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Charles's Law Calculator

Find a gas's volume after a temperature change.

Result

2.4000

How it is calculated

Charles's law says that for a fixed amount of gas at constant pressure, volume is proportional to absolute temperature: V₁ / T₁ = V₂ / T₂. The tool solves for the final volume, V₂ = V₁ × T₂ / T₁. V₁ is the starting volume in any volume unit (the answer uses the same unit), and T₁ and T₂ are the starting and final temperatures in kelvin. All three values must be above zero. To convert, K = °C + 273.15, so 27 °C is 300.15 K. Never use Celsius or Fahrenheit directly: the proportion only works from absolute zero. The result is rounded to four decimals. It assumes an ideal gas, a sealed amount of gas, and a pressure that stays the same.

Worked example

A balloon holds 2 L of gas at 300 K (about 27 °C). It is warmed at constant pressure to 360 K (about 87 °C). The ratio of temperatures is 360 / 300 = 1.2, so the volume grows by 20%. V₂ = 2 × 360 / 300 = 2.4. The engine gives a final volume of 2.4000 L. Doubling the kelvin temperature would double the volume, but doubling the Celsius value would not.

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Common mistakes

  • Entering temperatures in degrees Celsius. Going from 20 °C to 40 °C is not a doubling; in kelvin it is 293.15 K to 313.15 K, only about a 7% rise.
  • Applying the law when pressure changes or gas is added or leaks out; both break the constant pressure and fixed amount assumption.
  • Swapping T₁ and T₂, which inverts the ratio and gives a shrinking volume for a heated gas.
  • Using it near the point where the gas condenses, where real gases no longer behave like an ideal one.

About this calculator

Charles's law describes how a fixed amount of gas at constant pressure expands or contracts as its temperature changes: V₁/T₁ = V₂/T₂, where volumes and temperatures must be in an absolute scale (Kelvin) for the direct proportionality to hold. It's one of the classic gas laws — alongside Boyle's law (pressure-volume) and Gay-Lussac's law (pressure-temperature) — that together form the ideal gas law, PV = nRT.

Chemistry students and engineers use Charles's law to predict how a hot-air balloon expands as the air inside is heated, why a balloon taken outside on a cold day shrinks and fills out again once it is back in the warm, and to solve textbook problems converting a gas's volume between two temperatures at constant pressure.

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Comments (1)

  • Editorial team

    The law holds only for a sealed, fixed amount of gas whose pressure stays constant, such as air in a syringe or a loose balloon. In a rigid tank the volume cannot change, so pressure rises instead and this tool does not apply. To get a feel for scale, warming a gas from 20 °C by just 10 K enlarges it by only about 3.4%. Is your gas in a flexible container, and are your temperatures already converted to kelvin?

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