🧊 STP Calculator

Standard Temperature and Pressure (STP) is a reference condition defined as 0 °C (273.15 K) and 1 atmosphere (101.325 kPa). At STP, one mole of an ideal gas occupies 22.414 liters—the molar volume. Scientists and engineers use STP as a common baseline so that gas measurements from different experiments and locations can be compared directly.

This calculator helps you convert a gas volume measured at any temperature and pressure to the equivalent volume at STP, or vice versa. It also calculates the number of moles of gas using the molar volume. This is essential for stoichiometry in chemistry, gas law problems, and industrial process calculations.

To convert a volume from actual conditions (P₁, T₁) to STP (P₂ = 1 atm, T₂ = 273.15 K), the calculator uses the Combined Gas Law: V₂ = V₁ × (P₁/P₂) × (T₂/T₁). The number of moles is then found using n = V(STP) / 22.414 L/mol.

The table below shows common reference conditions used in science and industry:

This calculator uses the traditional STP definition (0 °C, 1 atm, 22.414 L/mol) as it is still the most widely used in textbooks and many industries.

1. Enter the actual volume of your gas sample.
2. Enter the actual temperature (in Kelvin) and pressure (in atm).
3. Click the Calculate button.
4. Review the equivalent volume at STP and the number of moles.
5. For reverse calculation (STP → actual), enter STP values (273.15 K, 1 atm) as the actual conditions.

What is the difference between STP and NTP?

STP (Standard Temperature and Pressure) is defined as 0 °C (273.15 K) and 1 atm. NTP (Normal Temperature and Pressure) uses 20 °C (293.15 K) and 1 atm. The molar volume differs: 22.414 L/mol at STP vs 24.055 L/mol at NTP.

Why is 22.414 L/mol important?

At STP, one mole of any ideal gas occupies exactly 22.414 liters. This molar volume lets you convert between volume and moles without knowing the specific gas identity—it works for nitrogen, oxygen, hydrogen, CO₂, and all ideal gases equally.

Does STP use 1 atm or 100 kPa?

There are two conventions. The traditional STP (pre-1982) uses 1 atm = 101.325 kPa, giving a molar volume of 22.414 L/mol. Since 1982, IUPAC redefined STP pressure as 100 kPa (0.987 atm), giving 22.711 L/mol. This calculator uses the traditional 1 atm definition, which is still common in textbooks.

How do I convert from Celsius to Kelvin?

Add 273.15 to the Celsius temperature. For example, 25 °C = 298.15 K. This conversion is essential because the gas laws require absolute temperature.

Can I use this for gas density calculations?

Yes. Once you know the volume at STP and the number of moles, you can calculate density: ρ = (n × M) / V, where M is the molar mass of the gas. For example, at STP, the density of O₂ (M = 32 g/mol) is (1 × 32) / 22.414 = 1.428 g/L.

Why are my STP and actual volumes different?

Gas volume depends on temperature and pressure. A gas sample at high temperature expands (more volume), and at high pressure it compresses (less volume). Converting to STP normalizes these conditions so that you can compare gas amounts across different environments.

Always convert temperature to Kelvin before calculation: K = °C + 273.15. The calculator uses 1 atm = 101.325 kPa for pressure conversion. STP molar volume (22.414 L/mol) applies to ideal gases—real gases have slightly different values (e.g. CO₂ = 22.26 L/mol, He = 22.40 L/mol at STP). For precise work with real gases, consult the specific gas's molar volume. If you need to calculate with varying amounts of gas, try the combined gas law calculator which handles pressure-volume-temperature relationships without requiring STP.