Charles's Law Calculator
Written by Thierno Sadou Diallo, formula verified per our methodology • Last checked on 9/6/2026
Charles's law is written V₁/T₁ = V₂/T₂ at constant pressure, where temperatures must be expressed in kelvins. For 2 L of gas at 20°C heated to 80°C, the final volume reaches about 2.409 L.
Explanation
Charles's law describes the behavior of an ideal gas during an isobaric transformation, meaning at constant pressure: the ratio between volume and temperature (in kelvins) stays unchanged. In practice, a gas heated at constant pressure sees its volume increase in proportion to its absolute temperature, and vice versa when cooled. This is the principle that explains, for example, why an inflatable balloon visibly shrinks in winter or when placed in a freezer. **The most common pitfall with this law**: the relationship only holds with temperatures expressed in kelvins (absolute temperature, where 0 K corresponds to absolute zero, −273.15°C), never directly in degrees Celsius. Calculating with raw Celsius values gives an incorrect result, because the ratio V/T only has physical meaning if T is measured from absolute zero. This calculator accepts temperatures in degrees Celsius, more intuitive to enter, and automatically performs the conversion to kelvins internally.
Example: heating 2 L of gas from 20°C to 80°C
Inputs
Initial volume: 2 L. Initial temperature: 20°C. Final temperature: 80°C.
Calculation
20°C = 293.15 K and 80°C = 353.15 K. Final volume = 2 × (353.15 ÷ 293.15) ≈ 2 × 1.2047 ≈ 2.409 L.
Result
The gas's volume goes from 2 L to about 2.409 L when heated from 20°C to 80°C.
Frequently asked questions
Why must kelvins be used instead of degrees Celsius?
Because Charles's law expresses a direct proportionality between volume and absolute temperature: doubling the absolute temperature doubles the volume. This proportionality doesn't exist with the Celsius scale, whose zero is arbitrary (the freezing point of water) rather than absolute zero. Using degrees Celsius directly in the formula would give a physically incorrect result.
What is absolute zero and why is it a limit for this calculation?
Absolute zero (0 K, i.e. −273.15°C) is the theoretical lowest possible temperature, where the thermal agitation of particles is minimal. At this temperature, an ideal gas's volume would tend toward zero according to this law, and dividing by a zero absolute temperature makes the calculation undefined — a real physical limit, not just a mathematical curiosity.
Does this law assume constant pressure?
Yes, that's the essential condition for Charles's law (an isobaric transformation). If pressure also varies, you need to use the full ideal gas law (PV = nRT), or our Boyle's law calculator if it's instead the temperature that stays constant.