First your longitude:
1. Convert the minutes portion of the longitude from minutes to decimal
by dividing by 60.2. For North America and locations of West longitude, subtract your longitude from 180 degrees. For location of East longitude, add 180 degrees.
3. Next divide this value by 20. The whole number result will be used to determine the first character for your grid, as follows:
0=A, 1=B, 2=C, 3=D, 4=E, 5=F, 6=G, 7=H, 8=I, 9=J, 10=K, 11=L, 12=M, 13=N, 14=O, 15=P, 16=Q, 17=R, 18=S.
4. For the third digit, multiply this last number by 10. The digit immediately before the decimal point is the third digit of your grid.
Now use your latitude.
1. If your latitude is north, add 90. If your latitude is South, subtract your latitude from 90.
2. Divide this number by 10. The whole number result will be used to determine the second character of your grid as follows:
0=A, 1=B, 2=C, 3=D, 4=E, 5=F, 6=G, 7=H, 8=I, 9=J, 10=K, 11=L, 12=M, 13=N, 14=0, 15=P, 16=Q, 17=R, 18=S.
3. Now, multiply this number by 10. The digit immediately before before the decimal point is the fourth digit of your grid.