °C, 1 atm): V = n × 24.055 L/mol.

The only input is the amount of substance n in moles. Each factor is the ideal-gas molar volume Vm = RT/P for that condition.

The principle

The ideal gas law PV = nRT gives V/n = RT/P. With R = 8.314462618 J/(mol·K), 273.15 K and 100 kPa, Vm = 22.711 L/mol; at 101.325 kPa the same temperature gives 22.414 L/mol; at 293.15 K and 101.325 kPa it is 24.055 L/mol. Avogadro's law is the intuition: equal volumes of ideal gases at the same temperature and pressure contain equal numbers of molecules, so the molar volume does not depend on which gas it is. IUPAC changed standard pressure from 1 atm to 1 bar in 1982, which is why both 22.4 and 22.7 L/mol appear in books.

Worked example

Input: n = 1 mol.
STP (0 °C, 1 bar): 1 × 22.711 = 22.711 L.
0 °C, 1 atm: 1 × 22.414 = 22.414 L.
NTP (20 °C, 1 atm): 1 × 24.055 = 24.055 L.

Each point is the calculator re-run with only “Moles” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Moles	Volume at STP (0 °C, 1 bar) (L)
0.25	5.67775
0.5	11.3555
0.75	17.03325
1	22.711
1.25	28.38875
1.5	34.0665
2	45.422

How to read the result

The three numbers differ by up to about 7%, which is larger than most measurement errors, so always state which condition you mean. Higher temperature increases the volume in proportion to absolute temperature, and higher pressure decreases it in inverse proportion. Another common reference, SATP (25 °C, 1 bar), gives about 24.79 L/mol. Real gases deviate slightly from these values; for gases such as nitrogen or oxygen near room conditions the difference is well under 1%.

Common mistakes

Using 22.4 L/mol automatically when the problem defines STP as 1 bar.
Applying a molar volume to a liquid or solid, such as water at 0 °C.
Mixing up "normal" and "standard" conditions; industrial "normal cubic metres" can use 0 °C or 20 °C depending on the field.

Keep in mind: These molar volumes are ideal-gas values. For easily condensed gases, high pressures or precise gas metering, use real-gas data or a compressibility factor and the reference condition required by the applicable standard.

Frequently asked questions

How do I convert litres at STP back to moles?

Divide the volume by the molar volume for that condition, for example 45.4 L ÷ 22.711 L/mol = 2.0 mol at IUPAC STP.

How do I find the volume at other conditions?

Use V = nRT/P directly, or scale a known value: V₂ = V₁ × (T₂/T₁) × (P₁/P₂), with temperatures in kelvin.

Which value should I use in school chemistry?

Use whichever your textbook or exam states. Many curricula still use 22.4 L/mol at 0 °C and 1 atm, while IUPAC uses 22.7 L/mol at 1 bar.

Sources: IUPAC Compendium of Chemical Terminology (Gold Book), "standard conditions for gases" · CODATA recommended value of the molar gas constant, NIST

Checked 8 Nov 2026. Standard textbook formulas in SI units, computed in your browser. For learning and estimates — real designs must follow your local codes and be checked by a qualified engineer.

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