Drill bit sizes: converting mm bits to inch fractions

Drill bit sets come in two incompatible systems: metric bits sized in millimeters, and imperial bits sized in fractions of an inch. They look interchangeable at a glance — a “6 mm” bit and a “1/4-inch” bit both look roughly quarter-inch — but they aren't the same size, and the difference is large enough to matter for anything that needs a precise fit.

Why 6 mm and 1/4 inch aren't equal

Converting exactly: 6 mm ÷ 25.4 = 0.23622 inches. A quarter inch is 0.25 inches. The gap — about 0.0138 inches, or 0.35 mm — is small on a ruler but significant for a drilled hole. A hole drilled 0.35 mm undersized can make a bolt or dowel bind or refuse to seat; a hole drilled that much oversized can leave a fastener loose or a dowel joint weak. Neither metric nor imperial bit sizes were designed to align with the other system's steps, so near-matches like this are common, not an exception.

Common near-matches, compared

Some pairs, like 4 mm and 5/32 in, are close enough (about 0.03 mm apart) that the difference is negligible for most woodworking or hobby use. Others, like 6 mm and 1/4 in, have a gap large enough to notice in a tight-tolerance joint — it's worth checking the exact figure rather than assuming “close in name” means “close enough.”

Number and letter drill sizes complicate it further

Beyond fractional inches, US drill bits are also sold in numbered (1 through 80, smallest to largest) and lettered (A through Z) sizes, mostly used for precise tap-drill and clearance-hole charts. A #7 bit, for example, is 0.201 inches, and doesn't correspond to any clean fraction — it converts to 5.106 mm. When a project references a numbered or lettered US bit size, converting to millimeters first (or consulting a dedicated drill-and-tap chart) is more reliable than trying to eyeball an equivalent from a fractional-inch set.

When to use the exact match, not the nearest one

For rough pilot holes in softwood, the nearest available bit — metric or imperial — is usually fine. For anything threaded, dowelled, or load-bearing, like fastener clearance holes referenced in a torque spec sheet or a furniture joint that needs to hold weight, use the exact matching bit size the manufacturer specified rather than substituting the nearest size from the other system. When a project mixes hardware from both systems — a common situation with imported fasteners or older US equipment retrofitted with metric parts — converting every dimension with a millimeters-to-inches converter first avoids compounding small mismatches across multiple holes.