Drill Bit Conversion Chart: mm to Inches, Fractions, and Which Bit to Actually Use

Nobody searches for a conversion out of curiosity. The question behind "6.4 mm to inches" is always the same one: *the drawing says one thing, my rack holds another, which bit do I put in the chuck?* So this chart has a fourth column that most conversion tables leave out — the bit you actually order — and a rule for when the difference between 6.35 and 6.4 mm is worth caring about.

Bookmarkable version: the drill size converter on its own page.

The six conversions that come up every week

In a hardware trade that runs on both systems, six numbers do most of the work. These are worth knowing cold:

Metric to decimal inch — every stock bit from 1 to 13 mm

Stock HSS drills come in 0.1 mm steps from 1.0 mm up. This is the full run, with the decimal inch and the nearest 64th:

Fractional inch to metric — 1/64" to 1"

The substitution rule: when 0.1 mm matters

Most of the time it does not. A hand-held drill in an unclamped workpiece produces a hole 0.1–0.3 mm oversize and slightly oval before anyone has done anything wrong; chasing a 0.05 mm conversion error inside that is theatre. The cases where it genuinely matters are specific and short:

• Tap drills — round up, never down. 0.1 mm under on an M8 or 1/4-20 tap drill is a measurable jump in torque and the most common reason taps snap in the hole.
• Reamed, bored or dowel holes — do not substitute. The tolerance is the point of the hole.
• Press fits and bearing seats — the substitution direction reverses: go under and open the hole to size, because you cannot put metal back.
• Sheet metal and thin tube — the hole ends up bigger than the bit anyway as the drill breaks through. Use a step drill and read the step, not the conversion table.

Non-standard metric sizes, and why your set skips them

A 19-piece metric set jumps 1.0, 1.5, 2.0 … 10.0 in half-millimetre steps. But tap and plug charts constantly call for 4.2, 5.1, 6.8, 8.5 and 10.2 mm — none of which are in that set. Those are not exotic sizes; they are the tap drills for M5, 1/4-20, M8, M10 and M12, five of the most-cut threads in the world. The set skips them because half-millimetre steps make a tidier index case, not because the sizes are unusual.

The practical consequence is that anyone who taps threads needs loose singles alongside their set. It is also the most reliable repeat sale on a hardware counter: 4.2, 5.1, 6.8, 8.5 and 10.2 wear out at three times the rate of their neighbours, because every threaded hole in the shop goes through one of them.

From our range

Industrial 25-Piece HSS Twist Drill Bit Set (1.0-13.0mm, Index Case) – OEM

1.0–13.0 mm in 0.5 steps, 25 pieces in an index case — the working set, with the tap sizes bought loose alongside.

From our range

Industrial Titanium Coated HSS Twist Drill Bits (DIN338, Metric Sizes) – OEM

Titanium-coated DIN 338 singles in every 0.1 mm size, for the tap drills the sets leave out.

Reading a bit whose markings have worn off

Shank markings disappear after a few hundred holes in a chuck. To identify the bit, measure across the margins just behind the point, not on the shank and not down the flute — the shank on a reduced-shank drill is a different diameter by design, and the flute body is relieved behind the cutting edges so it always reads undersize. A digital caliper on the two margins gives the true diameter; compare it against the tables above, and remember the bit has probably lost 0.02–0.05 mm to wear, so round up to the nearest catalogue size.

Buying for a market that runs both systems

India, Pakistan and the Gulf all run mixed. Metric fasteners, imperial pipe threads, machinery from three continents. A stockist who carries only one system sends customers to a competitor for half their holes. The cheapest fix is not a second full index — it is a metric set plus the eight imperial sizes that appear on drawings (1/8", 3/16", 1/4", 5/16", 3/8", 7/16", 1/2" and the letter F). We ship exactly that split as a mixed carton; tell us the market and we will build the size mix for it and quote it in 24 hours.