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Gas Moles Calculator

Enter pressure, volume and temperature to find the moles of gas.

How it is calculated

The tool rearranges the ideal gas law PV = nRT to n = P × V ÷ (R × T). Pressure P is entered in kPa, volume V in litres and temperature T in kelvin. Internally pressure is multiplied by 1000 to give pascals and volume divided by 1000 to give cubic metres, and R is 8.314 J/(mol·K). The molecule count is n × 6.022 × 10²³. Moles are shown with four decimals. All three inputs must be above zero. The tool does not convert from °C, atm or mmHg, and it treats the gas as ideal, so it is least accurate at high pressure or low temperature.

Worked example

A 5 L vessel holds a gas at 250 kPa and 300 K. Convert units: 250 kPa = 250,000 Pa and 5 L = 0.005 m³. Numerator: 250,000 × 0.005 = 1250. Denominator: 8.314 × 300 = 2494.2. Moles: 1250 ÷ 2494.2 = 0.5012 mol. Molecules: 0.5012 × 6.022 × 10²³ = 3.018 × 10²³. As a check, the standard 22.4 L at 273.15 K and 101.325 kPa gives 0.9994 mol, close to 1 mol, with the gap coming from rounding in 22.4.

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Common mistakes

  • Entering the temperature in °C. The field needs kelvin, so 27 °C is 300.15 K; typing 27 gives about eleven times too many moles.
  • Entering pressure in atm or bar. The field is in kPa, so 1 atm is 101.325, not 1.
  • Entering volume in cubic metres or millilitres. The field is in litres, so 0.005 m³ is 5.
  • Applying it to a gas near condensation or at very high pressure. The ideal model then differs noticeably from a real gas.
  • Using the answer as a mass. Moles must be multiplied by the molar mass of the specific gas to get grams, and the tool does not know which gas you have.

Sources

About this calculator

The ideal gas law, PV = nRT, links pressure (P), volume (V), amount of substance in moles (n), and absolute temperature (T) through the gas constant R. This calculator takes pressure in kilopascals, volume in litres and temperature in kelvin, and uses R = 8.314 J/(mol·K) (the same constant is 0.0821 L·atm/(mol·K) in litres and atmospheres). Rearranged to n = PV/RT, it lets you compute how many moles of gas are present in a container without ever weighing the sample.

This relationship is a staple of general chemistry and physics coursework and of real lab work — filling gas cylinders, calibrating vacuum systems, or checking stoichiometric yields in a gas-phase reaction. Once you have the mole count, multiplying by Avogadro's number (6.022×10²³) gives the equivalent number of individual gas molecules, which this calculator also reports.

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Comments (1)

  • Editorial team

    Pressure gauges on tyres, cylinders and compressors usually read gauge pressure, which is relative to the surrounding air. The gas law needs absolute pressure, so add roughly 101.3 kPa first. In a 5 L vessel at 300 K, a gauge reading of 200 kPa is about 301.3 kPa absolute and holds 0.6040 mol, whereas typing 200 gives only 0.4009 mol. Which gas or vessel are you working with?

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