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A 1/4-inch drill bit measures exactly 6.35 mm in diameter — not 6 mm and not 6.5 mm, although both of those metric bits sit close enough to be useful substitutes depending on the job. Below is the exact math, a size comparison, and a job-by-job guide to when each substitute is safe in wood and metal.
What we cover
The exact conversion (and why it never rounds)
Since 1959, the inch has been defined internationally as exactly 25.4 mm. Divide by four and you get 6.35 mm — a precise figure, not an approximation. For context, the imperial sizes on either side are 15/64 in (5.953 mm) and 17/64 in (6.747 mm).
Here’s the catch for anyone working from a metric set: no standard metric drill equals 6.35 mm. Metric indexes step in 0.1 mm increments (precision sets) or 0.5 mm increments (general sets), so 6.35 falls in a gap between 6.3 and 6.4, or between 6.0 and 6.5. One exception: if you own a letter-gauge index, letter drill E is 0.250 in — exactly 6.35 mm.
- 1/4 in = 6.350 mm (exact)
- 6 mm = 0.2362 in
- 6.5 mm = 0.2559 in
6 mm vs 6.5 mm: the two closest substitutes
A 6 mm bit is 0.35 mm undersize (−5.5%); a 6.5 mm bit is 0.15 mm oversize (+2.4%). On paper, 6.5 mm is the closer match — its error is less than half of 6 mm’s. But closer isn’t automatically better. The right choice depends on whether the hole needs to grip something or let it pass.
| Drill bit | Exact mm | Decimal inches | Difference vs 1/4 in | Typical result |
|---|---|---|---|---|
| 6 mm | 6.000 | 0.2362 | −0.35 mm (−5.5%) | Tight; undersize for 1/4 in hardware |
| 1/4 in (letter E) | 6.350 | 0.2500 | — | Exact |
| 6.5 mm | 6.500 | 0.2559 | +0.15 mm (+2.4%) | Close clearance for 1/4 in bolts |
| Letter F | 6.528 | 0.2570 | +0.18 mm | Machinist’s 1/4 in close clearance |
| 17/64 in | 6.747 | 0.2656 | +0.40 mm | Free/easy clearance for 1/4 in bolts |
Which substitute to grab: a decision matrix
The single most misunderstood point: a 1/4 in bolt measures a full 6.35 mm across its shank, so a 1/4 in hole won’t pass it cleanly. In soft pine you can sometimes force it; in steel it jams. For clearance, you actually want the substitute, not the exact bit.
| Your job | Material | Best bit | Why |
|---|---|---|---|
| Pass a 1/4 in bolt or machine screw | Steel, hardwood | 6.5 mm | +0.15 mm gives working clearance without noticeable slop |
| Loose fit for a 1/4 in bolt | Any | 17/64 in | +0.40 mm compensates for misaligned holes in assemblies |
| Press-fit a 1/4 in dowel pin | Steel, aluminum | Drill 6 mm, ream to 6.35 mm | Bits alone can’t hold press-fit tolerance; a reamer can |
| 1/4 in hardwood dowel joint, snug | Hardwood | 6 mm | Fibers compress; the dowel still seats tight, dry or glued |
| 1/4 in dowel joint with room for glue | Softwood | 6.5 mm | Prevents splitting and glue-starved joints |
| Clearance for an M6 bolt | Any | 6.5 mm | Sits between ISO fine (6.4 mm) and medium (6.6 mm) fits |
| Tap a 1/4-20 thread | Metal | Neither — use #7 (5.1 mm) | A 6+ mm hole leaves no material for the tap to cut |
| 1/4 in pop rivet | Sheet metal | 6.5 mm | Rivet spec calls for roughly 6.4–6.6 mm |
In wood, err toward 6 mm when the hole must hold something (dowels, pegs) — wood fibers compress and spring back, so a slightly undersize hole still grips. Err toward 6.5 mm when the hole must release something (bolt shanks, glue space). In metal there’s no compression to save you: clearance holes need oversize, and press fits need a reamer, period.
Check which “1/4” you actually mean: bit diameter vs hex shank
Half the confusion around this search comes from the shank. 1/4 hex shank drill bits are named for their shank, not their cutting diameter. The hex measures 6.35 mm across the flats — the same dimension as a 1/4 in bit, which is a coincidence that trips people up. A set sold as “1/4 hex drill bits” or “1/4 in hex shank bits” can contain cutting diameters anywhere from about 2 mm to 13 mm; read the size stamped on each bit, not the shank.
The 1/4 in hex is the universal quick-change standard — it drops into any impact driver or quick-change chuck, and 1/4 hex shank drill bits are sold by every major brand (Milwaukee Shockwave, DeWalt Impact Ready, Makita ImpactX, Bosch’s impact-rated lines, among others). Two practical caveats:
- Runout: an impact driver’s collet wobbles more than a drill chuck. If you need a hole that truly measures 6.35 mm, put a round-shank bit in a keyed or keyless chuck on a drill, not a hex bit in an impact driver.
- Converse trap: a 1/4 in diameter bit may have a reduced round shank — check the listing if your chuck capacity is small.
Getting a real 6.35 mm hole: accuracy realities
- Chuck runout adds size. A worn chuck or a hand drill held off-axis typically cuts 0.1–0.3 mm oversize — meaning your 6 mm bit may already be drilling a “6.3 mm” hole. This is why 6 mm often works fine as a casual 1/4 in stand-in.
- Point geometry matters. A 135° split point self-centers on metal and cuts closer to nominal; a brad point stops wander in wood and leaves clean shoulders that dowels seat against properly.
- Match the material. Plain HSS handles wood, aluminum, and mild steel. For stainless or hardened stock, step up to M35 cobalt — a dulled HSS bit rubs, heats, and cuts oversize anyway.
- Wear pattern. The outer margins dull first, and a worn bit cuts increasingly oversize holes while feeling “fine” to push. If dowels suddenly drop through, the bit is done — resharpen or replace.
- Buy singles for odd sizes. A single 6.5 mm or cobalt 6 mm bit costs only a few dollars. A metric index covering 1–13 mm in 0.5 mm steps generally runs in the $20–60 range; 0.1 mm-step machinist indexes cost more but close every gap except 6.35 itself.
FAQ
Is 6.5 mm the same as 1/4 inch?
No — it’s 0.15 mm larger (6.50 vs 6.35 mm). For clearance holes that’s an advantage; for press fits or precision dowel work, it’s too loose.
Can I get an exact 1/4 in hole from a metric set?
Not from standard sizes. Your options are a 6 mm bit used deliberately undersize, a 6.5 mm bit used oversize, a letter E drill (0.250 in), or reaming a 6 mm hole to 6.35 mm.
Is a 1/4 in hex shank also 6.35 mm?
Yes — 6.35 mm across the flats, which is why it fits every 1/4 in quick-change chuck worldwide. The cutting diameter is a separate spec stamped on the bit.
What size hole do I need to tap 1/4-20?
About 5.1 mm — a #7 wire-gauge drill (0.201 in) or 13/64 in. Neither 6 mm nor 6.5 mm leaves enough material to form threads.