How this instrument works
Standard temperature and pressure (STP) is a fixed reference point chemists use so that gas volumes measured anywhere can be compared on equal footing. At STP, one mole of any ideal gas occupies the same volume — the molar volume, Vm — regardless of what the gas actually is, because ideal-gas behavior depends only on the number of particles present, not their identity. This calculator multiplies your amount of gas (in moles) by the molar volume for whichever STP convention you select, giving the total volume directly.
There are two legitimate definitions of STP in circulation, and neither is wrong — they just answer the same question under two different pressure standards. IUPAC redefined standard pressure from 1 atm to exactly 100 kPa in 1982, which sets Vm at 22.711 L/mol at 0°C. Nearly every chemistry reference published or revised since then uses this figure. The older convention, still taught in countless general-chemistry courses and printed in older tables, defines standard pressure as 1 atm (101.325 kPa) and gives Vm = 22.414 L/mol — the familiar 'STP = 22.4 liters' figure you may already have memorized.
Which one you need depends on where the number is going. If you're checking a textbook problem or an older lab manual, use 22.414 L/mol. If you're working from a source that follows current IUPAC recommendations — most modern references, and anything citing the IUPAC Green Book or Gold Book — use 22.711 L/mol, the default here. The two answers differ by about 1.3%, small but not negligible once you chain several conversions together.
- Enter the amount of gas in moles into 'Amount of gas, n (mol).'
- Pick the STP convention that matches your source: the modern IUPAC 100 kPa standard (22.711 L/mol) or the traditional 1 atm standard (22.414 L/mol).
- Read the result in 'Gas volume at STP (L)' — it updates instantly as you change either input.
- If a textbook or assignment specifies which STP definition to use, match the dropdown to it rather than assuming the default.
Worked example — 2 mol of gas at the modern IUPAC standard
Say you're finding the volume that 2 moles of gas occupies at standard conditions under the current IUPAC definition. Enter 2 into 'Amount of gas, n (mol)' and leave the convention dropdown on its default, 22.711 L/mol. The calculator multiplies directly: V = 2 mol × 22.711 L/mol = 45.422 L.
Now switch to 1 mole under the traditional 1 atm convention (22.414 L/mol) instead, and the result reads 22.414 L directly — since n = 1, the volume equals the molar volume itself. That's the classic 'STP = 22.4 liters' figure taught in countless general-chemistry courses, about 1.3% smaller than the modern IUPAC figure for the same amount of gas.
Questions
Why does this calculator offer two different STP conventions?
Because chemistry genuinely uses two different definitions of 'standard pressure,' and both are still current in different contexts. IUPAC changed the official standard pressure from 1 atm to exactly 100 kPa in 1982, which shifted the molar volume of an ideal gas at 0°C from 22.414 L/mol to 22.711 L/mol. Modern references generally follow the 1982 definition, but a huge amount of existing coursework, textbooks, and older reference tables still use the pre-1982 value — so a calculator that only supported one convention would give the 'wrong' answer for roughly half its users.
Which STP value should I use — 22.711 or 22.414 L/mol?
Match whatever convention your source material uses. If a textbook, problem set, or exam specifies 'STP = 22.4 L/mol,' use the traditional 22.414 option. If you're working from a source that follows current IUPAC recommendations, or you have no other guidance, the modern 22.711 L/mol default is the technically current standard. When in doubt, check whether your source cites 1 atm or 100 kPa as its reference pressure — that tells you immediately which value applies.
Does this only work for ideal gases?
Yes — this calculator assumes ideal-gas behavior, where volume depends only on the number of moles and the standard molar volume, not on which specific gas you're measuring. Real gases deviate slightly from this at high pressure or low temperature because of intermolecular forces and finite molecular size, but at STP conditions the deviation is small for most common gases (a percent or two at most), which is why the ideal-gas molar volume remains the standard reference figure used throughout general chemistry.
Why does a 1.3% difference between conventions actually matter?
For a rough estimate, 1.3% barely registers. But in quantitative stoichiometry — converting between moles and liters across a multi-step calculation, or checking gas-law problems where several conversions compound — using the wrong Vm introduces a small, systematic error that can shift a final answer enough to mark it wrong on an exam or throw off a lab yield calculation. That's exactly why both values are kept precise to five significant figures here rather than rounded to 'about 22.4.'
Where does the 0°C reference temperature come from?
0°C (273.15 K) is the temperature component of the STP definition and hasn't changed between the traditional and modern conventions — only the pressure component (1 atm vs. 100 kPa) changed in 1982. Some fields use a different reference altogether, like NIST's 20°C 'normal conditions' or the natural-gas industry's 60°F standard, but 'STP' in a chemistry context specifically means 0°C, and that's the temperature both molar volumes on this page are calculated at.