About Thread Sizes

Thread Sizes answers one question quickly: what drill do I use for this tap? Most of the answers online are scanned manufacturer charts, a single number in a search box, or forum threads that disagree with each other. We wanted one page per thread that gives the drill, shows what the alternatives actually cut, and explains the choice in a few lines.

The numbers are not copied from any one chart. They are calculated from ASME and ISO thread standards, then checked against several published charts, and a value only goes on the site when at least two independent sources agree. The method, the formulas and the places where published charts disagree are all on How we calculate.

The site is written for people at the bench: machinists, mechanics, makers and anyone holding a tap and a drill index. Pages are built to load fast on a phone in the shop and to stay readable at arm's length, in light or dark mode.

If you spot a number that looks wrong, or a thread you would like added, use the contact page.