What Is the Ideal Gas Law Calculator?
The ideal gas law, PV = nRT, relates the pressure, volume, amount, and temperature of an ideal gas. This calculator solves the equation for whichever value you're missing — pressure, volume, moles, or temperature — and lets you enter temperature in either Celsius or Kelvin.
How to Use the Ideal Gas Law Calculator
- Choose which value you want to solve for: P, V, n, or T.
- Enter the other three known values (temperature in °C or K — your choice).
- Read the result, along with the formula shown with your numbers substituted in.
What P, V, n, R, and T Represent
Temperature must always be converted to an absolute scale (Kelvin) before it's used in the formula — that's why this calculator lets you type in Celsius but always converts to Kelvin internally.
Ideal Gas Law Assumptions
The ideal gas law is a model that assumes gas particles have negligible volume, experience no intermolecular attractive or repulsive forces, and collide elastically. Real gases only follow this model closely under typical laboratory conditions — moderate temperatures and low to moderate pressures.
Worked Example
A 10 L container holds 0.5 mol of gas at 20 °C (293.15 K). What is the pressure?
P = nRT ÷ V = (0.5 × 0.0821 × 293.15) ÷ 10 ≈ 1.203 atm
Understanding Your Results
The result panel shows the solved variable along with the formula substituted with your actual numbers, so you can verify the arithmetic yourself. The summary card also lists the temperature in both Celsius and Kelvin so you can double-check unit conversions at a glance.
Common Mistakes to Avoid
- Plugging Celsius directly into PV = nRT without converting to Kelvin first — this is the most common ideal gas law error.
- Forgetting that R = 0.0821 L·atm/(mol·K) only works when pressure is in atm and volume is in liters; using other units (like kPa or mL) requires a different value of R.
- Entering zero or negative values for pressure, volume, or moles, which are not physically meaningful for a real gas sample.
Frequently Asked Questions
R is the ideal gas constant. Its value depends on the units you use — when pressure is in atmospheres, volume in liters, and temperature in Kelvin, R = 0.0821 L·atm/(mol·K). Other unit systems use different numeric values of R (like 8.314 J/(mol·K)).
The ideal gas law is an approximation. It works well for real gases at typical laboratory temperatures and low to moderate pressures, but becomes less accurate at very high pressures or very low temperatures, where intermolecular forces and molecular volume become significant.
It assumes gas particles occupy negligible volume compared to their container, experience no attractive or repulsive forces between them, and undergo perfectly elastic collisions.
STP is commonly defined as 0 °C (273.15 K) and 1 atm of pressure — a reference condition often used to compare gas volumes and behavior, such as the fact that one mole of an ideal gas occupies about 22.4 L at STP.
The Kelvin scale is an absolute temperature scale that starts at absolute zero (0 K = -273.15 °C), the theoretical point of minimum possible thermal energy — so a physically valid Kelvin value can never be negative.