A drill size is the diameter of a drill bit, expressed in one of four naming systems: fractional (inches), metric (millimeters), number/wire gauge (#1 to #80), or letter (A to Z). Which drill size is “correct” for a given job depends entirely on what you’re drilling for — a pilot hole, a clearance hole, or a tap drill hole all call for different diameters on the exact same fastener.
The Four Drill Size Systems
Drill bits aren’t sold under one universal numbering scheme — shops stock bits from up to four overlapping systems, and mixing them up is a common source of “wrong size” mistakes:
| System | Range | Typical use |
|---|---|---|
| Fractional | 1/64″ to 1/2″ (and beyond, in 1/64″ steps) | General carpentry, household tasks, common English-unit shop work |
| Metric | Millimeters, commonly in 0.5 mm steps | Precision machining, anywhere metric fasteners are specified |
| Number (wire gauge) | #1 (largest, 0.2280″) down to #80 (smallest, 0.0135″) | Small-diameter precision holes — pilot holes, small tap drills |
| Letter | A (smallest, 0.234″) up to Z (largest, 0.413″) | Fills the gap between the largest number sizes and the start of fractional sizes |
Two directions worth memorizing, because they trip people up: in the number system, the drill gets smaller as the number goes up — a #1 bit is the largest in that set, #80 the smallest. In the letter system it’s the opposite — letter size increases as you move from A toward Z. If you’ve ever grabbed the wrong bit off a number-and-letter index set, this reversed direction is usually why.
“What Size Do I Need?” Depends on the Job, Not the Fastener
The same bolt or screw can call for three different drill diameters, depending on what the hole is for:
- Pilot hole — for a self-tapping screw going into wood or sheet metal. The bit diameter should match the screw’s solid shank (the core, not the outer thread diameter), so the threads have material to bite into as they cut their own path.
- Clearance hole — for a bolt that passes through material without threading it (the threads on the other side, or a nut, do the work). This hole needs to be larger than the fastener’s major diameter, so the bolt slides through freely.
- Tap drill hole — for cutting new internal threads with a tap. This hole needs to be smaller than the thread’s major diameter, so the tap has material left to cut into. Tap drill sizes follow their own reference tables rather than a simple rule of thumb, because the target is usually a specific thread-engagement percentage, not just “smaller than the bolt.”
Calling all three of these “the drill size for a 1/4-20 bolt” is where a lot of confusion starts — they’re three different numbers for the same fastener, and picking the wrong one either strips a tap, leaves a loose pilot hole, or binds a bolt that should have slid through freely.
If You’re Looking for a Specific Tap Drill Size
If what actually brought you here is “what bit do I need to tap this hole,” the naming-system explanation above is background — what you want is a lookup table for the specific thread you’re cutting:
- Standard machine threads (UNC, UNF, metric): see Tap Drill Size Chart: 1/4-20, M8, 3/8-16, and Other Common Sizes
- NPT pipe threads (tapered, a different system entirely): see NPT Tap Drill Size Chart (1/8″ to 3/4″)
Those two charts don’t overlap — UNC/UNF/metric machine threads and NPT pipe threads use unrelated tapping rules, even when the nominal size number looks the same (a 1/4-20 machine screw and a 1/4″ NPT pipe tap share “1/4” in the name and nothing else).
Related reading: Tap Drill Size Chart: 1/4-20, M8, 3/8-16, and Other Common Sizes · NPT Tap Drill Size Chart (1/8″ to 3/4″) · How to Measure Thread Size
