🌡 Charles's Law Calculator

Charles's Law, named after the French scientist Jacques Charles (1746–1823), states that the volume of a fixed amount of gas at constant pressure is directly proportional to its absolute temperature. The formula is V₁ / T₁ = V₂ / T₂, where temperature must always be in Kelvin. When you heat a balloon and it expands, or when cold weather makes a basketball look slightly deflated, you are seeing Charles's Law at work.

This calculator is ideal for chemistry students, meteorologists, engineers, and anyone studying thermodynamics. Enter any three of the four values—initial volume (V₁), initial temperature (T₁), final volume (V₂), and final temperature (T₂)—and the calculator returns the fourth. Temperature inputs must be in Kelvin. If your data is in Celsius, convert first using T(K) = T(°C) + 273.15.

The calculator uses the relation V₁ / T₁ = V₂ / T₂. If you know V₁, T₁, and V₂, it finds T₂ = (V₂ × T₁) / V₁. If you know V₁, T₁, and T₂, it finds V₂ = (V₁ × T₂) / T₁.

The table below shows how the volume of 1 L of gas at 273 K (0 °C) changes with temperature at constant pressure:

The ratio V/T stays constant at 0.00366 L/K—this is the signature of Charles's Law. The law also implies that extrapolating volume to lower temperatures reaches zero at −273.15 °C (absolute zero), a key insight that helped establish the Kelvin temperature scale.

1. Convert all temperatures to Kelvin (K = °C + 273.15).
2. Enter any three of V₁, T₁, V₂, and T₂ (T values in Kelvin).
3. Leave the fourth field blank—that is what will be calculated.
4. Click the Calculate button.
5. Review the result and verify it makes physical sense (higher T → higher V).

Why must temperature be in Kelvin?

Charles's Law requires absolute temperature because it is based on direct proportionality (V ∝ T). The Kelvin scale starts at absolute zero, making the proportionality valid. Celsius and Fahrenheit have arbitrary zero points (freezing point of water), so their ratios are meaningless for this law.

Can I enter Celsius and let the calculator convert?

No—you must convert to Kelvin yourself before entering values. This avoids ambiguity about which fields are in which units. Use K = °C + 273.15.

What happens at absolute zero (0 K)?

Charles's Law predicts that gas volume reaches zero at 0 K (−273.15 °C). In reality, all gases liquefy or solidify before reaching this temperature, so the law breaks down. Absolute zero is the theoretical lower limit of temperature.

Does this work for non-ideal gases?

Charles's Law is exact for ideal gases. Real gases like nitrogen, oxygen, and CO₂ follow it closely at moderate temperatures and pressures, but deviate when approaching their condensation points or at very high densities.

How is Charles's Law used in real life?

Hot air balloons rely on Charles's Law: heating the air inside the envelope increases its volume, making it less dense than the surrounding air, and the balloon rises. Weather balloons expand as they ascend because the air inside warms relative to the lower outside pressure (though pressure also changes, so the combined gas law is more accurate).

What if my calculated temperature is negative in Kelvin?

A negative Kelvin temperature is physically impossible. If your result is negative, check your inputs—you likely used Celsius instead of Kelvin, or made a unit error.

Temperature must always be in Kelvin for Charles's Law. If your data is in Celsius, convert using K = °C + 273.15. If it is in Fahrenheit, convert using K = (°F − 32) × 5/9 + 273.15. Never use Celsius or Fahrenheit directly—the ratios will be wrong because these scales have arbitrary zero points. Charles's Law assumes an ideal gas at constant pressure with a fixed amount of gas. At very low temperatures near condensation, real gases deviate from this law. For a more general tool that handles simultaneous pressure and temperature changes, try our combined gas law calculator.