Gas volumes only make sense together with a temperature and a pressure, because the same amount of gas fills twice the volume at half the pressure. To compare measurements, chemists and engineers convert them to an agreed reference state – standard temperature and pressure, STP. Since 1982 IUPAC defines STP as 273.15 K (0 °C) and exactly 100 kPa (1 bar), where one mole of ideal gas occupies 22.711 L. Many textbooks still use the older definition, 0 °C and 1 atm (101.325 kPa), with its famous molar volume of 22.414 L/mol.
Other references are common too: NTP (20 °C and 1 atm, 24.055 L/mol) in industry and for air flow, and SATP (25 °C and 1 bar, 24.790 L/mol) for thermodynamic tables. Always check which one a data sheet uses – the difference between 0 °C and 25 °C is about 9% in volume.
This calculator applies the combined gas law to move your gas from the measured conditions to the standard state of your choice, and the ideal gas law to count the moles. Real gases deviate slightly from ideal behaviour, usually by less than 1% near room conditions for N₂, O₂, H₂ and the noble gases.
How to use the STP Calculator
- Measured gas volume
- Measured temperature
- Measured pressure
- Standard conditions
- Custom standard temperature
- Custom standard pressure
- Molar mass of the gas (optional) — O₂ 31.998, N₂ 28.014, CO₂ 44.009, dry air 28.96 g/mol
Results update as you type, every field has its own unit menu, and everything is calculated in your browser.
How it works
Enter the measured volume, temperature and pressure and pick a standard. The volume is scaled by P/P_std and T_std/T (temperatures in kelvin); the amount of gas is n = PV/RT; the standard molar volume is R·T_std/P_std. With a molar mass, the mass of gas is n·M and its density at the standard conditions is M/V_m.
Worked example
5.00 L of gas is collected at 350 K and 850 Torr. Converted to the older STP (273.15 K, 760 Torr): V = 5.00 × (850/760) × (273.15/350) = 4.364 L. The amount is n = PV/RT = (113,322 Pa × 0.00500 m³) / (8.314 × 350 K) = 0.1947 mol – the same as 4.364 L ÷ 22.414 L/mol. If the gas is oxygen (31.998 g/mol), it weighs 6.23 g. With the IUPAC standard (1 bar) the corrected volume would be 4.422 L.
Common reference conditions for gases
| Name | Temperature | Pressure | Ideal molar volume |
|---|---|---|---|
| IUPAC STP (since 1982) | 0 °C (273.15 K) | 100 kPa (1 bar) | 22.711 L/mol |
| Older STP | 0 °C (273.15 K) | 101.325 kPa (1 atm) | 22.414 L/mol |
| NTP | 20 °C (293.15 K) | 101.325 kPa | 24.055 L/mol |
| SATP | 25 °C (298.15 K) | 100 kPa | 24.790 L/mol |
| 25 °C and 1 atm | 25 °C (298.15 K) | 101.325 kPa | 24.465 L/mol |
Related calculators
For general P–V–T–n problems use the ideal gas law calculator; to move a gas between two arbitrary states use the combined gas law calculator.
Tips
- Use kelvin for every temperature in thermodynamic formulas – the unit menus convert for you.
- Watch the units: ΔH is usually in kJ/mol and ΔS in J/(mol·K).
- Tabulated values are for standard conditions (1 bar, usually 25 °C).
- Everything is calculated in your browser.
Common mistakes to avoid
- Mixing kJ and J between ΔH and ΔS.
- Assuming ΔH and ΔS do not change with temperature over large ranges.
- Using Celsius in ln(T₂/T₁).
