It's pretty neat. Plug in the values for your particular problem, press the "Calc" button and the new page will give you the answer and show you how it came up with the answer, for the values you entered.

Linear interpolation is often used to convert a heading from Magnetic to Compass or vice versa. If you're using a table of your boat's deviations (see sample table below) and the heading you need isn't listed on the table, a linear interpolation from the two closest points listed is probably the best approximation of the deviation you need to complete your work.

You can enter the deviations with or without an East/West designation. (Not necessary if entries are both East or both West, but if they're different, you'd better be specific.)

Note: Remember, the calculator doesn't care whether you're interpolating a deviation for a Magnetic heading or a Compass heading, but it'll sure matter for your work so remember to pull your headings from the correct column.