What Is the Ideal Gas Law?

The Ideal Gas Law is one of the most fundamental equations in chemistry and physics. Expressed as PV = nRT, it describes the relationship between four key properties of an ideal gas:

P (Pressure)

The force the gas exerts on its container walls

V (Volume)

The space the gas occupies

n (Moles)

The amount of gas in moles

T (Temperature)

Measured in absolute units (Kelvin)

The equation is linked by R, the universal gas constant, whose numerical value depends on the units used (e.g., 0.08206 L·atm/(mol·K) or 8.314 J/(mol·K)).

Practical Application: This calculator lets you solve for any one of the four variables when the other three are known, making it a versatile tool for chemistry homework, lab calculations, and engineering problems.

How to Use the Ideal Gas Law Calculator

Enter 3 Known Values

Type numbers into any three of the four variable cards (Pressure, Volume, Moles, Temperature).

Select Units

Choose the appropriate unit from each dropdown (e.g., atm, L, mol, K).

View the Result

The fourth variable is calculated automatically in real time and displayed with a green "Result" badge.

Using Quick Fill Presets

Click the Quick Fill button to choose a standard condition:

STP

Standard Temperature and Pressure
0°C, 1 atm

NTP

Normal Temperature and Pressure
20°C, 1 atm

SATP

Standard Ambient Temperature and Pressure
25°C, 1 bar

After selecting a preset, simply enter one more value (Volume or Moles) to complete the calculation.

Additional Features

Decimals — Adjust the number of decimal places (2–6) using the dropdown in the toolbar
Unit Conversions — Expand the conversion table to see your result in every available unit
Common Gases — View a reference table of common gases and their molar masses
Reset — Clear all inputs and start fresh

Key Features

Solve for Any Variable

Enter any three of the four variables — Pressure (P), Volume (V), Moles (n), or Temperature (T) — and the calculator instantly determines the missing one. The smart input system tracks which values you've entered and automatically identifies which variable to solve.

Smart Detection: The calculator automatically recognizes which variable you're solving for based on your inputs — no need to manually select the target variable.

Flexible Unit Support

Each variable supports multiple measurement units with automatic conversion:

Pressure Units

Multiple pressure measurement options

atm (atmospheres)
Pa, kPa (pascals)
bar
mmHg (torr)
psi

Volume Units

Comprehensive volume measurements

L, mL (liters)
m³, cm³ (cubic meters)
ft³ (cubic feet)
gal (US gallons)

Moles Units

Amount of substance options

mol (moles)
mmol (millimoles)
kmol (kilomoles)

Temperature Units

All major temperature scales

K (Kelvin)
°C (Celsius)
°F (Fahrenheit)
°R (Rankine)

All conversions are handled internally so you can mix and match units freely without manual calculations.

Gas Constant Display

A dedicated badge shows the value of the gas constant R in units that match your current pressure and volume selections. For example:

Example 1

atm + L

R = 0.08206 L·atm/(mol·K)
Common in chemistry labs

Example 2

Pa + m³

R = 8.314 J/(mol·K)
SI unit standard

Formula and Unit Conversion Display

After each calculation, the app shows the rearranged formula used (e.g., P = nRT/V) along with the result in base units. Expand the Unit Conversions section to see the result converted to every supported unit simultaneously.

Instant Conversions: View your calculated result in all available units at once — perfect for cross-referencing or working with different measurement systems.

Common Gases Reference

A built-in reference table lists 10 common gases with their chemical formulas and molar masses for quick lookup:

Gas Name	Formula	Molar Mass (g/mol)	Common Use
Hydrogen	H₂	2.016	Fuel, balloons
Helium	He	4.003	Balloons, cooling
Nitrogen	N₂	28.014	Atmosphere (78%)
Oxygen	O₂	31.998	Respiration, combustion
Argon	Ar	39.948	Inert atmosphere
Carbon Dioxide	CO₂	44.010	Carbonation, fire extinguishers
Methane	CH₄	16.043	Natural gas, fuel
Ammonia	NH₃	17.031	Fertilizers, refrigerant
Sulfur Dioxide	SO₂	64.066	Preservative, bleaching
Chlorine	Cl₂	70.906	Disinfection, bleaching

Frequently Asked Questions

What is the ideal gas constant R?

R is the universal gas constant that appears in the equation PV = nRT. Its numerical value depends on the units used. Common values include:

0.08206 L·atm/(mol·K) — Most common in chemistry
8.314 J/(mol·K) — SI unit standard
62.364 L·mmHg/(mol·K) — Used with mmHg pressure

The calculator automatically shows the appropriate R value based on your selected units.

Why must temperature be in Kelvin?

The Ideal Gas Law requires an absolute temperature scale. Kelvin starts at absolute zero (−273.15°C), where molecular motion theoretically stops. Using Celsius or Fahrenheit directly would give incorrect results.

Automatic Conversion: This calculator handles the conversion for you — simply enter the temperature in your preferred unit and it converts to Kelvin internally.

What are STP, NTP, and SATP?

STP

Standard Temperature and Pressure
0°C, 1 atm
Classic IUPAC standard

NTP

Normal Temperature and Pressure
20°C, 1 atm
Room temperature standard

SATP

Standard Ambient T&P
25°C, 1 bar
Newer IUPAC recommendation

Use Quick Fill to set any of these conditions 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 approximately 22.414 liters.

Verify It Yourself: Enter P = 1 atm, n = 1 mol, T = 0°C (or 273.15 K) and solve for V to confirm this value.

Does this work for real gases?

The Ideal Gas Law is an approximation that works well for gases at low pressures and high temperatures (far from their condensation point).

Ideal Gas Law

Best For

Low pressure conditions
High temperatures
Classroom calculations
Everyday problems

Real Gas Equations

Required For

High pressure systems
Near condensation point
Extreme conditions
Precision engineering

Extreme Conditions: For real gases under extreme conditions, more accurate equations like the Van der Waals equation should be used.

For most classroom and everyday calculations, the ideal gas approximation is sufficiently accurate.

Why can't I enter a negative absolute temperature?

Absolute zero (0 K) is the lowest possible temperature in physics. At this point, molecular motion theoretically stops completely.

Physical Limitation: Negative Kelvin values are physically meaningless for classical gas behavior, so the calculator does not produce results when the temperature converts to 0 K or below.

This is a fundamental law of thermodynamics, not a calculator limitation.

Find	Formula
P	P = nRT / V
V	V = nRT / P
n	n = PV / RT
T	T = PV / nR

Gas	Formula	Molar Mass (g/mol)
Hydrogen	H₂	2.016
Helium	He	4.003
Nitrogen	N₂	28.014
Oxygen	O₂	31.998
Argon	Ar	39.948
Carbon Dioxide	CO₂	44.009
Methane	CH₄	16.043
Ammonia	NH₃	17.031
Sulfur Dioxide	SO₂	64.066
Chlorine	Cl₂	70.906