Inputs explained
| Field | What to enter |
|---|---|
| Measured gas volume | Units: L, mL, µL, m³, dm³, cm³ … |
| Measured temperature | Units: K, °C, °F |
| Measured pressure | Units: mmHg (Torr), atm, kPa, bar, hPa (mbar), psi … |
| Standard conditions | Choose one: IUPAC STP: 0 °C, 100 kPa (1 bar), Older STP: 0 °C, 101.325 kPa (1 atm), NTP: 20 °C, 101.325 kPa, SATP: 25 °C, 100 kPa … |
| Custom standard temperature | Units: °C, K, °F |
| Custom standard pressure | Units: kPa, atm, bar, hPa (mbar), mmHg (Torr), psi … |
| Molar mass of the gas (optional) | Units: g/mol, kg/mol, Da (u), kDa. O₂ 31.998, N₂ 28.014, CO₂ 44.009, dry air 28.96 g/mol |
What each result means
- Volume at standard conditions = [Measured gas volume]×([Measured pressure]×101325÷[Ps])×([Ts]÷[Measured temperature])
- Amount of gas = [Measured pressure]×101325×[Measured gas volume]÷1000÷(8.314462618×[Measured temperature])
- Molar volume at the chosen standard = 8.314462618×[Ts]÷[Ps]×1000
- Mass of gas = [Amount of gas]×[Molar mass of the gas (optional)]
- Density of the gas at the chosen standard = [Molar mass of the gas (optional)]÷[Molar volume at the chosen standard]
- Density at the measured conditions = [Molar mass of the gas (optional)]×[Measured pressure]×101325÷(8.314462618×[Measured temperature])÷1000
- Number of molecules = [Amount of gas]×6.02214076e23
- Volume correction factor V_std/V = [Volume at standard conditions]÷[Measured gas volume]
Formula
V_std = V · (P / P_std) · (T_std / T) · n = P·V / (R·T) · V_m = R·T_std / P_std (22.711 L/mol at 0 °C and 1 bar; 22.414 L/mol at 0 °C and 1 atm)Worked example
Inputs — Measured gas volume: 5 L; Measured temperature: 350 K; Measured pressure: 850 mmHg (Torr); Standard conditions: Older STP: 0 °C, 101.325 kPa (1 atm); Molar mass of the gas (optional): 31.998 g/mol.
Results — Volume at standard conditions = 4.3642 L; Amount of gas = 0.19471 mol; Molar volume at the chosen standard = 22.414 L/mol; Mass of gas = 6.2304 g; Density of the gas at the chosen standard = 1.4276 g/L; Density at the measured conditions = 1.2461 g/L; Number of molecules = 1.1726e23; Volume correction factor V_std/V = 0.87285.
Key terms
- Standard temperature and pressure (STP)
- A reference state for gases, 273.15 K and 100 kPa according to IUPAC.
- Molar volume (V_m)
- The volume occupied by one mole of gas, R·T/P for an ideal gas.
- Combined gas law
- P₁V₁/T₁ = P₂V₂/T₂ for a fixed amount of gas.
- SATP
- Standard ambient temperature and pressure: 298.15 K and 100 kPa.
Frequently asked questions
What is STP in chemistry?
IUPAC defines standard temperature and pressure as 273.15 K (0 °C) and 100 kPa (1 bar). Older books use 0 °C and 1 atm (101.325 kPa).
What is the molar volume of a gas at STP?
22.711 L/mol at 0 °C and 1 bar, or 22.414 L/mol at 0 °C and 1 atm, for an ideal gas.
What is the difference between STP and NTP?
NTP (normal temperature and pressure) is usually 20 °C and 1 atm, where the molar volume is 24.055 L/mol – 7.3% more than at the older STP.
What is SATP?
Standard ambient temperature and pressure: 25 °C (298.15 K) and 100 kPa, with a molar volume of 24.790 L/mol.
How many moles are in 1 L of gas at STP?
1/22.711 = 0.04403 mol at 0 °C and 1 bar, or 1/22.414 = 0.04461 mol at 0 °C and 1 atm.
What is the density of oxygen at STP?
M/V_m = 31.998/22.414 = 1.428 g/L at 0 °C and 1 atm (1.409 g/L at 0 °C and 1 bar).
Why did IUPAC change STP from 1 atm to 1 bar?
The bar (100 kPa) is a round SI-based value; the change in 1982 lowers the reference pressure by 1.3% and raises the molar volume to 22.711 L/mol.
What formula does the STP Calculator use?
V_std = V · (P / P_std) · (T_std / T) · n = P·V / (R·T) · V_m = R·T_std / P_std (22.711 L/mol at 0 °C and 1 bar; 22.414 L/mol at 0 °C and 1 atm).