STP Calculator (Standard Temperature and Pressure)
Convert a gas sample's volume to STP conditions, or convert between moles and gas volume at STP, in one calculator.
🌡️ What is STP (Standard Temperature and Pressure)?
Standard temperature and pressure (STP) is a fixed reference point chemists use so gas measurements from different labs, at different actual conditions, can be compared on the same basis. Traditionally STP means 0°C (273.15 K) and 1 atm (101.325 kPa), though the current IUPAC definition instead uses 0°C and 100 kPa. This calculator either converts a gas sample's measured pressure, volume, and temperature into what its volume would be at STP, or converts a known amount of gas in moles into a volume at STP.
Chemistry students meet STP constantly: comparing the volume of gas released in a reaction to a textbook value, checking whether a lab-measured gas volume matches the amount predicted by stoichiometry, or converting between moles and liters using the well-known 22.4 L/mol molar volume without redoing the full ideal gas law by hand every time. It is also the basis for defining gas density tables and vapor pressure references used throughout general and analytical chemistry.
A frequent point of confusion is that STP is not the same as room temperature. 0°C is noticeably colder than a typical lab or classroom at roughly 20 to 25°C, so a gas sample measured at room conditions genuinely does occupy a different volume than it would at STP, this calculator's Convert Gas to STP mode performs exactly that adjustment using the combined gas law. It is also easy to confuse STP with NTP (Normal Temperature and Pressure, usually 20°C) or SATP (Standard Ambient Temperature and Pressure, 25°C and 1 bar), each of which gives a different molar volume from STP's 22.4 or 22.7 L/mol.
This calculator is useful because manually rearranging and applying the combined gas law, or remembering both the traditional and IUPAC molar volume conventions, is a common source of arithmetic mistakes on chemistry homework and exams. Entering pressure, volume, and temperature in whatever units are given, and letting the calculator handle the unit conversion and formula, removes that source of error.
📐 Formula
📖 How to Use This Calculator
Steps
💡 Example Calculations
Example 1 — Gas Sample in Atmospheres
2 atm, 10 L, 25°C.
Example 2 — Gas Sample in Kilopascals
150 kPa, 5 L, 50°C.
Example 3 — Converting Moles to Volume at STP
2.5 mol of an ideal gas.
Example 4 — Small Sample in Moles
0.5 mol of an ideal gas.
❓ Frequently Asked Questions
🔗 Related Calculators
What is standard temperature and pressure (STP) in chemistry?
STP traditionally means 0°C (273.15 K) and 1 atm (101.325 kPa) of pressure. The current IUPAC definition instead uses 0°C and 100 kPa. Both are reference conditions used to compare gas volumes on a common, standard basis.
What is the molar volume of a gas at STP?
At the traditional STP definition (0°C, 1 atm), one mole of any ideal gas occupies 22.4 liters. Under the current IUPAC definition (0°C, 100 kPa), one mole occupies 22.7 liters instead. Both values come directly from the ideal gas law.
How do you convert a gas volume to STP?
Use the combined gas law, V(STP) = V1 x (P1 / P(STP)) x (T(STP) / T1), where P(STP) = 1 atm and T(STP) = 273.15 K. This rescales a gas sample's measured volume to what it would occupy at standard conditions, assuming the amount of gas stays fixed.
Why are there two different STP definitions?
The traditional definition, 0°C and 1 atm, dates back to older chemistry conventions and is still the one most widely taught in general chemistry. IUPAC redefined STP in 1982 to 0°C and 100 kPa (a rounder pressure value in SI units), which changes the molar volume from 22.4 L/mol to 22.7 L/mol. Both remain in common use, so it is worth checking which one a textbook or exam expects.
How many moles are in a given volume of gas at STP?
Divide the volume by the molar volume: moles = V / 22.4 (traditional) or moles = V / 22.7 (IUPAC). For example, 44.8 liters of any ideal gas at traditional STP contains exactly 2 moles.
Is STP the same as room temperature and pressure?
No. STP's 0°C is noticeably colder than typical room temperature (around 20 to 25°C). Room-temperature reference conditions instead use NTP (20°C, 1 atm) or SATP (25°C, 1 bar), each with a different molar volume from STP.
What is the combined gas law?
The combined gas law states that P1V1/T1 = P2V2/T2 for a fixed amount of gas, combining Boyle's law, Charles's law, and Gay-Lussac's law into one relationship. This calculator's Convert Gas to STP mode is this law solved for V2, with state 2 fixed at standard conditions.
Does STP apply to real gases exactly?
The 22.4 L/mol and 22.7 L/mol molar volumes are derived from the ideal gas law, which assumes no intermolecular forces and negligible molecular volume. Real gases deviate slightly from this, particularly at low temperature or high pressure, but the ideal approximation is accurate enough for almost all general chemistry purposes.
Why does this calculator ask for pressure and temperature units separately?
Gas law formulas require pressure and temperature in specific units, atmospheres and Kelvin. Since lab instruments and problem sets often report pressure in kPa or mmHg and temperature in Celsius, this calculator converts whichever unit you enter into the correct base units automatically before applying the formula.
Can I use this calculator to find moles of gas directly?
Yes. Convert Gas to STP mode also reports moles of gas present, computed from the ideal gas law using your original pressure, volume, and temperature. Moles to STP Volume mode instead starts from a known mole amount and finds the corresponding STP volume.
What happens if I enter a temperature below absolute zero?
The calculator rejects it and shows an error. Absolute zero, -273.15°C or 0 K, is the lowest physically possible temperature, so any input at or below it (in Kelvin) cannot represent a real gas sample.