water - Why is the absolute zero -273.15ºC?

I can't find an answer of why the lowest temperature is -273.15ºC. Is it deduced theoretically or is it experimental?

An explanation is that when any gas volume tends to zero, the temperature will be -273.15ºC (Charles law). But shouldn't this number have some kind of error? And this would only apply to ideal gases.

Answer

Well the real question should be why is there a °C (Celsius).

The Celsius scale is a "Centigrade" scale in that it uniformly divides the temperature range between the boiling point of water, and the freezing point of water into 100 equal parts, and then it arbitrarily calls the freezing point zero °C, and the boiling point becomes 100°C.

The Kelvin scale is referenced to the triple point of water, not the freezing point, and that Temperature is about 0.1°C (it might be 0.098°C but I am not sure about that).

Quite arbitrarily, it was decided that degrees on the Kelvin scale, should be identical in size to Celsius degrees, and experimentally the zero on the Kelvin scale (zero kelvins) is 273.16 Celsius degrees below the triple point of water, which makes it also -273.15°C