Eastings and Northings are used in the United Kingdom's Ordnance Survey Maps. Translating the units into their numerical equivalents and using the difference between the Eastings and the difference between the Northings of two points on the map allows you to measure distances, confirm historical reports and find potentially important locations that are otherwise unmarked.
Ignore the letters in the reference for now. Instead, look at the numbers in the reference. There will always be an even number of digits: 6, 8, 10 or 12.
If there are six digits in the reference, split the first three from the second three. The first three numbers are the reference for the Eastings and the second three numbers are the reference for the Northings. For example, "387148" becomes "387" and "148."
Use the National Grid to replace the grid letters with the corresponding numbers. As an example, Grid "SU" becomes the number "41." This is not the number 41; rather it is two separate numbers, "4, "representing the Eastings, and "1," representing the Northings.
Place the "4" in front of the first group of numbers: "4387." Place the "1" in front of the second group of numbers: "1148." This now represents "E 4387" and "N 1148." If the Easting and Northing numbers are three digits long, the unit of measurement is kilometres ; if 4 digits long, the unit of measurement is 100 meters. If 5 digits long, the measurement is in 10-meter increments; if 6 digits long, the measurement is in 1-meter increments.
Calculate the distance between two points by determining the Eastings and Northings of the two points. Subtract the smaller Easting from the larger, subtract the smaller Northing from the larger. Construct a right triangle using the difference of the Eastings for the base and the difference of the Northings for the height. Use the Pythagorean theorem for right triangles to determine the length of the hypotenuse. The hypotenuse is the distance between those two points, in the "units of measurement" specified by the number of digits in the Eastings and Northings.
Keep a copy of the National Grid handy when doing this work.