Combined Gas Law Calculator

STP Calculator

Convert between moles and volume of a gas at standard temperature and pressure. Switch between the classic (22.4 L/mol) and current IUPAC (22.7 L/mol) definitions instantly.

STP Solver

Molar volume in use: 22.4 L/mol

STP, NTP, and SATP Compared

DefinitionTemperaturePressureMolar Volume
Classic STP0°C (273.15 K)1 atm (101.325 kPa)22.4 L/mol
IUPAC STP0°C (273.15 K)100 kPa22.7 L/mol
NTP20°C (293.15 K)1 atm (101.325 kPa)24.06 L/mol
SATP25°C (298.15 K)100 kPa24.79 L/mol

"Standard conditions" is not one single thing. It is a family of reference points that different disciplines and even different decades of chemistry have settled on, each trading off round temperature numbers against round pressure numbers.

The practical takeaway: always check which definition a textbook, exam, or paper is using before quoting a molar volume, since 22.4 L/mol and 22.7 L/mol are both "STP" but are about 1.3% apart, usually negligible, but occasionally exactly the difference between a right and wrong answer key.

All four values in the table follow directly from the ideal gas law, Vm = RT/P, see our Molar Volume Calculator if you need molar volume at a temperature and pressure combination not listed here.

Worked Example: Gas Volume Produced in a Reaction

Problem: A reaction produces 0.65 mol of carbon dioxide gas. What volume does this occupy at classic STP?

Given: n = 0.65 mol, classic STP molar volume Vm = 22.4 L/mol. V = n × Vm V = 0.65 × 22.4 V ≈ 14.56 L

This kind of calculation is extremely common in stoichiometry problems, where a balanced chemical equation gives you moles of gas produced, and you need to report how much physical space that gas would occupy under standard laboratory reference conditions. If the reaction actually happens at room temperature rather than 0°C, use the Molar Volume Calculator instead, which lets you plug in any temperature and pressure rather than only the fixed STP or IUPAC STP conditions used here.

Common Mistakes When Working at STP

The single most common mistake is mixing up the two STP definitions, using 22.4 L/mol when a problem or textbook actually specifies the current IUPAC standard (22.7 L/mol), or vice versa. The two values are close enough that an error is easy to miss but large enough (about 1.3%) to matter on a graded assignment or a precise laboratory calculation. Always check which definition is in play before quoting a molar volume from memory.

A second mistake is applying an STP molar volume to a gas that is not actually at STP. Room-temperature reactions are common in real laboratories, but 22.4 L/mol and 22.7 L/mol both assume 0°C specifically. If the reaction happens at 20–25°C instead, use the Molar Volume Calculator's custom or room-temperature preset rather than the STP figures on this page.

A third pitfall is forgetting that STP, NTP, and SATP are three genuinely different reference conditions with three different molar volumes, assuming "standard conditions" always means the same numbers regardless of which acronym a source uses can introduce a meaningful error, especially between STP (0°C) and SATP (25°C), which differ enough in temperature to shift molar volume by more than 10%.

Finally, remember that these figures are ideal-gas approximations. Real gases, especially near their condensation point, deviate slightly from the predicted STP molar volume, though the deviation is small enough to ignore for the vast majority of educational and laboratory purposes.

STP Calculator FAQ

What is STP in chemistry?
STP stands for Standard Temperature and Pressure, a fixed reference condition used to compare gas measurements consistently. The classic definition, still widely taught, is 0°C (273.15 K) and 1 atm; the current IUPAC definition (adopted in 1982) is 0°C and exactly 100 kPa. Both use freezing-point temperature but differ slightly in pressure.
What is the molar volume of a gas at STP?
At the classic STP definition (0°C, 1 atm), one mole of an ideal gas occupies 22.4 L. At the current IUPAC STP definition (0°C, 100 kPa), it occupies 22.7 L. The difference comes entirely from the slightly lower pressure IUPAC uses (100 kPa is slightly less than 1 atm's 101.325 kPa), which by Boyle's Law means slightly more volume.
Why are there two different definitions of STP?
Historically, chemists defined standard pressure as exactly 1 atmosphere (101.325 kPa), which is where the familiar 22.4 L/mol figure comes from. In 1982, IUPAC redefined standard pressure as exactly 100 kPa, a rounder, more SI-consistent number, which shifted molar volume to 22.7 L/mol. Many textbooks and courses still teach the older 22.4 L/mol figure, so it's worth confirming which definition a given problem expects.
What is the difference between STP and NTP?
NTP (Normal Temperature and Pressure) is a separate, less standardized reference condition, most commonly defined as 20°C (293.15 K) and 1 atm, closer to typical room conditions than STP's freezing point. NTP is used more often in engineering and gas-flow contexts than in chemistry.
What is SATP?
SATP (Standard Ambient Temperature and Pressure) is defined by IUPAC as 25°C (298.15 K) and 100 kPa, closer to a real laboratory's room temperature than STP's 0°C, while keeping the same 100 kPa pressure convention. At SATP, molar volume is about 24.79 L/mol.
How do I convert moles to volume at STP?
Multiply the number of moles by the molar volume at your chosen STP definition: V = n × Vm. For example, 2.5 mol of gas at classic STP occupies 2.5 × 22.4 L = 56 L. This calculator does that multiplication (and its reverse, volume to moles) instantly.
Why do chemists bother defining standard conditions at all?
Gas volume depends heavily on temperature and pressure. The same 1 mole of gas can occupy anywhere from under 20 L to well over 25 L depending on conditions. Without a fixed reference point, comparing gas measurements taken in different labs, at different altitudes, or on different days would be meaningless. STP gives every chemist the same yardstick, so a molar volume, a gas density, or a reaction yield reported 'at STP' can be checked and reproduced anywhere.
Does STP apply to liquids and solids too?
The term is occasionally used loosely for solids and liquids, but it matters enormously more for gases, since gas volume changes dramatically with temperature and pressure while liquid and solid volumes barely change at all under normal conditions. In practice, 'STP' almost always refers to a gas-phase reference condition.
Is STP the same as room temperature?
No: STP's temperature (0°C / 273.15 K, the freezing point of water) is noticeably colder than typical room temperature (roughly 20–25°C / 293–298 K). This trips up many students, since 'standard' sounds like it should mean 'normal everyday conditions.' If a calculation genuinely needs room-temperature values, use SATP (25°C, 100 kPa) or the custom option on our Molar Volume Calculator instead of STP.

Related Calculators

⇄ Combined Gas Law Vm Molar Volume K Kelvin Converter atm Pressure Units