Ideal Gas Law Calculator (PV = nRT)
This ideal gas law calculator solves PV = nRT for whichever variable you need. Enter any three of pressure, volume, moles, and temperature, and it finds the fourth instantly. It is built for chemistry homework, lab work, and engineering problems where you need a fast, reliable answer.
Each variable accepts the units you actually work in, so you can mix atmospheres with liters or pascals with cubic meters without converting by hand. The calculator tracks which three values you have entered, shows the rearranged formula it used, and even lists your result in every supported unit.
How to Use the Ideal Gas Law Calculator
Enter three known values
Type numbers into any three of the four cards: Pressure, Volume, Moles, and Temperature. The calculator marks them with a blue Input badge.
Pick the right units
Choose a unit from each dropdown, such as atm, L, mol, and K. You can mix units freely; every conversion is handled internally.
Read the result
The fourth variable is calculated in real time and shown with a green Result badge, alongside the rearranged formula that was used.
Use Quick Fill for standard conditions
Open Quick Fill to load STP (0°C, 1 atm), NTP (20°C, 1 atm), or SATP (25°C, 1 bar), then enter one more value to finish the calculation.
Features
Solve for Any Variable
Enter Pressure, Volume, Moles, or Temperature as any three knowns and the missing one is found instantly.
Real-Time Calculation
Results update as you type, with smart tracking that detects which three variables you have entered.
Flexible Unit Support
Pressure in atm, Pa, kPa, bar, mmHg, or psi; volume in L, mL, m³, cm³, ft³, or gal; plus mole and temperature units.
Quick Fill Presets
Load STP, NTP, or SATP standard conditions with one click instead of typing them each time.
Dynamic Gas Constant R
A badge shows the value of R that matches your selected pressure and volume units, such as 0.08206 or 8.314.
Formula Display
See the rearranged equation used for each answer, such as P = nRT / V, with the result in base units.
Unit Conversion Table
Expand the conversions panel to view your result expressed in every supported unit at once.
Common Gases Reference
A built-in table lists ten common gases with their chemical formulas and molar masses for quick lookup.
All Formulas Panel
A reference table shows the rearranged PV = nRT formula for solving P, V, n, and T.
Adjustable Precision
Choose 2, 3, 4, or 6 decimal places to control how detailed your results appear.
Frequently Asked Questions
What is the formula PV = nRT?
PV = nRT is the ideal gas law. P is pressure, V is volume, n is the amount of gas in moles, T is absolute temperature, and R is the universal gas constant. It links all four properties of an ideal gas, so knowing any three lets you solve for the fourth.
How do I solve for moles (n)?
Rearrange the equation to n = PV / RT, then enter pressure, volume, and temperature. The calculator detects that moles is the missing value, applies that formula, and shows the result with a green Result badge.
What value of R should I use?
R depends on your units. Common values are 0.08206 L·atm/(mol·K), 8.314 J/(mol·K), and 62.364 L·mmHg/(mol·K). You do not need to pick one yourself; the R badge updates automatically to match your selected pressure and volume units.
Why must temperature be in Kelvin?
The ideal gas law needs an absolute temperature scale. Kelvin starts at absolute zero (−273.15°C), so it never goes negative for a real gas. You can enter temperature in °C, °F, or °R if you prefer; it is converted to Kelvin internally before the calculation.
What units does each variable use?
Pressure supports atm, Pa, kPa, bar, mmHg, and psi. Volume supports L, mL, m³, cm³, ft³, and gal. Moles use mol, mmol, or kmol, and temperature uses K, °C, °F, or °R. You can mix any combination, and the conversions are handled for you.
What are STP, NTP, and SATP?
STP (Standard Temperature and Pressure) is 0°C and 1 atm, the classic IUPAC standard. NTP (Normal Temperature and Pressure) uses 20°C and 1 atm. SATP (Standard Ambient Temperature and Pressure) is 25°C and 1 bar. Use Quick Fill to set any of these instantly.
What is the molar volume of an ideal gas at STP?
At STP (0°C, 1 atm) one mole of an ideal gas occupies about 22.414 liters. You can confirm this by entering P = 1 atm, n = 1 mol, and T = 0°C, then reading the calculated volume.
Does this work for real gases?
The ideal gas law is an approximation that holds well at low pressure and high temperature, far from a gas's condensation point. For real gases under extreme conditions, equations like Van der Waals are more accurate. For most classroom and everyday problems, the ideal gas approximation is close enough.
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