Use the free Tap Drill Size Selector below. It runs entirely in your browser, with no sign-up and no download.
The hole before the thread
Cutting an internal thread starts with a hole that is deliberately smaller than the finished thread. The tap then cuts the thread into the wall of that hole, and the size of the hole decides how much thread the tap actually forms. Too small a hole and the tap has to remove too much metal, which is slow, hard on the tool and liable to snap it in the work. Too large a hole and the thread is shallow, weak and liable to strip under load. The tap drill chart exists to land between those two failures.
This selector gives you the standard tap drill for metric and imperial taps. Pick your tap, and it returns the drill size and, for metric taps, the approximate thread engagement. The standard sizes aim for roughly 75 to 80 percent thread engagement, which is the sweet spot for most work: nearly full strength without the effort and breakage risk of a 100 percent thread.
How metric tap drills are chosen
For metric threads there is a handy rule: tap drill diameter equals the nominal diameter minus the pitch. A M6 thread with 1.0 millimetre pitch needs a 5.0 millimetre drill. A M10 thread with 1.5 millimetre pitch needs an 8.5 millimetre drill. The rule is not exact for every fine pitch, because thread form depth is 0.54127 times pitch rather than the full pitch, but the standard chart follows it closely, and the small rounding in the chart is what delivers the 75 to 80 percent engagement.
Imperial threads work the same way but the chart is less tidy, because drill sizes come from numbered and lettered gauge drills as well as fractions. A 1/4-20 tap takes a number 7 drill, which is 0.201 inches. A 3/8-16 tap takes a 5/16 inch drill. The selector lists the gauge or fraction name, because that is what you will actually reach for in the drill index.
A worked example
Suppose you need a M8 threaded hole in a steel bracket. The standard coarse pitch for M8 is 1.25 millimetres, so the selector returns a 6.8 millimetre drill. Drill the hole at 6.8 millimetres, chamfer the top edge so the tap starts square, and tap with cutting fluid, turning forward half a turn and back a quarter turn to break the chip. The result is a thread with about 77 percent engagement, strong enough for a bolted joint and kind enough to the tap.
If the same hole were in a hard material like stainless steel, many machinists would step up one drill size, to 7.0 millimetres, accepting slightly less engagement for a much lower risk of a broken tap. A broken tap is one of the hardest workshop problems to fix, because the tap is harder than most drills, so prevention is worth more than the small strength difference.
Tapping technique that saves taps
Start square. A tap that enters crooked cuts a crooked thread, and no amount of care later will straighten it. Use a tap guide, a drill press with the spindle locked, or at minimum a square held against the tap while you start it. Use cutting fluid suited to the material: oil for steel, something lighter for aluminium, and nothing at all is worse than the wrong fluid for cast iron, which is usually tapped dry.
Back off regularly to break chips, especially in blind holes, where chips pack at the bottom and jam the tap. In a blind hole, drill deeper than the thread needs to go, so chips have somewhere to sit and the tap does not bottom out. And use the right tap for the job: a taper tap starts easily, a plug tap threads deeper, and a bottoming tap finishes threads close to the bottom of a blind hole.
Frequently asked questions
Can I use the clearance drill size instead? No. A clearance hole is sized so a bolt passes through freely, which is larger than the tap drill. Drilling clearance size and then tapping gives almost no thread at all.
What about thread forming taps? Form taps displace metal instead of cutting it, so they need a larger hole than cutting taps, typically sized for about 65 percent engagement. Do not use cutting tap drill sizes for form taps.
Does the chart change for soft materials? For very soft materials like plastic or soft aluminium, a slightly smaller drill can give more engagement without much breakage risk, but the standard size is still a safe default for one-off work.
Spanish version
Calculadora de Broca para Machos de Roscar – Broca Correcta para Cada Macho y Rosca