Drill bit size chart.

The chart

About the systems. Number drills (#1–#80) and letter drills (A–Z) are US industrial sizes — they fill the gaps between fractional sizes and are common in machining. Fractional drills (1/64″ steps) are the household standard. Metric drills are the international standard outside the US and Canada. Most drill index sets include all four systems together.

Common applications

Common pitfalls

• Number drills count down as they get bigger. #80 is the smallest, #1 is the largest. Just like AWG — and trips up everyone the first time.
• Letter drills start where number drills end. A (0.234″) is bigger than #1 (0.228″). Useful when you've outgrown the number set but haven't reached a clean fraction.
• "Close enough" isn't always. A 6 mm and a 15/64″ drill are 0.0094″ apart — fine for most through-holes, but a tight fit for a press-fit pin won't tolerate the difference.
• Metric and US fractional don't line up. 1/4″ ≈ 6.35 mm, not 6 mm. If a metric spec says 6 mm and you only have inch drills, the closest size is 15/64″ (5.95 mm) or 1/4″ (6.35 mm) — pick based on whether you want clearance or interference.
• Drill bit diameter ≠ hole size. A real-world hole is typically 0.001″–0.005″ larger than the bit due to runout and bit flexing. Critical for precision work.

Common questions

What size drill bit is 1/4 inch in metric?

1/4 inch = 6.35 mm. The closest standard metric drill is 6.5 mm (slightly larger) or 6 mm (slightly smaller). For tapping, a 6 mm hole is too tight for a 1/4-20 tap; 6.5 mm gives clearance. For clearance holes, 6.5 mm is the safer choice.

What's the difference between letter and number drills?

Number drills run from #1 (0.228") to #80 (0.0135") — getting smaller as the number increases. Letter drills run from A (0.234") to Z (0.413") — getting larger as the letter advances. They were standardized to fill gaps in the fractional series. Use number drills for small holes, letter drills for mid-sized holes.

Why is the tap drill different from the bolt diameter?

A tap drill leaves enough material for the thread crests to form. For a 1/4-20 UNC, the tap drill is #7 (0.201") — about 75% of the bolt's nominal 1/4" (0.25") diameter. Using a 1/4" drill would leave no thread; the bolt would just spin. The 75%-thread engagement is a strength/ease-of-tapping compromise.

What's a 'pilot hole' and when do I need one?

A pilot hole is a small initial hole drilled to guide a larger drill or fastener. For wood screws, the pilot prevents splitting and reduces driving torque; for metal, it lets the cutting edge of a large drill engage properly. Rule of thumb: pilot diameter ≈ 70% of the major diameter for self-tapping screws, smaller for wood.

How do I drill through hardened steel without breaking bits?

First, verify it's actually hardened — many 'hard' steels can be drilled with standard HSS bits and good technique. For true hardened steel (Rockwell 50+), you need cobalt M42 or carbide-tipped bits, slow speed (200-400 RPM), heavy pressure, and constant cooling. Hardened steel work-hardens further if you let the bit rub instead of cut; let it bite.

Sources

• Number drills (#1–80): ANSI B94.11M — Twist Drills.
• Letter drills (A–Z): ANSI B94.11M.
• Fractional drills: ANSI B94.11M, in 1/64″ increments.
• Metric drills: ISO 235 — Parallel shank twist drills.
• Tap drill values: Machinery's Handbook, 31st ed., for 75% thread engagement.

Disclaimer. Pilot, tap, and clearance hole sizes vary with material and intended fit. For precision or production work, always verify against your specific application.

See also