Ideal Gas Law Calculator
Calculate pressure, volume, moles, temperature using PV = nRT
Ideal Gas Law Calculator tool
R = 0.0821 L·atm/(mol·K)
Results
Pressure (atm)
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Pressure (kPa)
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Pressure (mmHg)
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Pressure (bar)
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Ideal Gas Law Calculator: key facts
- What it does
- Calculate pressure, volume, moles, temperature using PV = nRT
- Category
- Science Calculators
- Cost
- Free, with no account, sign-up, or install.
- Your data
- Runs entirely in your browser — the files and text you enter are never uploaded to a server.
- Last reviewed
- . Report an incorrect result.
Understanding the Ideal Gas Law Calculator
The Ideal Gas Law (PV = nRT) relates pressure, volume, amount of gas in moles, and temperature for an ideal gas. The Ideal Gas Law Calculator solves for any of these four variables when you know the other three, supporting multiple unit systems for pressure (atm, kPa, mmHg, torr, bar), volume (L, mL, m³), and temperature (K, °C, °F).
Students solve gas law homework problems, chemists calculate gas quantities for reactions, and engineers analyze gas behavior in industrial processes. The calculator automatically uses the correct gas constant R value based on your chosen units, eliminating a common source of calculation errors.
Because calculations run entirely in your browser using JavaScript, you can solve gas law problems without any data leaving your device. The tool explains when real gases deviate from ideal behavior in the FAQ section.
Getting a result
- Select what you want to calculate from the dropdown: pressure, volume, moles, or temperature.
- Enter the three known values in their respective input fields.
- Set the pressure, volume, and temperature units so the matching gas constant is applied.
- Click Calculate to solve for the unknown variable using PV = nRT.
- Read the answer, which is presented across several equivalent units.
PV = nRT, and the constant it depends on
The ideal gas law ties pressure, volume, quantity, and temperature into one equation, and the tool solves it for any of the four. The constant R is what makes the units work, and its numerical value depends on which units you use: this calculator applies 0.0821 L·atm/(mol·K), which is why pressure is handled in atmospheres and volume in litres.
Temperature must be absolute. Kelvin, not Celsius — the equation involves multiplying and dividing by temperature, and a scale with an arbitrary zero produces nonsense. Entering 0 for a freezing-point temperature rather than 273.15 is the single most common error with this law.
The default values are not arbitrary. One mole at one atmosphere and 273.15 K occupies 22.4 litres, the molar volume at standard temperature and pressure, and it is worth recognising because it makes a useful sanity check on any gas calculation.
- The defaults — 1 atm, 22.4 L, 273.15 K — solve to 0.999 mol, which is the molar volume at STP recovered to within rounding.
- Two moles at 300 K in a 10 L vessel exert 4.93 atm.
- Doubling the absolute temperature of a fixed quantity in a fixed volume doubles the pressure — which is why sealed containers must never be heated.
Reasons to use it here
- Ideal gas law calculations run entirely in your browser—no server communication occurs, ensuring your chemistry calculations remain private and the tool works offline.
- Solves for any of the four variables (pressure, volume, moles, temperature) when you know the other three, making it versatile for different problem types.
- Automatically selects the correct gas constant R value based on your chosen units, eliminating a common source of calculation errors.
- Handles unit conversions between different pressure (atm, kPa, mmHg, torr, bar), volume (L, mL, m³), and temperature (K, °C, °F) systems automatically.
Where real gases depart from the model
The law describes an idealisation: point particles with no volume of their own and no forces between them beyond perfectly elastic collisions. Real molecules occupy space and do attract one another, so deviations appear wherever those assumptions matter — at high pressure, where molecular volume becomes a significant fraction of the container, and at low temperature, where intermolecular attraction is no longer negligible against thermal motion.
In practice the approximation is good for most gases at around room temperature and atmospheric pressure, often within a percent or two, which is why it remains the workhorse. It fails badly near the point of condensation and for gases with strong intermolecular forces such as water vapour or ammonia. The van der Waals equation and other equations of state introduce correction terms for exactly these cases.
The law also says nothing about which gas it is. A mole of helium and a mole of carbon dioxide occupy the same volume under the same conditions, so recovering a mass requires the molar mass separately.
Frequently Asked Questions
What is the ideal gas law?
The ideal gas law is PV = nRT, where P is pressure, V is volume, n is the number of moles of gas, R is the gas constant, and T is temperature in Kelvin. It relates the four state variables of an ideal gas and is a good approximation for many real gases under standard conditions.
What pressure, volume, and temperature units does it accept?
The calculator supports atmospheres, kilopascals, mmHg, torr, and bar for pressure; liters, milliliters, and cubic meters for volume; and Kelvin, Celsius, and Fahrenheit for temperature. The gas constant R is automatically selected based on your units.
What is the gas constant R?
The gas constant R has different values depending on the units used. Common values include 0.0821 L·atm/(mol·K), 8.314 J/(mol·K), 62.36 L·mmHg/(mol·K), and 8.314 kPa·L/(mol·K). The calculator automatically uses the correct value for your chosen units.
When does the ideal gas law apply?
The ideal gas law applies to ideal gases under standard conditions. Real gases deviate from ideal behavior at high pressures and low temperatures when intermolecular forces become significant. However, the law is a good approximation for many situations at moderate conditions.
Why must temperature be in Kelvin?
The ideal gas law requires absolute temperature because gas volume is directly proportional to absolute temperature. Celsius and Fahrenheit can be used for input, but the calculator converts them to Kelvin for the calculation to ensure accuracy.
