The Maidenhead Grid Locator System is used to identify geographic positions for amateur radio communication and logging. It divides the Earth into grid squares based on latitude and longitude.

4-character locators (e.g. FN31) represent a ~70x100 mile area. 6-character locators (e.g. FN31pr) narrow it down to about 3x4 miles.

This tool lets you convert between latitude/longitude and grid square for logging, APRS, contesting, or satellite work.