Screw Size Chart: Metric Machine Screw Head Dimensions M1.6–M10 (Pan, Countersunk, Slotted, Socket Head)

It is the nominal thread diameter d in millimetres: M5 is a 5 mm thread, not a 5 mm head. The head is always wider than the thread - at M5, dk = 9.5 mm on a cross recessed pan head, 9.3 mm on a countersunk head and 8.5 mm on a socket head cap screw. Order by the M number plus the length, and confirm dk and k against the drawing or the counterbore.

By the head profile and the recess. Pan head: flat bearing face with a domed top, and wider than a cheese head at the same size (M5: dk = 9.5 mm against 8.5 mm) - for a head that sits on the surface. Countersunk head: a 90° cone with no bearing face - for a head that must finish flush, where a seat can be machined (M5: dk = 9.3 mm, kc = 2.7 mm). Cheese head: a plain cylinder with a flat top (M5: dk = 8.5 mm, k = 3.3 mm). Socket head cap: a cylinder driven from inside with a hex key (M5: dk = 8.5 mm, k = 5 mm, hexagon socket s = 4 mm) - for a recessed head or high tightening torque at a low head height.

They share the same head diameter dk - at M5 both are 9.5 mm - but the head height and the drive differ. GB/T 67 is k = 3 mm at M5 with a slot width n = 1.2 mm for a flat blade; GB/T 818 is k = 3.7 mm at M5 with a No.2 recess for a PH2 bit. The cross recess carries more torque without the bit slipping out of the slot, which is why it dominates machine screws; the slotted form still appears on older drawings and on some consumer-facing items, and the two are not interchangeable in height even though the head diameters match.

Cross recessed pan head (GB/T 818): No.0 (PH0) at M1.6 and M2, No.1 (PH1) at M2.5 and M3, No.2 (PH2) at M4 and M5, No.3 (PH3) at M6, No.4 (PH4) at M8 and M10. Recess numbering is per head type, so it cannot be carried across: in the cheese head series (GB/T 822) M2.5 is No.1 and M3, M4 and M5 are all No.2. Slotted (GB/T 67): the slot width n runs from 0.4 mm at M1.6 to 2.5 mm at M10 (1.2 mm at M5). Socket head cap (GB/T 70.1): the hexagon socket s runs from 1.5 mm at M1.6 to 8 mm at M10 (4 mm at M5).

The standard gives each head dimension as a nominal value that equals the top of the tolerance band, plus a separate minimum. These tables print the nominal (= max) figure, so a delivered screw measures at it or just under - an M5 cross recessed pan head is dk 9.5 mm and k 3.7 mm on that basis. Countersunk heads carry a third figure, the theoretical maximum that includes the whole 90° cone, which is the one to use when the seat is machined rather than the nominal.

Not as a substitute. Both series are 60° threads, but the diameters and thread counts differ, and the closest inch size is still a few tenths of a millimetre away - M5 is d = 5 mm with a 0.8 mm pitch, while the nearest inch machine screw, No.10 UNC, is 4.826 mm with a 1.058 mm pitch. A near-diameter screw starts and then jams and damages the thread. Use the metric to inch table on this page to read which callout a drawing means, and order the thread the joint is made for.