Tap Drill Size Calculator (Metric & Unified)

How it works

Cutting tap: drill = major − 1.299 × pitch × % (handbook charts; 1.0825 × pitch on the basic profile) · pitch = 1 ÷ TPI for unified threads · forming tap: drill = major − 0.0068 × % × pitch · thread % for a drill = (major − drill) ÷ (1.299 × pitch) · basic minor diameter = major − 1.0825 × pitch · pitch diameter = major − 0.6495 × pitch · blind-hole depth = thread depth + chamfer threads × pitch + 0.3 × drill

A tapped hole is drilled larger than the thread’s theoretical minor diameter, so the tap cuts only part of the full thread height. The share it cuts is the percentage of thread. Tap drill charts and Machinery’s Handbook measure it against a double thread depth of 1.299 × pitch, and at 75% that gives the familiar sizes: 8.5 mm for M10 × 1.5 and a #7 drill for 1/4-20. Above 75% a thread gains only a few percent of strength, but tapping torque and tap breakage rise steeply, so hard, tough or gummy materials are tapped at 55–65%. Some engineering texts measure against the basic profile height (1.0825 × pitch) instead, and the same drill then reads as a higher percentage. Both conventions are available under More options. Forming taps push metal into shape instead of cutting it, so their hole is close to the pitch diameter. They use their own formula and are usually run at about 65%. The calculator rounds the exact size to the nearest metric drill and the nearest fractional, letter or number drill, and shows the thread % each one really gives. It also works in reverse for a drill you already own, and lists clearance holes for the screw and the drill depth for a blind hole.

Worked example

M10 × 1.5 at 75% with a cutting tap: 10 − 1.299 × 1.5 × 0.75 = 8.54 mm. The standard 8.5 mm drill gives (10 − 8.5) ÷ (1.299 × 1.5) = 77% thread, and the nearest inch drill is R (0.339 in). A forming tap at 75% needs 10 − 0.0068 × 75 × 1.5 = 9.24 mm. For a 1.5 × D blind hole with a plug tap, drill 15 + 4 × 1.5 = 21 mm to full diameter, plus about 2.6 mm for the drill point. For 1/4-20 UNC: 0.25 − 1.299 × 0.05 × 0.75 = 0.2013 in, so use a #7 drill (0.201 in, 75%). The screw clears through an F drill (close fit) or a 17/64 in drill (free fit).

Sources: Machining Doctor — tap drill formulas for cutting and forming taps · Little Machine Shop — tap and clearance drill chart (75% and 50%) · Engineers Edge — theoretical hole size for cold-form tapping

Frequently asked questions