Combined Gas Law Calculator

Molar Volume Calculator

Find the volume of one mole of an ideal gas, Vm = RT/P, at STP, room conditions, or any custom temperature and pressure, plus the total volume for any amount of gas you specify.

Molar Volume Solver

Molar Volume (Vm = RT/P)

Why Molar Volume Is the Same for Every Ideal Gas

Molar volume follows directly from Avogadro's Law: equal volumes of any ideal gas at the same temperature and pressure contain equal numbers of moles. Flip that statement around and it says equal numbers of moles occupy equal volumes. Regardless of whether the gas is hydrogen, carbon dioxide, or neon.

This is only true because the ideal gas model ignores molecular size and intermolecular forces. A mole of gas is treated as a mole of point particles, so the identity of the gas never enters the equation Vm = RT/P. Real gases deviate slightly, more so at high pressure or near their condensation point, but the approximation is excellent for everyday calculations.

Molar volume is most often quoted at standard temperature and pressure (STP), but as this calculator shows, you can compute it at literally any combination of temperature and pressure by simply plugging into Vm = RT/P.

One Mole, Same Volume, Any Gas He N₂ CO₂ 1 mol each, same T & P → same volume

One mole of any ideal gas fills the same volume at a given temperature and pressure.

Worked Example: Molar Volume at Room Temperature

Problem: What is the molar volume of an ideal gas at 25°C and 1 atm, and what volume would 0.40 mol occupy?

Given: T = 25°C = 298.15 K, P = 1 atm = 101,325 Pa. Find: Vm, then V for n = 0.40 mol. Vm = RT/P = (8.314462618 × 298.15) / 101,325 Vm ≈ 0.02446 m³/mol = 24.46 L/mol V = n × Vm = 0.40 × 24.46 Vm ≈ 24.46 L/mol, so V ≈ 9.78 L

Compare this to the STP figure of 22.4 L/mol: at the warmer room temperature of 25°C instead of STP's 0°C, molar volume is roughly 9% larger, exactly as Charles' Law would predict for that temperature increase at constant pressure. Try the calculator above with the "Room" preset to reproduce this result instantly.

Common Mistakes When Working With Molar Volume

The most frequent mistake is quoting "the" molar volume of a gas without specifying the temperature and pressure it applies to. Molar volume is not a fixed property of a substance the way molar mass is. It depends entirely on conditions, and 22.4 L/mol is only correct at classic STP specifically. Quoting it for room temperature, or for the IUPAC definition of STP, will be off by a meaningful margin.

A second mistake is forgetting to convert temperature to Kelvin before computing Vm = RT/P . Since T appears directly (not as a ratio between two states), an unconverted Celsius value plugged into this formula produces a nonsensical result rather than just a slightly wrong one.

A third pitfall is applying the ideal molar volume figure to a gas under conditions where it is no longer a good approximation, very high pressure, or a temperature close to the gas's boiling point, both cause real molar volume to deviate from the ideal RT/P prediction because intermolecular forces and molecular size stop being negligible.

Finally, be careful with the amount unit when converting between moles and volume: if a problem gives millimoles or kilomoles rather than moles, convert to moles first (or adjust Vm accordingly), since forgetting a factor of 1,000 in either direction is an easy slip when working quickly.

Molar Volume FAQ

What is molar volume?
Molar volume (Vm) is the volume occupied by one mole of a substance, most commonly used for gases: Vm = RT/P, derived directly from the ideal gas law PV = nRT with n = 1. For an ideal gas, molar volume depends only on temperature and pressure, not on which gas it is. This is a direct consequence of Avogadro's Law.
What is the molar volume of a gas at STP?
At the older, still widely-taught STP definition (0°C / 273.15 K and 1 atm), the molar volume of an ideal gas is 22.4 L/mol. Under the current IUPAC STP definition (0°C and 100 kPa), it is 22.7 L/mol. Use our dedicated STP Calculator to switch between these definitions, or enter either pressure directly here.
Does molar volume depend on which gas you use?
For an ideal gas, no, one mole of hydrogen, oxygen, nitrogen, or any other gas occupies the same volume at the same temperature and pressure, because Vm = RT/P contains no term for molecular mass or identity. This is exactly what Avogadro's Law predicts. Real gases deviate slightly from this at high pressure or low temperature, but the ideal approximation is accurate for most classroom and laboratory purposes.
How do I find the molar volume at room temperature instead of STP?
Simply enter 25°C (298.15 K) and 1 atm into the calculator above instead of the STP defaults. At room temperature, an ideal gas's molar volume is about 24.5 L/mol, noticeably larger than at STP because volume increases with temperature at constant pressure (Charles' Law).
How is molar volume used to find the number of moles in a sample?
If you know a gas sample's volume at a specific temperature and pressure, dividing that volume by the molar volume at those same conditions gives you the number of moles: n = V/Vm. This is a common technique in stoichiometry problems involving gas-phase reactions.
What is the SI unit of molar volume?
The SI unit is cubic meters per mole (m³/mol), though liters per mole (L/mol) is far more commonly used in chemistry because it produces more convenient numbers, 0.0224 m³/mol versus 22.4 L/mol for the same STP quantity.
How does molar volume change with altitude or weather?
Since Vm = RT/P, molar volume rises whenever pressure falls (higher altitude, low-pressure weather systems) or temperature rises, and falls whenever pressure increases or temperature drops. A weather balloon's payload of gas that occupies a modest molar volume at ground level can expand to many times that volume by the time it reaches the thin, cold-but-not-cold-enough-to-matter upper atmosphere, purely from the drop in pressure.
Does molar volume apply to gas mixtures, like air?
Yes: as long as the mixture behaves approximately ideally (true for air under everyday conditions), the molar volume of a gas mixture is the same as for a pure gas at the same temperature and pressure, since Avogadro's Law makes no reference to chemical identity. One mole of air (a mix of roughly 78% N₂, 21% O₂, and trace other gases) occupies very nearly the same 22.4 L at STP as one mole of pure nitrogen or pure oxygen would.
How is molar volume related to gas density?
Density equals mass divided by volume, so a gas's density at a given temperature and pressure is its molar mass divided by its molar volume: ρ = M / Vm. Because molar volume is the same for every ideal gas at fixed T and P, density differences between gases (why helium balloons float and CO₂ sinks) come entirely from differences in molar mass, not from any difference in how much space a mole of each gas occupies.

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