STP Calculator for Gas Volume, Mass and Moles

Standard temperature and pressure provides a reference condition for comparing gas quantities. In this calculator, STP means a temperature of 0 degrees Celsius, or 273.15 kelvin, and an absolute pressure of 1 atmosphere. Under the ideal gas model, one mole occupies approximately 22.414 liters at these conditions. Enter an amount in moles to calculate its ideal volume, and enter the substance's molar mass to calculate the corresponding mass. Gas volume follows from the ideal gas law, pressure times volume equals moles times the gas constant times absolute temperature. Substituting 1 atmosphere and 273.15 kelvin gives the molar volume used here. Mass equals moles multiplied by molar mass. Particle count equals moles multiplied by the Avogadro constant, 6.02214076 times 10 to the power 23 entities per mole. Depending on the substance, an entity may be an atom, molecule, formula unit, ion, or another specified particle. The term STP is not entirely universal. Some standards use 100 kilopascals rather than 1 atmosphere, producing a molar volume near 22.71 liters instead of 22.414 liters. Always confirm the convention used by a textbook, laboratory, regulator, or data source before comparing results. This page states its convention explicitly so calculations remain reproducible. The pressure is absolute, not gauge pressure, and temperature must be interpreted on an absolute scale in gas-law equations. Real gases depart from ideal behavior because particles occupy space and interact. The ideal approximation is often useful at low pressure, but accuracy varies by substance and conditions. At 0 degrees Celsius, some compounds may condense and therefore cannot be treated as an ideal gas throughout the calculation. This calculator does not apply compressibility factors or phase-equilibrium corrections. Use a molar mass that matches the actual chemical formula and composition. For a gas mixture, an average molar mass can estimate total mass, while particle counts may need species-specific composition. Keep sufficient digits during intermediate work and round the reported result according to the precision of the measured amount and molar mass. For laboratory or engineering decisions, use the applicable standard definition and validated property data.