Engineering

Thread & Fastener Calculator

Turn a thread designation into its pitch, tap drill and clearance hole, and find the nearest equivalent in the other thread system.

The thread on the drawing

Thread designationMetric: M8, M8x1.25. Inch: 1/4-20 (UNC), 1/4-28 (UNF), #10-32.

Result

Drill 6.75 mm

M8x1.25 - major diameter 8.000 mm, pitch 1.250 mm (20.32 threads per inch).

SystemMetric (ISO 261)

Major diameter8.000 mm

Pitch1.250 mm

Threads per inch20.32

Tap drill6.75 mm

Tap drill (metric)6.75 mm

Clearance hole≥ 9.00 mm

Nearest equivalent

Metric coarseM8 × 1.25

Inch UNC5/16-18 (18 TPI)

A nearest equivalent is not an interchangeable one. To swap systems you also change the engagement, the head and the torque - confirm which system the mating part is tapped for.

What to know before tapping

M8 coarse is 1.25 mm pitch, 1250 µm thread depth per turn. Tap drill 6.75 mm gives roughly a 75% thread, which is the normal target: a full thread is stronger on paper and strips more often in practice because the tap has no room for chips.
Thread engagement: 1×D of full thread is the usual target in steel, and about 2×D in aluminium or brass because the threads are weaker than the bolt. Treat those as starting points, not a design calculation.

Pitches and thread counts come from the published ISO 261 and UNC/UNF series; the tap drill is computed from them by the standard formula. Confirm against your tap supplier’s chart and the drawing before cutting metal.

What you provide

Thread designation (M8x1.25, 1/4-20, #10-32)