Make sure you have your northing expressed in kilometers. Every northing measurement should be at least four digits. If there are two little numbers next to it, those are the first two digits. If there are more than four digits (up to seven), put a decimal point in so that the first four are to the left of it.
Convert kilometers into degrees latitude. Divide the northing measurement by 111.13, the number of kilometers in a longitudinal degree. The result is the number of degrees north of the equator, the latitude.
If you would like your answer expressed in degrees, minutes, and seconds, you must make two more calculations. Take the decimal portion of your answer and multiply it by 60 to get the minutes. Now, take the decimal portion of that and multiply it by 60 to get the seconds.
If you would like a more accurate conversion, you need a more in-depth and complicated calculation. The formulas necessary would take pages to lay out and explain. Fortunately, there are handy in-depth explanations available. You may find these using the Resources link.
