What the numbers mean.

All metric threads have a prefix of M, this means metric. All numbers there after are represented in millimeters

The number(s) directly after M represent the Nominal Diameter of the bolt and the number after the X represent the pitch of the thread.

*Nominal Diameter - The true bolt diameter is a little smaller and the true nut diameter a little larger
*Pitch is distance between threads

Example for a bolt with a thread of M6 x 1.

M6 x 1   Nominal Diameter = 6mm
M6 x 1   Pitch = 1mm


How to work out "Tap Drill Sizes" for metric threads.

Tap Drill Size = Nominal Diameter - Pitch 


Formula for a bolt with a thread size of M6 x 1.

Tap Drill Size = Nominal Diameter - Pitch
Tap Drill Size = 6 - 1
Tap Drill Size = 5mm


Common Tap Drill Sizes (Metric Coarse)

M3 x 0.5 = 2.5mm
M4 x 0.7 = 3.3mm
M5 x 0.8 = 4.2mm
M6 x 1 = 5mm
M8 x 1.25 = 6.75mm (Use 17/64 inch = 6.747mm) alternativley use (use 6.7mm or 6.8mm)
M10 x 1.5 = 8.5mm
M12 x 1.75 = 10.25mm (use 10.2mm or 10.3mm)


Common Tap Drill Sizes (Metric fine)

M8 x 1 = 7mm
M10 x 1.25 = 8.75mm (Use 11/32 inch = 8.731mm) alternativley use (use 8.7mm or 8.8mm)
M12 x 1.25 = 10.75mm (Use 27/64 inch = 10.716mm) alternativley use (use 10.7mm or 10.8mm)


Additional info.

The thread angle is 60°. The thread depth is 0.614 x pitch.

There are three kinds of ISO metric threads:

• coarse
  
• fine
  
• extra fine