Number and Letter Drill Bit Sizes: Complete Guide and Chart

What You’ll Need

• Drill bit set with number/letter sizes (or individual bits)
• Drill or drill press
• Tap and drill size reference chart (included below)
• Calipers or micrometer (to verify bit diameter if needed)
• Cutting fluid (for metalwork)
• Safety glasses

Safety Precautions

• Small number bits (#60 and smaller) are extremely fragile. Use slow, steady pressure — they snap with almost no lateral force. Never use high-speed drilling in hard materials with these micro bits.
• Use a drill press for small number bits whenever possible. Hand drills introduce lateral movement that snaps small bits. A drill press with a slow, consistent downfeed is much safer.
• Wear eye protection. When small bits snap, fragments travel at high velocity.
• Secure workpieces firmly. Small bits in metal require the workpiece to be clamped — never hand-held.

What Are Number Drill Bit Sizes?

Number drill bits (also called wire gauge drills) use a numbering system derived from the old Brown and Sharpe wire gauge standard. The system runs from #1 through #80, where a higher number means a smaller diameter. This counterintuitive relationship trips up many beginners.

Number bits are common in metalworking, electronics assembly, model engineering, and wherever you need precise hole sizes that fractional inch bits don’t cover. The most common use is drilling tap clearance holes and tap drill holes — the pre-tap hole that determines thread engagement.

Number Drill Sizes: Most Used Reference Chart

What Are Letter Drill Bit Sizes?

Letter drill bits bridge the gap between number bits and the larger fractional sizes. They run from A (the smallest at 0.234″) through Z (the largest at 0.413″), increasing in diameter alphabetically. Letter sizes are most commonly used for drilling clearance holes for bolts, tap holes for coarse thread fasteners, and precision machining applications.

Letter Drill Sizes: Reference Chart

Number vs Letter vs Fractional vs Metric: How They Compare

For metric conversions, see our Metric Drill Bit Conversion Chart.

Common Tap and Drill Size Combinations

The most practical use of number and letter drill bits is selecting the correct tap drill size — the hole diameter that gives approximately 75% thread engagement when tapping (threading) a hole. Here are the most commonly used combinations in US shops:

Where to Buy Number and Letter Drill Bits

Number and letter drill bits are less commonly stocked in big-box hardware stores than fractional sets. For the best selection:

• Sets #1–#60: Available as complete indexed sets at industrial supply stores (MSC, McMaster-Carr, Grainger) and online.
• Individual sizes: Order individually from online industrial suppliers for sizes you use most.
• Material: For metalwork, choose cobalt or black oxide HSS number bits — not the cheap uncoated carbon steel bits that come in cheap index sets. Cheap number bits are notoriously inconsistent in diameter.

Pro Tips

• Label your bits. Individual number bits look nearly identical. Store them in a dedicated index (a numbered drill index) so you can find the right size quickly without measuring each one.
• Verify critical sizes with calipers. Budget number drill bits can be 0.005″ off their nominal size. For precision tap work, verify the actual diameter before drilling.
• Use a center punch before drilling small number bits in metal. Even a small drill press wants a starting point — center punch prevents walking.
• Start with a center drill. For very small bits (#40 and smaller) in metal, start the hole with a short, rigid center drill before switching to the number bit. This prevents bit deflection and breakage.

Frequently Asked Questions

What is the most commonly used number drill bit?

#7 (for 1/4″-20 tap), #29 (for #8-32 tap), #25 (for #10-24 tap), and #21 (for #10-32 tap) are the most frequently used in general metalworking and fabrication. Having these four covers the majority of common fastener threading jobs.

Why do number drill sizes go backward (higher = smaller)?

The number system is derived from the wire gauge system where wire diameter decreases as gauge number increases — the same counterintuitive relationship seen in wire gauge (#12 wire is thicker than #14). It’s a historical convention that stuck in manufacturing and machining industries.

Are letter drill bits the same as fractional drill bits?

No — letter drill bits have specific decimal equivalents that don’t correspond to any standard fractional size. For example, drill F = 0.2570 inches, which is between 1/4″ (0.250″) and 17/64″ (0.266″). Letter sizes exist precisely to fill those in-between gaps needed for accurate tap drill sizing.

Can I use number drill bits in a regular cordless drill?

Yes — most number bits have standard 1/8″ or larger shanks that fit standard chucks. Very small bits (#60 and under) require slow speed and very steady hand pressure. A drill press is strongly recommended for #50 and smaller to avoid snapping the bit.

How do I know which number bit to use for a tap size?

Use a tap drill chart (like the one above). For standard 75% thread engagement, the chart lists the correct bit for every common US thread size. For metric taps, the formula is: Tap Drill Size = Tap Diameter − (1 × Thread Pitch). See our drill bit sizes explained guide for a full reference.

Conclusion

Number and letter drill bits are precision tools designed primarily for metalworking and threading operations. Once you understand the system — numbers count up as holes get smaller; letters fill the gap above fractional — it becomes a straightforward reference. Print a tap drill chart, keep it in your shop, and you’ll always have the right bit for the thread you’re cutting.

Related guides on Power Tools Today:

• Drill Bit Sizes Explained: Fractional, Metric, and More
• Metric Drill Bit Conversion Chart
• Drill Bit Coating Types Guide
• Types of Drill Bits: Complete Guide
• Fractional vs Metric Drill Bits