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.

 A: 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
A: "Kelvin" and "degrees Celsius" are defined such that there are 273.16 degrees between absolute zero and the triple point temperature of water. Degrees Celsius are defined as $K - 273.15$.
These definitions have been in place since 1967.  
A: It is indeed defined as the temperature for which an ideal gas reaches a zero volume. Or that's how the definition is based. This number has an error altough I can't seem to find it anywhere, they probably did a lot of experiments so that the error is much smaller than the value -273.15°C that's used for 0K.
A: You are outlining the earliest attempts to understand the value of absolute zero. However, once the concept of absolute zero was realized, scientists would have created other methods to probe this value including trying to create an environment with a temperature as close to absolute zero as possible.
In 2018, the Institute of Photonic Sciences in Spain were able to cool atoms to 0.00036 Kelvin based on measurements of the frequency of photons emitted from the atoms at that temperature. They used a method of laser cooling to achieve this low temperature.
So, to answer your question, it was originally deduced theoretically but later confirmed by various experimental methods.
Then metrology steps in as the unit of measurement for the Kelvin is defined as 1/273.16th of the difference between absolute zero and the triple point of water.
