Drill Bit Size Chart: Number, Letter, Fractional & Metric

What Are the Four Drill Bit Sizing Systems?

Inch-system drill bits are sized in three parallel ways, and each system exists for a practical reason:

• Fractional sizes step through 1/64 inch increments from 1/64 up to 1 inch and beyond. They are the oldest and most intuitive system — the fraction is the diameter — but the 1/64 step is too coarse for the small end of the range.
• Number sizes run from #80 (smallest, 0.0135 inch) to #1 (largest, 0.228 inch). They are associated with the wire gauge numbers used for drawing metal wire, and they provide intermediate diameters between the fractional sizes, with steps finer than 1/64 inch.
• Letter sizes run from A (0.234 inch) to Z (0.413 inch), continuing the series above #1 up to 13/32 inch and slightly beyond.
• Metric sizes are simply the diameter in millimeters. Standard sets step 0.5mm apart through the common range, with 0.1mm steps in the smallest sizes.

The systems overlap in coverage rather than replace each other. Between 0.228 inch (#1) and 0.234 inch (letter A) there is no fractional step at all. Between 13/64 inch and 1/4 inch, the standard fractional sizes are 7/32 and 15/64 inch, and number sizes such as #6, #5, #4, and #3 provide additional intermediate diameters. A workshop drawing that says "drill 5.2mm" and a shelf organized in inch sizes are reconciled by the charts below.

How to Read This Drill Bit Size Chart

Every row in this drill bit diameter chart gives the callout size plus its decimal-inch and millimeter equivalents, so the tables work in both directions. The conversion behind them is fixed: 1 inch = 25.4mm exactly, so a #7 drill at 0.201 inch is 5.11mm, and a 6mm drill is 0.2362 inch.

Two reading notes. First, the decimal-inch values for number and letter sizes are the industry's standard reference figures — they are not derived by formula, and a few steps in the number series are closer together than others (the gauge scale descends from wire drawing, not from arithmetic). Second, the millimeter values are rounded to two decimal places for readability. For precision applications, use the unrounded nominal diameter and verify the drill manufacturer's dimensional tolerances and the required finished-hole tolerance.

Fractional Drill Bit Chart

Fractional sizes from 1/64 through 1 inch, with decimal and millimeter equivalents:

Fractional drills are available in larger diameters beyond 1 inch in steps of 1/64 inch or 1/32 inch depending on the manufacturer. The values above are nominal reference sizes; ground and rolled drills of the same nominal size sit within the manufacturer's tolerance band around these figures.

Number Drill Bit Chart

The number drill bit size chart below lists sizes from #1 (largest) down to #80 (smallest):

Note that the steps are not uniform: #30 to #31 jumps 0.0085 inch while #36 to #35 moves only 0.0035 inch. That irregularity is inherited from the wire gauge scale, and it is why matching a drill to a hole spec means reading the chart rather than counting steps.

Letter Drill Bit Chart

Letter sizes from A to Z:

Letter drills take over where the number series ends (#1 at 0.228 inch) and run just past 13/32 inch. In tapping work they appear constantly: the 3/8-24 tap takes a Q drill, the 5/16-24 takes an I, and the 7/16-14 takes a U. The full thread-to-drill pairings are in our tap drill chart.

Metric Drill Bit Sizes

Common metric drill sizes from 0.5 to 13mm in 0.5mm steps:

Metric drill sets in the small sizes are finer than this table: 0.1mm steps are standard below 2mm, and 0.25mm steps appear through the 2–3mm range. The 0.5mm table above covers the diameters most sets carry in quantity; for a metric tap drill size such as the 5.0mm drill for an M6 thread, the dedicated tap drill chart lists the pairing directly.

Drill Bit Lengths: Jobber, Stub, and Long Series

Diameter is only half of a drill bit callout — the other half is length, and choosing it wrong costs as many drills as choosing the diameter wrong. Twist drills come in standardized length series that share the same diameter geometry but differ in flute length and overall reach:

• Jobber length is the default. The flute length runs roughly 9–14 times the diameter, a proportion that balances reach against rigidity for general-purpose holes. Most "assorted drill bit sets" are jobber series.
• Stub (screw-machine) length drills are shorter and notably stiffer. The short flutes resist wandering and deflection, which makes them the choice for drilling on a lathe, in shallow holes, or in hard material where a long drill would flex and snap.
• Long and taper-length drills extend the flute for deep holes and for reaching across clamps and fixtures. The trade-off is rigidity: long drills need lighter feed, careful pecking, and often a pilot hole, or they wander and break.

The failure pattern is familiar: a long drill used where a stub would do flexes in the gap, chips pack in the deep flutes, and the bit snaps at the hole entrance. Length should come from the hole — depth first, then the shortest series that reaches it with clearance for chips and for the drill chuck.

Numbered Drills vs. Wire Gauges: Related but Not the Same

Number drill sizes are associated with historical wire-gauge numbering systems, but they follow their own standardized diameter series. American Wire Gauge (AWG) specifies electrical conductor sizes, whereas number drill designations specify nominal drill diameters. The two systems are not interchangeable: an AWG table sizes the copper conductor inside electrical cable, while a number drill chart sizes cutting tools, and the values in the two tables do not correspond.

The practical caution goes the other way: searching for "12 gauge chart" returns both wire tables and drill charts, and mixing them up produces a hole badly out of spec. When a drawing specifies a numbered drill, size it from the number chart above; when it specifies wire or current capacity, that is an electrical question with its own tables.

Drill Bit Types at a Glance

The charts above size the diameter; the tool itself comes in point and flute geometries suited to different work:

Material choice (HSS versus cobalt versus carbide) changes how each of these performs in hard or abrasive stock, and that selection deserves its own treatment. What the type choice does not change is the diameter: a #7 brad point and a #7 twist drill cut the same 0.201-inch hole. That diameter-first logic carries over to milling, too — JLCMC stocks end mills sized by the same fractional and metric diameters when the work moves from making holes to milling pockets and profiles.

FAQ about Drill Bit Size Chart

About This Drill Bit Size Chart

A drill bit size chart is really four charts wearing one title: fractional for the familiar fractions, number and letter sizes for the fine steps the fractions skip, and metric for drawings already in millimeters. The decimal-inch column is the common language between them, and the 25.4 multiplier is the only conversion worth memorizing.

Diameter is the chart's job; everything else is selection. Length series change reach and rigidity without touching diameter, tool geometry changes how the hole gets cut, and the tap drill pairings connect these sizes to the threads they serve. Once the right hole is drilled, the hardware that goes into it — matched screws and inserts for thin or repairable material — is the same fastening ecosystem our threaded inserts guide covers in depth.