Tap Drill Size Chart: 1/4-20, M8, 3/8-16, and Other Common Sizes

The tap drill size is the hole diameter you drill before cutting internal threads — get it wrong and you either strip the tap trying to cut too much material, or end up with threads too shallow to hold load. For the three sizes covered most often on a shop floor: a 1/4-20 UNC tap needs a #7 drill (0.2010″), an M8×1.25 tap needs a 6.8 mm drill, and a 3/8-16 UNC tap needs a 5/16″ drill (0.3125″). This applies to standard machine-thread taps (UNC/UNF and metric) — not NPT pipe taps, which follow a different tapered-thread rule and aren’t covered here.

Why the Drill Size Isn’t the Same as the Thread Size

A tap drill hole is smaller than the thread’s major diameter, because the tap itself cuts the rest of the material into thread profile as it advances. The standard target is roughly 75% thread engagement — enough thread contact to reach close to full fastener strength without making the tap fight excessive material. The rule of thumb for metric threads: tap drill diameter ≈ major diameter − pitch (for M8×1.25, that’s 8 − 1.25 = 6.75 mm, rounded in practice to 6.8 mm). Inch threads don’t reduce to as clean a formula — they’re read off a standardized reference table instead, which is why a chart is more useful than a calculation for UNC/UNF sizes.

Tap Drill Size Chart — Common Sizes

On the M8×1.25 row: this is the one size in the chart where sources don’t fully agree at the decimal level. One reference calculates it directly from the diameter-minus-pitch rule and lands on 6.8 mm; another combined UNC/metric chart lists 6.9 mm for the same size. The 0.1 mm gap is normal — it comes from which rounding convention a given source applies to the 75% engagement target, not from one source being wrong. Either drill will function; 6.8 mm is the more commonly cited value and the one backed by the direct calculation, so use that as your default unless your shop standard specifies otherwise.

If You’re Tapping Harder Material (or Want an Easier Cut)

75% engagement is the standard target for general-purpose work in aluminum, mild steel, and similar materials. In harder materials, some shops deliberately go to a lower engagement percentage — the tap cuts easier and is less likely to break off in the hole, at the cost of somewhat lower thread strength. The two sizes covered here both have documented alternates:

• 1/4-20: drop to 13/64″ (0.2031″) for an easier cut at roughly 50% engagement, if you don’t have a #7 bit on hand or you’re working harder material.
• 3/8-16: drop to letter drill ‘O’ (0.3160″) for roughly 65–70% engagement in harder materials.

Don’t go the other direction (smaller than the 75% drill) expecting a stronger thread — undersized tap drilling is the most common cause of a tap snapping off in the hole, not a way to gain strength.

Tap Drill Hole vs. Clearance Hole — Don’t Confuse the Two

The sizes above are for cutting new internal threads (tapping). If instead you’re drilling a hole to pass a bolt through without threading it — a clearance hole — the diameter needs to be larger than the bolt’s major diameter, not smaller. For an M8 bolt passed through (not tapped), a clearance hole runs around 9.0 mm — noticeably larger than the 6.8 mm tap drill for the same thread size. Mixing these up (drilling a clearance-hole-sized hole where you meant to tap it, or vice versa) is an easy and completely avoidable mistake — check which operation you’re actually doing before you pick up the bit.

Related reading: More on metric vs. SAE thread naming and sizing conventions will link here as those pages are built out for this hub.

Danny Ni Engineering & Mechanical Systems Writer

Danny Ni is an engineering-focused technical writer at SYZ Machine, specializing in mechanical components, linkage systems, and real-world application engineering. His work covers aftermarket vehicle parts, industrial joints, and mechanical principles, translating complex engineering concepts into practical insights for engineers, fabricators, and industry buyers.

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