# Proof of X-LFP/UFP-LFP=constant for all scales of temperature

I came across this formula to convert from one unit of temperature to another. I could not find any proof for this though. In the formula, 'X' is the temperature value to be converted, 'UFP' is the upper fixed point and 'LFP' is the lower fixed point of the temperature scale.

What might be misleading about your formula is that the 'constant' is not a universal constant like the Universal Gas Constant $R$ in the Ideal Gas Law $pV=nRT$. The constant in your formula always depends on what 2 intervals you are comparing, just as the ratio of heights depends on what building you are measuring.