Charles's law calculator
Heat a gas at constant pressure and it expands in proportion to its absolute temperature. Leave the temperature in Celsius and the same arithmetic says a 20°C to 40°C warming doubles the volume. It does not: it adds about 7%. Nothing you type here leaves your browser — it is worked out on your own device, so there is nothing to send and nothing to log.
Why it has to be absolute
Charles's law says V₁/T₁ = V₂/T₂. That is a statement about a ratio of temperatures, and a ratio only means something when zero means zero.
Zero Celsius does not mean zero anything. It is the freezing point of water, which is a fact about water and not about temperature. Putting Celsius into the formula asks what fraction 40 is of 20 and answers "twice", when the physical quantities are 313.15 K and 293.15 K — a ratio of 1.068.
| 1 L at 20°C, heated to 40°C | 1.068 L |
|---|---|
| The same sum in Celsius | 2 L |
And at 0°C it divides by zero. Nothing physical happens to a gas at zero Celsius; it is an ordinary 273.15 K. The zero is an accident of where somebody put the mark on the thermometer, and a formula that cares about it is a formula being used wrong.
The line points at a place no gas reaches
Extrapolate Charles's law downward and the volume reaches zero at −273.15°C. That is where absolute zero came from historically — several gases, extrapolated, all pointing at the same temperature, which is a strong hint that the temperature rather than the gas is what matters.
No gas gets there. Every real gas condenses to a liquid long before, and a liquid does not obey Charles's law at all. The straight line is real over the range gases stay gases, and the point it aims at is real, and nothing travels the last part of the journey.
Zero Celsius is 273.15 K exactly, by the definition of the degree Celsius rather than by measurement. That is one of the few numbers on this site that can be stated without a source, because it is a definition rather than a finding.
What has to stay constant
The pressure, and the amount of gas. A sealed rigid container does not obey this law — there the volume is fixed and the pressure rises instead, which is Gay-Lussac's law and the reason an aerosol can carries a warning about heat. The everyday case this page fits is a balloon or a piston free to move against the atmosphere.