About this tool
The Tap Drill & Thread Size Calculator gives you the drill diameter to use before cutting a thread, plus the pitch, pitch diameter and clearance-hole sizes, for both metric (ISO) and imperial Unified (UNC/UNF) fasteners. Choose a standard size from the list — M2 through M20 coarse and fine, and #4 through 1/2 inch UNC/UNF — or switch to custom mode to enter any major diameter and pitch or TPI. Set the desired percentage of thread engagement (75% is the usual default) and everything recalculates instantly, entirely in your browser.
The tap-drill diameter uses the standard thread-engagement relationship: hole = major diameter − (%engagement / 76.98) × pitch, where the pitch is the metric pitch in millimetres or 1/TPI in inches. This is the exact form behind the familiar shortcut of subtracting the full pitch for a metric coarse thread, which corresponds to roughly 77% engagement. The pitch diameter is major − 0.6495 × pitch. Clearance holes for the bolt shank are reported at close, normal and loose fits (about 1.07, 1.10 and 1.16 times the nominal diameter). The result is shown in both millimetres and inches, along with the nearest standard number, letter or fractional drill.
As a worked example, an M6 × 1.0 thread at 75% engagement needs a tap drill of 6 − (75 / 76.98) × 1.0 ≈ 5.03 mm, so a 5.0 mm drill is used; a 1/4-20 UNC thread at 75% works out to 0.201 in, which is a #7 drill. Higher engagement percentages call for a smaller tap drill and a stronger but harder-to-tap thread, while lower percentages ease tapping at a small loss of strength; 65–75% is the practical range for most work.