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What drill bit sizes mean
A drill bit’s size describes the diameter of its cutting end, not its length or the size of the drill that holds it. The same diameter may be marked as a fraction of an inch, a numbered size, or a metric measurement. Knowing how those systems relate helps you choose a bit for a pilot hole, clearance hole, or wall anchor—and avoid a hole that is too loose or too tight.
Check the marking on the shank, packaging, or size chart. A bit marked 1/4 inch is 6.35 millimeters across; a 6 mm bit is slightly smaller. That difference can matter when drilling for a snug-fitting anchor or fastener.
Fractional inch sizes
Fractional bits are marked in fractions such as 1/8, 3/16, and 1/2 inch. They are common in household and construction work, and the fraction is the diameter in inches. Larger fractions mean wider holes: 3/8 inch is wider than 5/16 inch.
To compare a fractional size with a metric one, multiply inches by 25.4. For example, 3/8 inch is 9.525 mm, usually rounded to 9.5 mm. Fractions are easy to recognize, but a basic set may skip useful intermediate sizes. That can be inconvenient when a screw or anchor needs a close fit.
Numbered drill bit sizes
Numbered sizes run from #80, one of the smallest common sizes, through #1, then continue upward to #60 in many sets. The scale runs in reverse at the small end: a #60 bit is larger than a #80, while #1 is larger than #60. Above #1, numbered bits get larger as the number rises. A #30 bit, for example, is larger than a #40.
Numbered bits are useful for metalworking and jobs that call for a specific tap-drill or pilot-hole size. The number alone does not tell you the diameter intuitively, so use a chart or a drill gauge rather than guessing. Numbered sets include sizes that fractional sets often omit, but a complete set may cost more and be harder to sort.
Metric sizes
Metric bits are marked in millimeters, often in steps of 0.5 mm or 0.1 mm. They are standard for many wall plugs, machine screws, and products sold with metric instructions. A 6 mm bit makes a nominally 6 mm hole; actual results can vary with the material, bit condition, and drilling technique.
Do not assume that the nearest metric and fractional sizes are interchangeable. A 1/4-inch bit is 6.35 mm, not 6 mm. If an anchor package calls for a 6 mm hole, use a 6 mm bit when possible. A 1/4-inch hole may be loose enough to reduce the anchor’s grip, especially in soft masonry or drywall.
Common size comparisons
This table shows approximate equivalents. They are useful for comparing sets, but rounding does not make two sizes identical. For a specified anchor or fastener, follow its instructions rather than substituting a nearby size.
| Fractional size | Approximate millimeters | Closest common numbered size |
|---|---|---|
| 1/8 inch | 3.18 mm | #30 (3.26 mm) |
| 3/16 inch | 4.76 mm | #14 (4.62 mm) |
| 1/4 inch | 6.35 mm | #3 (6.35 mm) |
| 5/16 inch | 7.94 mm | #P (7.94 mm) |
| 3/8 inch | 9.53 mm | #O (9.25 mm) |
Number-letter labels are commonly used alongside numbered sizes for larger diameters, but charts and available sets vary. Confirm the actual diameter from a reliable chart or gauge before using a specialty size.
Choosing a bit set for the job
For general household repairs, a fractional set with common sizes from about 1/16 to 1/2 inch is often enough. If you regularly install metric hardware or follow metric anchor instructions, choose a metric set instead of relying on close fractional substitutes. If you drill metal or need precise pilot holes, a numbered set fills gaps between the more widely spaced sizes.
A combined fractional and metric drill bit set is convenient, but check its size range and whether the case clearly labels each bit. Cheap sets can be fine for occasional wood or plastic work. For frequent drilling in metal or masonry, bits that dull quickly or chip can waste more time than their low price saves.
Match the bit type to the material as well as the diameter. Standard twist bits work for wood and many metals; masonry bits are made for brick, block, or concrete. A hammer drill uses a hammering action for masonry, but that action is not appropriate for ordinary twist bits or most work in tile. For tough concrete, use a suitable masonry bit and let the tool do the work rather than forcing it.
Pilot holes, screws, and wall anchors
A pilot hole should guide a screw without removing so much material that the threads lose grip. For a wood screw, the pilot diameter is generally close to the screw’s solid core diameter, especially in hardwood; a smaller pilot may split the wood, while a larger one can leave weak thread engagement. Check the screw maker’s chart when the fit matters. Test in scrap wood if the material or screw is unfamiliar.
For wall anchors, use the diameter stated on the package. A hole that is too small can crush or deform a plastic plug as you drive it in. A hole that is too large can let the plug spin or pull out under load. In masonry, drill to the specified depth and clear dust from the hole; dust left inside can stop the anchor from seating fully.
If you are choosing anchors for a project, a wall anchor assortment with a size guide can help cover common light-duty jobs. Still, check the load rating and wall material. A light-duty plastic plug is not a substitute for a rated fastener in a heavy shelf installation.
How to verify a bit’s size
Use a drill bit gauge or caliper if the marking has worn off. A caliper measures the cutting diameter; avoid measuring a tapered tip or a damaged cutting edge. If you do not have a gauge, place the bit against a ruler for a rough check, but do not rely on that method for close-tolerance work.
Before drilling, clamp the work where practical, mark the location, and start at low speed to keep the bit from wandering. With a cordless drill, use the clutch setting for driving screws, not for drilling a clean hole. With an impact driver, use only bits and accessories designed for its quick-change chuck; it is generally a poor choice for accurately drilling holes with ordinary round-shank bits. A correctly sized, sharp bit is safer and more likely to give a clean result than pushing a dull one harder.