The common specifications for screws are 4mm, 5mm, 6mm, 8mm, 10mm, 12mm, 14mm, 16mm, 18mm in diameter.

Metric thread outer diameter is in millimeters, such as 6,8,10,12,18,20 mm, etc. Pitch is also in millimeters, such as 0.5,0.75,1,1.5,2,3, and so on.

Inch thread outside diameter is in inches, (each inch is equal to 25.4 millimeters) such as 3/16, 5/8, 1/4, 1/2, and so on, so with the metric calipers to measure the outside diameter readings are often with irregular decimal.

Inch pitch is expressed in terms of how many teeth per inch. Calipers set at 25.4 millimeters, a ruler to align the tip of the threads, another ruler to align the tip of the threads such as the tip of the threads is the British threads, such as the tip of the threads are not aligned should be metric threads.

Measurement of pitch is best to thread the tip of the inverted print in white chalk, chalk on the print is more clear, easy to measure. Measurement of metric pitch should measure a length, such as 10,15,20, millimeters, etc., count the number of teeth, calculate the pitch with inches as the unit of specification of the thread for the imperial thread, such as: G1. metric units of millimeters to specify the specification of the thread for the metric thread.

Such as: M30. imperial is an inch (2.54 centimeters) within the number of teeth to set, generally 55 degrees angle. Metric system is the pitch of the two teeth is how many MM to set the pitch, generally 60 degrees.