Check which drill to use for a tapped hole and what diameter the rod for a die should have. The calculator covers metric coarse and fine, BSP pipe (G and R) and UNC, UNF and Whitworth BSW inch threads.
How do I choose a drill for a metric thread?
The simplest rule: drill diameter = thread diameter − pitch. For M6 × 1 that is 5 mm, for M8 × 1.25 – 6.8 mm, for M12 × 1.75 – 10.2 mm. Fine threads have a smaller pitch, so the drill is larger, e.g. M12 × 1.5 → 10.5 mm.
BSP pipe threads: G and R
A G thread is parallel and seals on a gasket or O-ring, while an R thread is tapered and seals on the thread itself (with PTFE tape or paste). A slightly smaller hole is drilled for a taper thread. Pipe threads have 11–28 threads per inch, depending on size.
Rod for a die
The rod for an external thread should be 0.1–0.2 of the pitch thinner than the nominal diameter, with a chamfered end. A rod that is too thick tears the thread and wears the die quickly.
How we calculate
For metric threads the tap drill ≈ nominal diameter − pitch (e.g. M10 × 1.5 → 8.5 mm). For inch and pipe threads we give drills from standard tables. The rod for a die should be slightly thinner than the nominal diameter – about 0.1–0.2 of the pitch less – so the die does not tear the thread. Minor (hole) diameter of a metric thread D1 = d − 1.0825 × P.
Frequently asked questions
Which drill for an M8 thread?
For an M8 × 1.25 thread drill a 6.8 mm hole. For M6 – 5 mm, M10 – 8.5 mm, M12 – 10.2 mm.
Which drill for a 1/2 inch pipe thread?
For a parallel G 1/2" (BSP) pipe thread a 19 mm drill is used, and for a taper R 1/2" about 18.25 mm. The external G 1/2" thread is cut on a 21.3 mm pipe.
Which drill for an M6 thread?
For M6 × 1 a 5 mm drill is used, for the fine M6 × 0.75 – 5.2 mm.
What is the difference between UNC and UNF?
Both are American 60° inch threads. UNC has a coarser pitch (e.g. 1/4"-20), UNF a finer one (1/4"-28), so it needs a larger drill.
How do I tap stainless steel?
Use a tap for stainless steel (HSS-E or cobalt), cutting oil and low speed. Every half to one turn, back the tool off to break the chip.