Drill bit sizes are stamped or engraved on the shank (the smooth, upper portion that your chuck grips), and they use one of four systems: fractional inches, metric millimeters, numbered gauges, or letter gauges. Knowing which system you’re looking at is the first step to picking the right bit for your project.
Where to Find the Size on a Drill Bit
Look at the shank, the smooth cylindrical section above the flutes (the spiral grooves). The size is stamped there, often along with a material code like “HSS” (high-speed steel). On smaller bits, these markings can be tiny and hard to read. A magnifying glass helps.
Over time, the stamped numbers wear off from use and repeated chucking. If you can no longer read the marking, a pair of digital calipers is the most reliable way to measure the bit’s diameter. Place the jaws across the widest point of the cutting end, not the shank, since some bits have a reduced shank that’s narrower than the cutting diameter. Calipers are inexpensive and worth owning if you work with drill bits regularly. A drill index, which is a case with labeled holes for each size, also lets you identify a mystery bit by finding the hole it fits into.
The Four Sizing Systems
Fractional Inch Sizes
This is the system most people in the U.S. encounter first. Sizes are expressed as fractions of an inch: 1/8″, 3/16″, 1/4″, and so on. The smallest standard increment is 1/64 of an inch (about 0.016″), and sets typically range from 1/16″ up to 1/2″ or 1″ for home use. A bit stamped “3/8” has a cutting diameter of 0.375 inches, or about 9.5 mm.
The fractions follow a logical pattern. Each step up adds 1/64 of an inch. So between 1/4″ (which equals 16/64″) and 5/16″ (which equals 20/64″), you’ll find 17/64″, 9/32″ (18/64″), and 19/64″. If fractions aren’t intuitive for you, converting to decimals helps: just divide the top number by the bottom number. For example, 7/32 = 7 รท 32 = 0.2188 inches.
Metric Sizes
Metric bits are stamped in millimeters, like “4.0” or “6.5.” These are straightforward since the number is the diameter. Common sets range from about 1 mm to 13 mm in half-millimeter or one-millimeter steps. If you need to convert, 1 inch equals 25.4 mm. A 6 mm bit is roughly equivalent to a 15/64″ bit (0.236″), and a 10 mm bit is close to 25/64″ (0.394″).
Number Gauge Sizes (Wire Gauge)
Number gauge bits run from #80 (the smallest, at 0.0135″) to #1 (the largest, at 0.228″). The counterintuitive part: higher numbers mean smaller holes. A #80 bit is thinner than a sewing needle, while a #1 bit is just under 1/4 inch. These sizes fill the gaps between fractional sizes, which is why machinists and hobbyists use them for precision work like tapping threads or drilling pilot holes in electronics.
Some common reference points to help you calibrate:
- #60 = 0.040″ (about 1 mm)
- #50 = 0.070″ (about 1.8 mm)
- #40 = 0.098″ (close to 3/32″)
- #30 = 0.1285″ (close to 1/8″)
- #20 = 0.161″ (close to 5/32″)
- #10 = 0.1935″ (close to 13/64″)
- #1 = 0.228″ (close to 15/64″)
Letter Gauge Sizes
Letter gauge bits pick up where number gauge bits leave off. They run from A (0.234″) to Z (0.413″), with each letter representing a slightly larger diameter. Unlike the number system, the progression here is alphabetical and intuitive: A is the smallest, Z is the largest. Letter E, for example, is exactly 0.250″, which is identical to a 1/4″ fractional bit. Letter sizes exist to provide finer increments between the common fractional sizes in that range, which jump from 1/4″ to 5/16″ (a gap of 1/16″).
How the Systems Overlap
All four systems describe the same physical thing: a diameter. They overlap extensively. A #7 bit (0.201″) sits between 13/64″ (0.2031″) and a letter C bit (0.242″). A complete drill index interleaves all three imperial systems, fractional, number, and letter, into a single sequence sorted by ascending diameter. This gives you nearly every increment from 0.0135″ up to 1″ without large gaps.
When a project calls for a specific size you don’t own, knowing the decimal equivalent lets you find the closest substitute. If you need a #29 bit (0.136″) for a tap drill and don’t have one, a 9/64″ fractional bit (0.1406″) is close but slightly oversize. Whether that’s acceptable depends on your tolerance requirements.
Reading Bits With a Reduced Shank
Most standard bits have a shank the same diameter as the cutting end. But some larger bits, called Silver and Deming bits, have a cutting diameter of 1/2″ or more while the shank is ground down to 3/8″ or 1/2″ so it fits in a standard drill chuck. The size stamped on these bits refers to the cutting diameter, not the shank. If you see “3/4″ stamped on a bit with a 1/2” shank, the hole it drills will be 3/4 inch wide. Always read the stamped size as the hole size, and if you’re measuring with calipers, measure across the widest point of the cutting lips.
What the Color Tells You
Drill bit color has nothing to do with size, but it’s a common source of confusion since colored bits are sometimes organized differently. Color indicates the coating or material. Black bits have a nitride coating that reduces friction. Gold or yellow-brown bits are either cobalt alloy or titanium-coated. Bright silver bits are plain high-speed steel with no coating. None of these colors change how you read the size. The stamped number, fraction, or letter on the shank is still your size indicator regardless of color.
Practical Tips for Identifying Unlabeled Bits
If you’ve inherited a coffee can of loose bits with no markings, here’s the fastest way to sort them out. First, get a set of digital calipers. Close the jaws gently on the cutting end and read the measurement. If your calipers show inches, compare the decimal reading to a chart. If they show millimeters, divide by 25.4 to get inches, or simply match it to standard metric sizes.
A drill index case is even faster for bits in common sizes. These cases have labeled holes for every size in a set. Drop the bit into holes until you find the one it fits snugly. Indexes come in fractional-only, number-only, or combination versions that include all three imperial systems. A 115-piece index covers fractional sizes from 1/16″ to 1/2″, number gauges from #60 to #1, and letter gauges from A to Z, giving you a built-in identification tool for nearly any bit you encounter.