The measurement unit for temperature in International System of Units ( SI ) is kelvin. The temperature in Celsius, the commonly used system for measuring temperature is Kelvin – 273. To convert from Kelvin to Celsius is very simple, it has not a complicated formula, but the change was needed to consider water freeze temperature at 0 degrees. This is why Celsius system was introduced and it is the commonly used temperature measurement system in the world. Other important temperature scales are Fahrenheit and Rankine. Fahrenheit is most used in UK. It has a complicated formula. The Rankin scale is calculated based on Fahrenheit scale.