Tap Drill Size Chart (Inch & Metric) — With the Right Carbide Drill One Click Away

Standing at the machine wondering what to drill before you tap? You’re in the right place. Below is the full inch and metric tap drill chart, and because we stock every one of these sizes in solid carbide, each row links straight to the drill. Look it up, click it, and you’re done.

Quick Lookup: Type Your Tap Size

Inch Tap Drill Chart — UNC & UNF (75% Thread)

Every “Shop” link opens that exact size in USA-made solid carbide jobber drills—all flute and coating options for that diameter on one page.

Metric Tap Drill Chart — Coarse Series

The closest inch-size drill is within 0.001" of the metric tap drill, a substitution machinists make every day when the metric size isn’t in the crib. The thread percentage shifts by a point or two, which is fine for general work.

Why 75% Thread—and When to Drill Bigger

The charts above give roughly 75% of full theoretical thread engagement, the standard for general-purpose tapping. Counterintuitive but true: going from 75% to 100% thread adds only about 5% to the joint’s strength—while roughly tripling the torque on your tap. That’s why nobody drills for 100% thread on purpose.

In tough materials—stainless, titanium, and work-hardening alloys—many shops deliberately drop to 50–65% thread engagement: drill the next size up from the chart, and your taps stop snapping. In soft, forgiving materials like aluminum or brass, stay at 75% for maximum thread strength.

The Hole Matters More Than the Tap

Most “bad tap” problems are really bad-hole problems. Three things fix most of them:

Drill size discipline. A drill that wanders or cuts oversize changes your real thread percentage. Solid carbide’s rigidity holds hole size far better than HSS—we ran the full comparison in Solid Carbide vs. HSS.

Short and rigid beats long and flexible. On CNC work, a short screw machine drill with a 135° self-centering split point starts on location without a spot drill and drills straighter than a jobber length—use the shortest drill that reaches.

Speeds and feeds, not feel. Carbide wants to be run, not babied. Dial in RPM and feed with our free Speeds & Feeds Calculators before you bury a drill in the corner of the chart.

FAQ

What happens if my tap drill is too small?
Thread percentage climbs, tapping torque climbs faster, and taps break — usually in the most expensive part of the table. If a tap feels like it’s fighting you, check the hole size first.

Too big?
Thread engagement drops and so does stripping strength. That strategy is legitimate up to a point (see the 50–65% note above); beyond that, threads pull out under load. Measure the hole, not the drill—worn drills cut small; bent setups cut big.

Do form (roll) taps use the same chart?
No. Form taps displace material instead of cutting it, so they need a noticeably larger hole than this chart shows. Use the form-tap manufacturer’s recommendation—drilling a form tap to a cut-tap chart is a classic broken-tap recipe.

Is it OK to substitute inch drills for metric tap drills?
Yes, when the diameters are close—that's precisely what the “closest inch sub” column shows. Within about 0.001", the thread percentage change is negligible for general work. When it comes to critical threads, the true metric size is the one to use.