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 45 common threads: numbered screws, inch UNC and UNF, and metric coarse, with fine-pitch metric drills on the metric chart.

How the numbers are checked

The chart drills are cross-checked against two published charts. Every thread percentage comes from one formula, and that formula matches the table in a tap maker's chart for all 28 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. It is checked against the sources and fixed on the page.