Here is a pressure calculator. You can use it for any sort of pressures, but in meteorology, the most useful application is for conversions between millibars (mb or mbar)-- the metric atmospheric pressure values commonly used on weather maps and storm systems, and inches of mercury (in Hg)-- the US atmospheric pressure units commonly seen on in-home barometers.

Simply select one set of units on the left, enter a number in, and then select a set of units on the right, and the conversion will be made instantly!