Tap Drill Size Chart: Metric & SAE Thread Tap Drill Sizes

Metric Coarse Tap Drill Sizes: M3 to M24

Standard coarse-pitch metric threads with the recommended tap drill in millimetres and inches, plus the subtraction that produces it.

The final column exists to show that the rule genuinely holds: every recommended tap drill equals the major diameter minus the pitch, rounded to the nearest available drill. M12 × 1.75 gives 10.25mm and the chart calls for 10.2mm, the nearest stocked size. That is why you can derive any metric tap drill without a chart at all, and why metric threading is markedly less error-prone than inch threading, where no such clean relationship survives the unit conversion.

Thread Engagement Percentage Explained

Tap drill size is really a choice about how much of the theoretical thread profile you actually cut.

The counter-intuitive result in those three cards is that chasing 100 per cent engagement is actively harmful. Thread strength climbs steeply from 50 to 75 per cent and then almost flattens, because past that point the bolt itself becomes the weakest element and cutting deeper threads cannot make the joint any stronger. Tapping torque, meanwhile, keeps climbing. A larger tap drill within the 65 to 75 per cent band is nearly always the better engineering choice, especially in stainless steel or when tapping a blind hole by hand.

Metric Fine Tap Drill Sizes

Fine threads take a noticeably larger tap drill than coarse threads of the same diameter, because the shallower thread form removes less material: M10 × 1.5 needs 8.5mm while M10 × 1.25 needs 8.8mm and M10 × 1.0 needs 9.0mm. Using the coarse tap drill for a fine thread leaves the hole undersized by several tenths of a millimetre, which is enough to bind the tap and snap it off inside the work. Always confirm the pitch before drilling. The bolt size chart lists the standard coarse pitch for every metric diameter as a starting point.

UNC Tap Drill Sizes

Inch tap drills are named rather than calculated, and they range across all three inch drill series: 1/4-20 calls for a #7 from the number series, 5/16-18 calls for an F from the letter series, and 3/8-16 calls for a plain 5/16" fraction. That mix is exactly why a full inch drill index carries all three sets, and why the drill bit size chart is worth having open alongside this one when you are working in inch threads.

UNF Tap Drill Sizes

Comparing this table against the UNC one directly above shows the same pattern the metric tables show: for any given diameter the fine thread takes the larger drill. 1/2-13 UNC needs 27/64" at 0.4219", while 1/2-20 UNF needs 29/64" at 0.4531". A difference of about a thirty-second of an inch in the same nominal size. Pick the wrong row and the tap either cuts air or seizes.

Selecting the Correct Tap Drill

One last check that costs nothing: after tapping, run the actual fastener in by hand. It should turn freely for its full depth with no wobble. If it binds, the hole was undersized or the tap wandered; if it rocks, the hole was oversized and the thread will strip under load. Once the fastener is in place, sizing the tool that tightens it is a separate question, head size, not thread size, and the metric to SAE socket converter covers that side of the job.

More Conversion Tools

The rest of the fastener, thread and drill references on this site, every one of them is one click away from here.