How to read this chart
American twist drills are labeled four different ways, and all four systems are mixed together in actual shop use. This chart lists every jobber-length size from the smallest number drill up through a full inch, sorted by actual diameter, so you can look a size up regardless of which naming system it was called out in. Search by any designation: a number like 7 or #7, a letter like F, a fraction like 5/16, a metric size like 6.5mm, or a decimal like .201. Then use the series chips to narrow the list to just one naming system.
Four series, one physical scale
Number drills (#97 down to #1), letter drills (A through Z), fractional drills (in 64ths of an inch), and metric drills all measure the same thing: hole diameter. They overlap because drill manufacturers standardized each series independently, at different points in the industry's history, and none of them was ever replaced. They just got layered on top of each other. The result is that a single physical drill often has more than one valid name. The clearest example in this table is E and 1/4": both are exactly 0.2500", so a drawing that calls out "letter E" and a drawing that calls out "1/4 inch" are asking for the identical bit. This chart shows that row once, labeled E / 1/4", rather than as two separate rows, because it is one drill, not two sizes that happen to coincide.
A handful of number-drill sizes also line up with a "nominal" metric designation from the same source table. For instance, #97 is listed alongside 0.15 mm, and #13 alongside 4.70 mm. Those rows show both designations for the same reason: it's one physical size with two names in the standard.
Why number drills run backwards
The most common point of confusion for anyone new to a drill index: number sizes get smaller as the number gets bigger. #97 (0.0059") is the tiniest drill in the whole numbered series, and #1 (0.2280") is the largest. That's the reverse of how "bigger number" reads in almost every other context. This isn't arbitrary; numbered drill sizing descends from an old wire-gauge convention, where a higher gauge number meant thinner wire, and that backwards logic simply carried over when the same numbering was applied to twist drills.
Where letter drills pick up
Once the number series runs out at #1 (0.2280"), the letter series takes over immediately above it. Letter drill A is 0.2340", already bigger than the largest number drill. From there, B, C, D, E… climb in size up to Z at 0.4130". So if you're moving up through sizes in a drill index, the practical order is: numbers #97→#1 (smallest to largest), then letters A→Z (continuing to get larger), interleaved with fractional and metric sizes that fall in between at their own points on the same physical scale.
Why this chart goes to #97, not #80
Most consumer drill indexes stop at #80, because #80 (0.0135") is about the smallest bit a typical hobbyist set or hardware-store index bothers to stock. Anything smaller is thin enough to snap under normal hand-drill pressure and mostly shows up in precision, jewelry, or electronics work. The Machinery's Handbook standard this chart is sourced from doesn't stop there: it documents the full numbered series down to #97 (0.0059"), because that full range is genuinely in industrial use for fine pilot holes, PCB work, and precision tooling. If your particular index or supplier only stocks #80 and up, everything from #81 to #97 on this chart is still a real, standardized size. It's just a rarer one to find on the shelf.
About the "Closest Fraction" column
Every other column in this table, the decimal inch and millimeter values, comes directly from the source standard, unchanged. Closest Fraction is different: it's calculated, not sourced. For each row, this tool rounds the decimal-inch value to the nearest 64th of an inch (the finest fractional graduation typically used in shop measurement) and reduces that fraction to its simplest form. It's there to give you a fast mental anchor ("that's about 5/16"), not a substitute for the actual decimal or metric spec on a print. The smallest drills on this chart (down to #97 at 0.0059") are tinier than 1/64" (0.015625") itself, but "nearest" still means nearest: those rows read 1/64", not a fraction smaller than the smallest one this scale has.
How to pick the nearest available drill, and when not to round
If you need a hole for a bolt, dowel, or reamer and don't have the exact called-out size on hand, the safe default is to go to the next size up, not the nearest size by absolute difference: undersized holes bind or crack material, and oversized holes are usually just a slightly looser fit. Scan the Decimal (in) column for your target value and read up the list until you clear it. For tap drilling specifically, don't round at all: tap drill sizes are chosen deliberately to hit a target percentage of thread engagement, and swapping in "the closest bit you own" instead of the specified tap drill changes how much material the tap actually cuts, which affects thread strength. The tap drill chart lists the correct drill for every Unified thread size by percent thread engagement, drawn from this same verified dataset. When precision actually matters (tapping, reaming to a press fit, matching an existing hole), treat the decimal or metric value as the real spec and the Closest Fraction column as a label, not a substitute.