Four Systems, One Drill Press
Walk into almost any American machine shop and you'll find drill indexes from four completely different sizing systems sitting side by side. Fractional inch sets. Wire gauge sets numbered from #80 to #1 and then A through Z. Metric sets in millimeters. And if you work in tooling or do precision tapping, you know all four by heart — because the hole you need might fall between the cracks of any single system.
This guide covers all four systems, where they overlap, and how to pick the right drill for the job without guessing.
Fractional Inch Drills
Fractional drills are the everyday workhorse of American shops. They run from 1/64" up through 1/2" in 1/64" increments in a standard 29-piece set, and larger sets extend well beyond that. The naming is exactly what it says: 1/4", 3/8", 7/16" — no translation required.
Fractional drills dominate general fabrication, structural work, and any application where the target diameter maps cleanly to a fraction. They're widely stocked, easy to source, and most machinists can read a fractional size instantly without consulting a chart.
The limitation: fractional sizes jump in 1/64" steps, which is .015625" per step. For precision tapping and close-tolerance work, you often need a size that falls between two fractions — which is where the other systems come in.
Wire Gauge (Number) Drills: #80 to #1
Number drills run from #80 (the smallest, at .0135") up to #1 (.2280"). The numbering is counterintuitive — higher numbers are smaller drills. A #60 drill is .0400"; a #30 drill is .1285".
Number drills fill the gap between fractional sizes, especially in the small-diameter range below 1/4". They're essential for tap drilling. A 10-32 tap, for example, calls for a #21 drill (.1590") — a size that doesn't exist in the fractional system. Number drills exist precisely because thread standards and hole tolerances don't always align with 1/64" steps.
If you do any tapping, you need number drills. Full stop.
Letter Drills: A Through Z
Letter drills pick up where number drills leave off. They range from A (.2340") through Z (.4130"), filling in fractional gaps in the 1/4" to 7/16" range.
Like number drills, letter drills are used heavily for tapping. A 1/2"-13 tap calls for a 27/64" drill — which is a fractional size — but a 7/16"-14 tap calls for a U drill (.3680"), a letter size. Without letter drills, you either guess with an adjacent fractional size or you drill undersize and ream, which adds steps.
Letter drills are also useful when reaming to a precise diameter. Drilling close but not exact, then reaming to finish, is standard practice for tight-tolerance holes — and letter drills give you more options for that rough-drill step.
Metric Drills
Metric drills run from as small as 0.5mm up through 20mm and beyond in standard sets, typically in 0.1mm or 0.5mm increments depending on the range. For shops doing metric work — European designs, automotive, medical, or aerospace with metric fasteners — a metric drill index is not optional.
One millimeter equals .03937 inches. A 6mm drill is .2362", which falls between 15/64" (.2344") and 1/4" (.2500"). Neither fractional size is correct for a 6mm hole. Substituting a close inch drill might work for a clearance hole but will cause problems for a tapped hole or a precision fit.
As more American shops work to international prints and mixed metric/inch designs, metric drill sets have become standard equipment rather than specialty items.
Where the Systems Overlap
Some sizes across systems are close enough to be interchangeable for clearance holes — but not for tapping or precision fits. Here are a few common cross-references:
- 1/4" (.2500") vs. 6.35mm (.2500") — identical
- #7 drill (.2010") vs. 5.1mm (.2008") — essentially the same
- A drill (.2340") vs. 5.94mm (.2339") — nearly identical
- #29 drill (.1360") vs. 3.45mm (.1358") — very close
For clearance holes, these differences are irrelevant. For tapping, even a .002" variance can affect thread engagement enough to matter in structural or high-vibration applications.
Choosing the Right System
Here's a practical decision tree most machinists use without thinking about it:
- Clearance hole, inch fastener: fractional drill, usually one or two sizes over the bolt diameter
- Tapped hole, inch thread: tap drill chart → usually a number or letter drill
- Tapped hole, metric thread: metric drill at (diameter − pitch)
- Precision bore or reamed hole: letter or number drill to rough size, then ream to finish
- Very small hole below 1/4": number drill
Keeping Your Sets Complete
A gap in your drill index costs more than the missing drill. When a machinist can't find the right size, the next move is usually to grab the closest thing available — which means an oversized hole, a stripped tap, or scrap. Incomplete sets are a hidden source of quality problems and frustration.
Resharpening worn drills is one of the most cost-effective ways to maintain a complete set. A drill that's been reground still cuts to the correct diameter. A drill discarded and replaced costs several times more than a resharpen. For shops running number and letter drills heavily — the sizes that get used hardest for tapping — resharpening economics are especially strong.
MachinistPost resharpens HSS drills starting at $3.00 per drill via mail-in service ($20 minimum order). Any carrier. If keeping your drill index complete and accurate matters to your shop, it's worth considering.
The Takeaway
No single drill sizing system covers every situation in a modern machine shop. Fractional drills handle general work. Number drills cover tapping and small precision sizes. Letter drills bridge the mid-range. Metric drills are required for metric work. Understanding all four — and knowing which chart to reach for — is a basic competency that separates a well-run shop from one that's constantly improvising.
Keep a tap drill chart on the wall. Keep your sets complete. And when a drill starts pushing instead of cutting, don't throw it away — regrind it.