What is the ideal gas law formula?

The ideal gas law is PV = nRT, where P is pressure, V is volume, n is the amount of substance in moles, R is the universal gas constant, and T is the absolute temperature in kelvin. Rearranged, P = nRT/V, V = nRT/P, n = PV/RT and T = PV/nR.

What value of the gas constant R does this calculator use?

It works internally in SI base units (pascals, cubic metres, moles, kelvin) using R = 8.314462618 J/(mol·K), the CODATA value. That is exactly equivalent to R = 0.082057366 L·atm/(mol·K) when pressure is in atmospheres and volume in litres, so both conventions give the same answer.

Why must temperature be in kelvin?

The ideal gas law is only valid with an absolute temperature scale, so the gas constant relation holds. The calculator accepts °C or °F for convenience but converts them to kelvin (K = °C + 273.15) before solving. Using a Celsius value directly would give nonsense, including division by zero at 0 °C when solving for other variables.

Why does 1 mole of gas give 22.414 litres at STP?

Putting P = 1 atm, n = 1 mol and T = 273.15 K into V = nRT/P gives 22.414 L, the molar volume of an ideal gas at the old IUPAC standard temperature and pressure. At the newer STP definition of 100 kPa and 273.15 K the molar volume is about 22.711 L — change the pressure unit to kPa and enter 100 to see it.

How accurate is the ideal gas law for real gases?

It is an excellent approximation for most gases at moderate temperatures and low to moderate pressures. Accuracy drops near a gas's condensation point or at high pressure, where intermolecular forces and molecular volume matter; for those cases a real-gas equation such as van der Waals is more precise.

Is my data sent anywhere?

No. Every calculation runs locally in your browser with JavaScript. Nothing you type is uploaded, logged, or stored on a server.

Ideal Gas Law Calculator

The ideal gas law calculator solves the relationship PV = nRT for whichever quantity you are missing — pressure, volume, amount of substance, or temperature — while letting you work in the units you actually have. Pick the variable to find, enter the other three with their units, and the answer appears instantly alongside fully shown working. It is built for chemistry and physics students, lab technicians, and anyone sizing a gas volume or checking a pressure who wants the steps, not just a number.

How it works

The ideal gas law links the four state variables of an ideal gas through a single constant. In symbols it is PV = nRT, where P is pressure, V is volume, n is the amount of substance in moles, T is the absolute temperature, and R is the universal gas constant. Depending on what you need, the calculator rearranges it: P = nRT / V, V = nRT / P, n = PV / RT, or T = PV / nR.

To stay exact across unit choices, the tool converts every input into SI base units first — pressure to pascals, volume to cubic metres, amount to moles, temperature to kelvin — and uses R = 8.314462618 J/(mol·K). That is the CODATA value, equal to 0.082057366 L·atm/(mol·K) when you prefer atmospheres and litres. After solving in base units it converts the result back into your selected output unit and also shows the SI value, so the conversion path is transparent. Temperatures entered in °C or °F are converted with K = °C + 273.15 before any division, which is essential because the law is only valid on an absolute temperature scale.

Worked example

Suppose you have 2.00 mol of an ideal gas in a 10.0 L container at 25 °C and want the pressure. First convert: T = 25 + 273.15 = 298.15 K, V = 10.0 L = 0.0100 m³. Then apply P = nRT / V:

P = (2.00 mol × 8.314462618 J/(mol·K) × 298.15 K) / 0.0100 m³ ≈ 495,800 Pa,

which is about 4.89 atm (495,800 ÷ 101,325). Switch the pressure unit to atm and the tool reports the same figure directly. The “Show working” panel lists each substitution so you can check it by hand.

Formula note: PV = nRT with R = 8.314462618 J/(mol·K). Equivalent constant: 0.082057366 L·atm/(mol·K) and 8.314462618 m³·Pa/(mol·K). Always use absolute temperature (kelvin).

Solve for	Rearrangement	Needs
Pressure	P = nRT / V	n, T, V
Volume	V = nRT / P	n, T, P
Moles	n = PV / RT	P, V, T
Temperature	T = PV / nR	P, V, n

Every figure is computed in your browser — nothing is uploaded or stored.