Gas Law Calculator

Gas Law Calculator — calculate gas law using standard chemistry formulas. Worked example with units.

A gas law calculator applies the ideal gas law and its component laws (Boyle's, Charles's, Gay-Lussac's, Avogadro's, and the combined gas law) to relate the pressure, volume, temperature, and moles of a gas. The ideal gas law (PV = nRT) is a fundamental model of gas behaviour that works well at moderate pressures and temperatures, with deviations occurring for real gases at high pressures or near the liquefaction point.

Used in chemistry and physics courses, engineering calculations for compressed gas systems, HVAC, pneumatic systems, atmospheric science, and any application involving gas behaviour under changing conditions.

The four state variables are linked, so changing one quantity changes at least one of the others unless the amount of gas and temperature, pressure, or volume are held constant. Before calculating, identify which values belong to the same sample and state. A sealed container can be treated differently from a gas that flows continuously through a pipe, and a gas mixture may need partial pressures rather than one undifferentiated pressure.

Remember that the gas constant has several equivalent forms. Use 0.08206 L·atm/(mol·K) with litres and atmospheres, or 8.314 J/(mol·K) with cubic metres and pascals. Mixing a pressure unit from one form with a volume unit from the other changes the answer by a large conversion factor.

For a useful cross-check, compare the ideal gas law calculator with the molarity calculator when a question moves between gas state variables and solution concentration.

Select the law to apply: Boyle's (P and V at constant T), Charles's (V and T at constant P), Gay-Lussac's (P and T at constant V), or the Ideal Gas Law (all four variables).
Enter known values with their units — the calculator handles unit conversion between atm, kPa, bar; litres, mL, m³; Celsius (converted to Kelvin), Fahrenheit, Kelvin.
The calculator solves for the unknown variable.
For real gas corrections, the van der Waals equation modifies the ideal gas law with substance-specific a and b constants.

Gas law formulas

Ideal Gas Law: PV = nRT, where R = 8.314 J/(mol·K) = 0.08206 L·atm/(mol·K)

Boyle's Law (constant T, n): P₁V₁ = P₂V₂

Charles's Law (constant P, n): V₁/T₁ = V₂/T₂ (T in Kelvin)

Gay-Lussac's Law (constant V, n): P₁/T₁ = P₂/T₂

Combined Gas Law: P₁V₁/T₁ = P₂V₂/T₂

van der Waals: (P + a(n/V)²)(V − nb) = nRT

Interpreting gas law results

Standard conditions reference

STP (Standard Temperature and Pressure): 0°C (273.15 K) and 1 atm — 1 mole of ideal gas occupies 22.414 L. SATP (Standard Ambient): 25°C (298.15 K) and 1 bar — 1 mole occupies 24.789 L. NTP (Normal Temperature and Pressure, US engineering): 20°C and 1 atm — 1 mole occupies 24.055 L. Always convert Celsius to Kelvin before applying gas laws: K = °C + 273.15.

For example, one mole at 25°C and 1 atm occupies about 24.47 L under the ideal model. If a measured sample gives a different value, check whether the pressure is absolute, whether the container volume includes fittings or dead space, and whether the temperature has equilibrated. The discrepancy is useful evidence about measurement or non-ideal behaviour, not a reason to change the gas constant until the units have been checked.

Chemistry tips and best practices

Always convert temperature to Kelvin — gas laws require absolute temperature; using Celsius gives wrong answers.
For high-pressure applications (>10 atm) or polar gases (CO₂, H₂O vapour, NH₃), the van der Waals equation is significantly more accurate than the ideal gas law.
Gauge pressure (what tyre gauges and most industrial gauges read) = absolute pressure − atmospheric pressure. Always convert to absolute pressure (add ~1 atm or 101.325 kPa) before applying gas laws.
Dalton's Law of Partial Pressures: in a gas mixture, each component exerts pressure independently — P_total = P₁ + P₂ + P₃ + ...

Earth's atmospheric pressure at sea level is 101,325 Pa (1 atm) — equivalent to a column of air 100 km tall pressing down.
A standard SCUBA tank holds about 12 litres at 200 bar — expanding to ~2,400 litres at atmospheric pressure using Boyle's Law.
The partial pressure of oxygen in air at sea level is ~21 kPa (21% × 101.325 kPa). At the summit of Everest (~340 mbar atmospheric pressure), O₂ partial pressure falls to ~71 mbar — less than a third of sea level.

Common mistakes to avoid

Using Celsius instead of Kelvin — a temperature of 0°C is 273.15 K, not 0 K; using 0 makes the equation unsolvable.
Forgetting to convert gauge pressure to absolute pressure — a tyre at 32 psi gauge is at approximately 46.7 psi absolute.
Applying the ideal gas law to steam near 100°C at atmospheric pressure — water vapour near its condensation point deviates significantly from ideal behaviour.

Gas law calculations are based on the ideal gas approximation. For engineering design of pressure vessels, gas pipelines, HVAC systems, and compressed gas equipment, consult a licensed mechanical or chemical engineer and follow applicable codes (ASME, ISO, OSHA).