Tap Drill Size Calculator
Find the correct drill bit size for tapping a given thread.
Tap drill size
5 mm
Usage examples
Checking drill size before tapping
Before cutting a thread, check the exact drill bit size matching the thread designation ahead of time.
Reducing tap breakage by lowering engagement
For hard materials or blind holes, lower the engagement percentage (e.g. 65%) to reduce cutting resistance and the risk of a broken tap.
Looking up a drill designation for an imperial drawing
Quickly look up the drill number, letter, or fraction for a UNC/UNF imperial thread callout on a drawing.
Frequently asked questions
What thread engagement percentage should I use?
75% is the most widely used standard. Higher engagement (e.g. 90%) makes a stronger thread but increases cutting resistance and tap breakage risk, so it's often lowered to 60-65% for hard materials or blind holes.
What if the calculated metric drill size isn't a standard drill?
Round the metric result to the nearest available drill size (commonly in 0.1mm steps). The imperial UNC/UNF table already shows only standardized drill designations (number, letter, or fractional drills), so no rounding is needed there.
What formula is used to calculate the tap drill size?
Metric mode uses 'major diameter − pitch×(engagement/75)', verified to exactly match the well-known simplified rule 'major diameter − pitch' at 75% engagement. Imperial UNC/UNF uses the standard machinist's handbook tap drill table at 75% engagement.
Does this cover every thread standard?
No, only the most common sizes are included (metric M3–M24, and common UNC/UNF imperial designations). Specialty threads or pipe threads (like NPT) aren't covered — consult a dedicated reference for those.