About MachinistData
MachinistData is a free reference site that gives machinists, tradespeople and DIYers the correct drill size to use before tapping a hole. We cover UNC, UNF, ISO metric and NPT pipe threads, with full charts, an interactive calculator and the formula behind every size.
What we publish
Every thread has its own reference page showing the tap drill in number, letter, fractional, decimal-inch and millimetre forms, plus the worked formula and close-fit or free-fit clearance sizes. The site is organised into three families — inch machine screws (UNC/UNF), ISO metric threads, and NPT pipe threads — so you can browse by system or jump straight to a specific size.
Our tap drill values are based on standard engineering practice: approximately 75% thread engagement for inch machine screws, the major-minus-pitch rule for metric threads, and the pipe minor diameter at the gauge plane for NPT taper threads.
Who it's for
MachinistData is built for machinists, tool makers, engineers, students, and anyone who needs a quick, reliable answer when they're about to drill and tap a hole. We keep the site plain, fast and free of clutter because the goal is a confident, accurate answer without distraction.
Our approach
We believe a reference tool should be trustworthy and simple. We aim for accurate, clearly explained data, an interface that doesn't get in the way, and content that respects your time. If something on the site can be clearer or more useful, we'd like to know.
Correctness and safety
Machining reference data is engineering tools, and we present it as such. Tap drill sizes depend on the material, the percentage of thread engagement and the thread standard, so we encourage you to verify critical joints against the relevant standard or your manufacturer's specification.
Get in touch
Questions, corrections or feedback about the charts, the calculator or the site in general? Please contact us.