STP Calculator
Enter your gas volume, temperature and pressure, then choose a standard-condition definition (Old STP, New STP, NTP, or SATP). The calculator converts your gas to the chosen standard, shows the number of moles, molar volume, and gas density, and walks through every step of the math.
Formula
Worked example
A gas sample occupies 5 L at 25 °C (298.15 K) and 1 atm (101 325 Pa). At Old STP (273.15 K, 101 325 Pa): V_STP = 5 × (273.15 / 298.15) × (101 325 / 101 325) = 4.581 L. Moles: n = (101 325 × 0.005) / (8.3145 × 298.15) = 0.2043 mol. Molar volume = 22.414 L/mol.
What is STP (Standard Temperature and Pressure)?
Standard Temperature and Pressure (STP) is a reference state chemists use so that gas volumes measured in different laboratories, at different altitudes, and on different days can be compared fairly. Because the volume of an ideal gas depends on temperature and pressure, a reading of "5 litres of nitrogen" is meaningless without stating the conditions. By converting to a shared reference point, gas volumes become directly comparable. The most widely taught definition (Old STP) sets T = 273.15 K (0 °C) and P = 1 atm (101.325 kPa). IUPAC updated its official definition in 1982 to use 100.000 kPa (1 bar) instead of 1 atm, raising the molar volume from 22.414 to 22.711 L/mol. Many textbooks and exam boards still use the older value, so it is important to check which definition applies to your course or problem.
The four standard-condition definitions
This calculator supports the four definitions most commonly encountered in science and engineering: (1) Old STP (IUPAC pre-1982): 273.15 K and 101.325 kPa, molar volume 22.414 L/mol. (2) New STP (IUPAC post-1982): 273.15 K and 100.000 kPa, molar volume 22.711 L/mol. (3) NTP (NIST Normal Temperature and Pressure): 293.15 K (20 °C) and 101.325 kPa, molar volume 24.055 L/mol. (4) SATP (Standard Ambient Temperature and Pressure): 298.15 K (25 °C) and 100.000 kPa, molar volume 24.790 L/mol. Room-temperature lab work often uses SATP or NTP because they represent more practical working conditions than 0 °C.
How the combined gas law converts volumes to standard conditions
The combined gas law states that for a fixed amount of ideal gas, PV/T is constant. This gives the conversion: V_std = V × (T_std / T) × (P / P_std). The initial volume V is multiplied by a temperature ratio (cooling or heating the gas to T_std) and a pressure ratio (compressing or expanding to P_std). Decreasing temperature shrinks the gas; increasing pressure also shrinks it. The moles of gas remain constant throughout the conversion, which is why n = PV/(RT) gives the same result before and after.
Gas density and molar mass at standard conditions
The density of an ideal gas at any standard condition is simply its molar mass divided by the molar volume: rho = M / V_mol. At Old STP, a gas with molar mass 28 g/mol (nitrogen, N2) has a density of 28 / 22.414 = 1.249 g/L. At New STP the molar volume is larger, so the density is lower: 28 / 22.711 = 1.233 g/L. This relationship lets you identify unknown gases: measure the density at STP and multiply by the molar volume to get the molar mass. Enter a molar mass above to see the density for your chosen standard.
Standard condition definitions compared
| Name | Temperature | Pressure | Molar volume (L/mol) | Defined by |
|---|---|---|---|---|
| Old STP | 273.15 K (0 °C) | 101.325 kPa (1 atm) | 22.4141 | IUPAC before 1982 |
| New STP | 273.15 K (0 °C) | 100.000 kPa (1 bar) | 22.7109 | IUPAC after 1982 |
| NTP | 293.15 K (20 °C) | 101.325 kPa (1 atm) | 24.0546 | NIST |
| SATP | 298.15 K (25 °C) | 100.000 kPa (1 bar) | 24.7896 | IUPAC |
Four widely used standard-condition definitions, each giving a different molar volume for an ideal gas.
Frequently asked questions
What is the molar volume of a gas at STP?
Under the old IUPAC definition of STP (0 °C, 1 atm), one mole of an ideal gas occupies 22.414 L. Under the current IUPAC definition (0 °C, 100 kPa, in use since 1982) the molar volume is 22.711 L/mol. Many chemistry courses and textbooks still use 22.4 L/mol, so check which version your course requires.
What is the difference between STP, NTP and SATP?
All three are standard reference states for gases, but they differ in temperature and sometimes pressure. Old STP uses 0 °C and 1 atm. New STP uses 0 °C and 100 kPa. NTP (NIST Normal) uses 20 °C and 1 atm. SATP uses 25 °C and 100 kPa. Because temperature and pressure both affect gas volume, each definition gives a different molar volume: 22.41, 22.71, 24.05, and 24.79 L/mol respectively.
How do I convert a gas volume to STP?
Use the combined gas law: V_STP = V x (273.15 / T) x (P / 101325), where T is your starting temperature in Kelvin and P is your starting pressure in Pascals (use 100000 Pa for new STP). This scales the volume for the temperature change and the pressure change separately, then multiplies them together. If you know the moles of gas, you can also use V_STP = n x 22.414 (old STP) directly.
Why did IUPAC change the STP definition in 1982?
IUPAC changed the pressure reference from 1 atm (101.325 kPa) to 1 bar (100.000 kPa) to align with the SI unit system, in which the bar is a more convenient pressure unit than the atmosphere. The temperature remained at 273.15 K. The practical effect is a slightly larger molar volume: 22.711 L/mol versus 22.414 L/mol. The old value is still widely taught because most published data tables pre-date the change.
Can I use this calculator for real gases, not just ideal gases?
The combined gas law and the ideal gas law (PV = nRT) assume no intermolecular forces and negligible molecular volume. Real gases deviate from these assumptions, especially at high pressures (above a few atm) or temperatures near the boiling point of the gas. For those conditions, the Van der Waals equation or a real-gas equation of state will give more accurate results. For most chemistry and engineering problems at or near atmospheric pressure, the ideal-gas approximation is accurate to within about 1 percent.
How do I calculate the density of a gas at STP?
Gas density at STP equals molar mass divided by molar volume: rho = M / V_mol. For old STP: rho = M / 22.414. For example, carbon dioxide (M = 44.01 g/mol) has a density of 44.01 / 22.414 = 1.964 g/L at old STP. Enter the molar mass in the optional field above to get the density automatically.
What temperature and pressure are considered standard?
It depends on which authority you follow. The IUPAC currently defines STP as 0 °C (273.15 K) and 100 kPa. NIST (NTP) uses 20 °C and 1 atm. SATP uses 25 °C and 100 kPa. The old pre-1982 IUPAC value of 0 °C and 1 atm is still the most commonly used in general chemistry courses worldwide.