Celsius and Kelvin share the same degree size; Fahrenheit and Rankine share a smaller one (1 °F = 5/9 °C). The scales differ in where they put zero: 0 °C is the freezing point of water, 0 °F was Fahrenheit's coldest brine, and 0 K and 0 °R are absolute zero. That is why converting a temperature means scaling and shifting, and why a temperature difference converts differently from a temperature reading.
Use the converter above for any pair. The most searched pairs have their own pages with worked examples, a reference table of everyday temperatures and tips for estimating in your head.