Download the NGRCalc from the Resource section. This is a free program that you can use as many times as you want. Simply input the ordinance grid coordinates, and the program will give you the equivalent latitude and longitude. You can also reverse the process and put in latitude and longitude to get Ordinance Survey coordinates.
Go to Fielden Maps. You will find a free Java-based program that can convert Ordinance Survey co-ordinates into the traditional latitude and longitude coordinates you're used to. Find the "Great Britain National Grid" section, input the Ordinance Survey coordinates and then click "Convert." The longitude and latitude numbers will change to reflect the conversion.
Visit Movable Type, linked in the Resource section. Movable Type is a Java-based program that you can use for free, as of February 2011. Type your Ordinance Survey coordinates into the "OS Grid Ref" box and then press "Enter." The latitude and longitude numbers will be converted.
