About TapDrillSize

TapDrillSize is an independent reference site for tap drill sizes, made by one developer. It has no ads.

What it does

The site gives the correct drill for a thread, shows what the drills on either side of it do to the thread, and says where each number comes from. It covers 47 common threads: numbered screws, inch UNC, UNF and UNEF, and metric coarse, with fine-pitch metric drills on the metric chart.

How the numbers are checked

The inch chart drills are checked against two published charts. Metric drills are the major diameter minus the pitch, checked against a published chart where it lists the size. Every thread percentage comes from one formula, and that formula matches the tables in tap makers' charts (OSG, and YG-1 for the extra-fine sizes) for all 30 inch threads here. The method page shows the formula, the sources, and where the guides disagree.

Automated tests in the site's repository re-run these checks on every build.

What it is not

This is reference material, not engineering advice. Check your tap manufacturer's chart before cutting expensive parts.

Report a wrong number

If you spot a mistake, use the Contact link in the footer or email contact@tapdrillsize.com with the thread and the figure. We check it against the sources and fix the page.