Drill Sizing Systems Explained

Fractional Drills: What Most Shops Start With

Fractional drills are sized in 1/64-inch increments from 1/64" up through several inches. This is the system most people learn first. A standard 29-piece set covers 1/16" through 1/2" in 1/64" steps, and that handles maybe 80% of general shop work.

The limitation of fractional drills is the step size. The jump between 1/64" increments is 0.0156", which is too coarse for many tap drill requirements. For example, the ideal tap drill for a 1/4"-20 UNC thread at 75% engagement is 0.2010". The closest fractional drill is 13/64" (0.2031") or 3/16" (0.1875"), neither of which is close enough for precision work. You need a #7 drill (0.2010") to hit the target.

Fractional drills are also the most commonly available. Every hardware store, industrial supplier, and toolbox has them. For clearance holes, body holes, and general drilling where the exact size is not critical to the thousandth, fractional drills are perfectly fine.

Number Drills (#1 through #60)

Number drills originated from the wire gauge system used in the 19th century. The numbering is counterintuitive: #1 is the largest (0.2280") and #60 is the smallest (0.0400"). Larger number means smaller drill. This catches everyone at least once.

The number drill series fills the gaps between fractional sizes. Where fractional drills jump in 0.0156" steps, number drills provide intermediate sizes that are essential for tap drill selection. The spacing between consecutive number drills varies, some gaps are less than 0.002", while others exceed 0.005". The series is not linear or uniform.

Common number drills every machinist should know:

• #7 (0.2010"): Tap drill for 1/4"-20 UNC
• #21 (0.1590"): Tap drill for 10-24 UNC
• #25 (0.1495"): Tap drill for 10-32 UNF
• #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"): Tap drill for 2-56 UNC

If you work with inch-based tapped holes smaller than 1/4", you will use number drills constantly. They are the backbone of small-hole tap drill work.

Letter Drills (A through Z)

Letter drills fill the gap between the #1 number drill (0.2280") and 17/64" fractional (0.2656"). The series runs from A (0.2340") to Z (0.4130"), with each successive letter being slightly larger. Unlike number drills, the lettering is intuitive: A is smallest, Z is largest.

The letter drill series was developed specifically because the number drills stopped at 0.228" and fractional drills at that size range jump in steps that are too coarse for tap drill work. Key letter drills include:

• F (0.2570"): Tap drill for 5/16"-18 UNC
• H (0.2660"): Close clearance for 1/4" bolts
• I (0.2720"): Tap drill for 5/16"-24 UNF
• Q (0.3320"): Tap drill for 3/8"-24 UNF
• U (0.3680"): Tap drill for 3/8"-16 UNC (alternative to 5/16")
• W (0.3860"): Tap drill for 7/16"-20 UNF

In daily shop work, you will reach for letter drills less often than number or fractional drills. But when you need one, you really need it, because no fractional or number drill hits that size. This is especially true for tap drills in the 5/16" to 7/16" range.

Metric Drills and CNC Work

Metric drills are sized in millimeters, typically in 0.1mm increments for common sizes and 0.5mm increments for larger sizes. The system is straightforward: a 6.0mm drill is 6.0mm. No backwards numbering, no letters, no fractions. This simplicity is why metric has become the dominant system in CNC programming and international manufacturing.

Common metric drill sizes and their inch equivalents:

• 3.3mm (0.1299"): Tap drill for M4 × 0.7
• 4.2mm (0.1654"): Tap drill for M5 × 0.8
• 5.0mm (0.1969"): Tap drill for M6 × 1.0
• 6.8mm (0.2677"): Tap drill for M8 × 1.25
• 8.5mm (0.3346"): Tap drill for M10 × 1.5
• 10.2mm (0.4016"): Tap drill for M12 × 1.75

In a mixed shop (inch and metric work), you will inevitably end up with both drill systems. A 5.0mm metric drill (0.1969") is close to but not identical to a 13/64" fractional (0.2031"). For tap drill work, "close enough" is usually not good enough. Using a 13/64" drill for an M6 tap hole gives about 66% thread engagement instead of the target 75%. It will work, but the threads are weaker.

If your shop does any metric work at all, invest in a dedicated metric drill set (1.0mm to 13.0mm by 0.5mm) at minimum. For CNC work, a set with 0.1mm increments through 10mm is worth the cost.

When to Use Which System

Tap drill selection: Use whatever system gives you the closest match to the calculated tap drill size. The tap drill formula gives you a decimal diameter. Look it up across all four systems and pick the one that hits closest. For UNC/UNF threads, the answer is usually a number or letter drill. For metric threads, use metric drills.

Clearance holes: Fractional drills are usually fine. Per ASME B18.2.8, clearance hole sizes for hex bolts are specified as Normal, Close, and Loose fit. These almost always land on fractional sizes: a 1/4" bolt uses a 9/32" normal clearance hole, a 5/16" bolt uses 11/32", and so on.

Reaming: Ream pilot holes should be 1/64" (0.015") under the finished size for most materials. For precision reamed holes, use whatever drill system gets you closest to the target pilot diameter.

CNC programming: If the print is metric, use metric drills. If the print is inch, use inch drills. Converting between systems in the program adds a rounding step that can put you off target. Keep your tooling consistent with the dimensioning system on the drawing.

The bottom line: carry all four systems and stop worrying about which is "best." Each one exists because no single system provides every size you need. The 115-piece combined set (fractional + number + letter) plus a separate metric set covers virtually everything.