Converting inches to millimetres is a simple math process when you start with the correct conversion. Because 1 inch equals 25.4 millimetres, you also easily can determine how many millimetres equal measurements that are less than one inch, using arithmetic to make this conversion. Start with the inch conversion, and perform the calculations to find the comparable number of millimetres in 7/8 inch.
Start with the premise that 1 inch equals 25.4 millimetres.
Divide 25.4 millimetres by 8 to reach 3.175. This means that 1/8-inch is equal to 3.175 millimetres.
Multiply 3.175 by 7 to get 22.225. This means that 7/8-inch is equal to 22.225 millimetres.