In 1916, the Society of Automotive Engineers, or SAE, emerged to oversee the development of standards for the transportation industry, including component parts such as bolts. Today, SAE threads represent a major system for classifying screw and bolt sizes. However, the metric system represents an international standard. Metric screws are named according to the diameter of the shank in millimetres; the pitch, or the distance between threads; and the length of the screw or bolt. Although no metric screw is exactly like an SAE screw, you can calculate the closest equivalent by converting these measurements to inches and threads per inch (TPI).

Look at the first number in the screw or bolt's name to determine the diameter.

For example, an M8 screw has an 8mm diameter.

Divide this number by 25.4. The answer is the diameter in inches.

The 8mm screw has a diameter of approximately 0.315 inches.

Multiply the diameter by 64 to calculate how many sixty-fourths of an inch the screw measures.

A 0.315 inch shank equals 20.16/64 inch.

Round the numerator to the nearest whole number.

20.16/64 is not a standard screw size, but 20/64 is.

Reduce the fraction to lowest terms, if necessary.

A 20/64 inch screw is commonly called 5/16 inch.

Look at the second number in the screw or bolt's name to determine the pitch.

For example, an M8 x 1.25 screw has 1.25 millimetres between the crest of one thread and the crest of the next.

Divide 1 by the pitch of the metric screw to determine the number of threads per millimetre.

A pitch of 1.25 equals 0.8 threads per millimetre.

Multiply this number by 25.4. The answer is the number of threads per inch.

0.8 threads per millimetre equals 20.32 threads per inch.

Round the answer to the nearest whole number.

For example, no screw has 20.32 TPI, but many screws have 20 TPI.

Consult a chart of SAE screw and bolt sizes to determine the actual TPI of the nearest screw size.

For instance, a 5/16 inch SAE screw usually has 24 TPI rather than 20.

Look at the last number in the screw or bolt's name to determine the length.

For example, an M8 x 1.25 x 12 screw is 12mm long.

Divide this number by 25.4. The answer is the length in inches.

The 12mm screw measures approximately 0.472 inches.

Multiply the length by 4 to calculate how many fourths of an inch the screw measures.

A 0.472 inch screw equals 1.888/4 inch.

Round the numerator to the nearest whole number.

1.888/4 is not a standard screw size, but 2/4 is.

Reduce the fraction to lowest terms, if necessary.

A 2/4 inch screw more accurately called 1/2 inch.