ISO 888: Thread lengths for ISO metric fasteners (bolts)

Calculation of thread lengths

Typical, unless specified in a product standard, dimensions for thread lengths (b) is calculated as follow:

for l <= 125 mm: b = 2d + 6mm
for 125 mm < l <= 200 mm: b = 2d + 12mm
for l > 200mm: b = 2d + 25mm

Where l is lenght of the bolt and b is the threaded part of the bolt.

For bolt, where lengths l <= 0.5d the bolt should be fully threaded.

Typical thread lengths (b) for popular sizes of fasteners

Diameter	M12	M16	M20	M22	M24	M27	M30	M33	M36
l <= 125	30	38	46	50	54	60	66	72	78
125 < l l <= 200	36	44	52	56	60	66	72	78	84
l > 200	–	57	65	69	73	79	85	91	97