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. Choose spindle speed and feed using the bit manufacturer’s instructions for the bit diameter and work material; small-diameter bits can require higher RPM but very light, carefully controlled feed.
- Use a drill press for small number bits whenever possible. Hand drills introduce lateral movement that snaps small bits. A drill press with secure workholding and diameter/material-appropriate spindle speed and controlled feed can improve accuracy; tiny bits do not universally require low RPM.
- 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
| Drill # | Decimal (inches) | Nearest Fraction | Common Use |
|---|---|---|---|
| #1 | 0.2280″ | 15/64″ | Tap drill, clearance holes |
| #2 | 0.2210″ | 7/32″ | Machining, metalwork |
| #3 | 0.2130″ | — | Tap drill for 1/4-28 UNF (not 1/4-20) |
| #4 | 0.2090″ | — | Machining |
| #7 | 0.2010″ | 13/64″ | Tap drill for 1/4-20 UNC ✓ |
| #10 | 0.1935″ | — | Common machining size |
| #21 | 0.1590″ | 5/32″ | Tap drill for #10-32 UNF |
| #25 | 0.1495″ | — | Tap drill for #10-24 UNC ✓ |
| #29 | 0.1360″ | — | Tap drill for #8-32 UNC ✓ |
| #36 | 0.1065″ | — | Tap drill for #6-32 UNC ✓ |
| #43 | 0.0890″ | — | Tap drill for #4-40 UNC ✓ |
| #50 | 0.0700″ | — | Small fastener tap drill |
| #60 | 0.0400″ | — | PCB work, micro drilling |
| #80 | 0.0135″ | — | Smallest standard size |
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
| Letter | Decimal (inches) | Nearest Fraction | Common Use |
|---|---|---|---|
| A | 0.2340″ | 15/64″ | Close to #1, precision machining |
| B | 0.2380″ | 15/64″ | Machining |
| C | 0.2420″ | — | Not the standard 5/16-18 tap drill; use letter F (0.2570 in.) |
| D | 0.2460″ | — | Machining |
| F | 0.2570″ | — | Tap drill for 5/16-18 UNC ✓ |
| I | 0.2720″ | — | Tap drill for 5/16-24 UNF |
| J | 0.2770″ | 9/32″ | Clearance hole applications |
| N | 0.3020″ | — | Letter N size only; standard 3/8-16 UNC tap drill is 5/16 in. (0.3125 in.) |
| Q | 0.3320″ | — | Tap drill for 3/8-24 UNF |
| S | 0.3480″ | — | NOT a clearance hole for a 3/8 in. bolt; letter S (0.3480 in.) is smaller than the bolt diameter |
| U | 0.3680″ | — | Tap drill for 7/16-14 UNC ✓ |
| W | 0.3860″ | 25/64″ | Machining |
| Z | 0.4130″ | 13/32″ | Largest letter size |
Number vs Letter vs Fractional vs Metric: How They Compare
| System | Range | Primary Use | Common In |
|---|---|---|---|
| Number (#1–#80) | 0.0135″–0.2280″ | Tap drill sizes, micro holes | Metalworking, machining, electronics |
| Letter (A–Z) | 0.234″–0.413″ | Tap drill and clearance holes | Metalworking, machining |
| Fractional (1/64″–4″) | 1/64″–4″+ | General purpose | Woodworking, construction, DIY |
| Metric (0.3mm–100mm) | 0.3mm–100mm+ | General purpose | Everywhere outside the US |
For metric conversions, see our Metric Drill Bit Conversion Chart.
Common Tap and Drill Size Combinations
A practical use of number and letter bits is preparing holes for threading. The listed tap-drill combinations are common starting sizes for cutting taps, not a universal 75% engagement result in all materials. For example, a 3/8-16 UNC tap commonly uses 5/16 in. (0.3125 in.), not letter N. Always check the tap manufacturer’s size guidance for the work material. Here are the most commonly used combinations in US shops:
| Thread Size | Tap Drill | Decimal | Thread Type |
|---|---|---|---|
| #4-40 | #43 | 0.0890″ | UNC |
| #6-32 | #36 | 0.1065″ | UNC |
| #8-32 | #29 | 0.1360″ | UNC |
| #10-24 | #25 | 0.1495″ | UNC |
| #10-32 | #21 | 0.1590″ | UNF |
| 1/4″-20 | #7 | 0.2010″ | UNC |
| 1/4″-28 | #3 | 0.2130″ | UNF |
| 5/16″-18 | F | 0.2570″ | UNC |
| 3/8″-16 | 5/16″ | 0.3125″ | UNC |
| 7/16″-14 | U | 0.3680″ | UNC |
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?
Not every small number drill has a 1/8-inch or larger shank. Verify that the actual bit shank is within your drill chuck’s gripping range. For fragile small bits, secure the workpiece and follow the bit maker’s recommended speed/feed; a drill press with suitable chuck or collet may improve accuracy.
How do I know which number bit to use for a tap size?
Use a tap drill chart (like the one above). The chart shows selected common cutting-tap sizes rather than every US thread, and the preferred drill diameter varies with the material, tap and target thread engagement. 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. Check the actual tap manufacturer’s drill guidance. Cross-reference LittleMachineShop tap/clearance drill chart and Tapmatic drill-selection chart; this article’s # and letter tables list selected sizes, not the complete #1–#80/A–Z series.
