Gas Law Calculator
The tool applies the ideal gas law, PV = nRT, where P is absolute pressure (Pa), V is volume (m³), n is amount of substance (mol), T is absolute temperature (K), and R = 8.314462618 J/(mol·K).
Enter Values
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How to use this calculator
- Fill in any three of pressure (Pa), volume (m³), amount of gas (mol) and temperature (K).
- Leave the one quantity you want to find blank, then press Calculate.
- Use absolute pressure and absolute (kelvin) temperature — convert °C to K by adding 273.15 first.
How it works
The tool applies the ideal gas law, PV = nRT, where P is absolute pressure (Pa), V is volume (m³), n is amount of substance (mol), T is absolute temperature (K), and R = 8.314462618 J/(mol·K). Given any three quantities it rearranges the equation to solve for the fourth: P = nRT/V, V = nRT/P, n = PV/(RT) or T = PV/(nR).
The ideal gas law assumes gas particles have negligible volume and no intermolecular forces. It is very accurate for real gases at ordinary temperatures and low-to-moderate pressures, and less accurate near condensation (high pressure or low temperature), where a real-gas equation of state is needed.
Worked example
One mole at 0 °C in a 22.4 L vessel. With n = 1 mol, T = 273.15 K and V = 0.0224 m³, leaving pressure blank gives P = nRT/V = (1 × 8.314462618 × 273.15) ÷ 0.0224 = 101,388.1904 Pa (about 101.39 kPa, close to 1 atm — the classic molar-volume result).
Common mistakes
- Using gauge pressure or degrees Celsius. The formula needs absolute pressure (add atmospheric pressure to a gauge reading) and absolute temperature in kelvin (°C + 273.15).
- Mixing unit systems — this tool expects SI base units (Pa, m³, mol, K). Convert litres to cubic metres (1 L = 0.001 m³) and kPa/atm to pascals before entering.
- Leaving zero or more than one field blank. Enter exactly three positive values so there is a single unknown to solve for.
Frequently asked questions
What is the ideal gas law?
It is PV = nRT: the product of a gas's pressure and volume equals the amount of gas (in moles) times the gas constant R times the absolute temperature. It links the four state variables of an ideal gas in one equation.
What value of R does this calculator use?
R = 8.314462618 J/(mol·K), the SI value of the ideal gas constant. That is why inputs are in pascals, cubic metres, moles and kelvin — the units are consistent with this R.
Can I use it for Boyle's, Charles's or the combined gas law?
Yes indirectly: those laws are special cases of PV = nRT. For a fixed amount of gas, solve for one state, then the other, keeping the constant quantities fixed — the ratios PV/T stay constant.
Do I have to enter kelvin and pascals?
Yes. Convert Celsius to kelvin (°C + 273.15) and any pressure to pascals (1 atm = 101,325 Pa, 1 kPa = 1,000 Pa) and volume to cubic metres (1 L = 0.001 m³) before entering, so the result is correct.
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