Millimeters to Inches for Drill Bit Sizes (mm to in)

Matching a Metric Hole Size Against an Imperial Drill Index

A metric drawing asks for an 8 mm hole. The index on the bench runs in sixteenths and sixty-fourths of an inch, and none of them is 8 mm. Somewhere in that set there is a bit that will do the job, and one that will ruin it — the difference is a few hundredths of a millimetre, which is precisely why the number has to be converted rather than eyeballed.

The same problem shows up in sheet stock quoted by gauge, in bar sold by fractional inch, and in a 3D printer that refuses to think in anything but millimetres while the model was drawn in inches.

An inch is exactly 25.4 mm, so inches = mm ÷ 25.4. An 8 mm hole is 8 ÷ 25.4 = 0.3150 in, which lands almost exactly on a 5/16" bit at 0.3125 in.

Why It Matters

Drill Indexes

Imperial sets step in sixty-fourths, so every metric size falls somewhere between two bits and you have to know which side.

Sheet and Bar Stock

Material is sold in fractional inches or by gauge number in some markets and in whole millimetres in others.

3D Printing

Nozzle diameters, layer heights and bed sizes are all millimetre figures, whatever units the source model arrived in.

Reading a Metric Dimension on an Inch-Based Machine

Enter the millimetre figure

Type the dimension straight off the drawing or the slicer. The decimal inch value builds as you type, carried far enough to compare against a bit chart.

Find the fraction it sits between

Sixty-fourths are 0.015625 in apart. Divide your decimal by that and round to see which sixty-fourth you are nearest.

Choose which way to err

For a clearance hole, go slightly over. For a hole that will be tapped or reamed, go under and open it out afterwards — you can always remove more material.

Coming the other way is the everyday case in a mixed shop: a drawing gives 3/16" and the CNC post-processor wants millimetres. The swap arrows put inches in the top field and return millimetres below.

inches = mm ÷ 25.4 • A 0.4 mm printer nozzle is 0.0157 in, and a 0.2 mm layer is 0.0079 in — numbers that show exactly why the trade never left millimetres.

Metric Sizes and the Bit You Would Actually Reach For

The gap in the last column is the error you accept by substituting. Negative means the imperial bit cuts undersize, which is recoverable; positive means it cuts oversize, which is not.

Metric size	Decimal inches	Nearest bit	Gap
3 mm	0.1181 in	1/8" (3.175 mm)	+0.175 mm
5 mm	0.1969 in	13/64" (5.159 mm)	+0.159 mm
6 mm	0.2362 in	15/64" (5.953 mm)	−0.047 mm
8 mm	0.3150 in	5/16" (7.938 mm)	−0.062 mm
10 mm	0.3937 in	25/64" (9.922 mm)	−0.078 mm
12 mm	0.4724 in	15/32" (11.906 mm)	−0.094 mm

Metric Figures Read Back as Decimal Inches

The decimal appears on each keystroke and runs to eight places, which is enough to place a size between two sixty-fourths without ambiguity.

Flip It to Post an Imperial Drawing

The swap arrows reverse the pair, which is what a mixed shop needs when the print is in fractions and the machine control is metric.

Copy a Decimal Into a CAM Setup

Each field yields the digits alone, ready to paste into a tool table or a slicer field that would reject anything with a unit attached.

Reach Centimetres for Stock and Bed Sizes

Sheet and print-bed dimensions are easier to read a scale up. Search the unit list for Centimeter and the same value appears there.

The one substitution to avoid is a 1/8" bit for a 3 mm hole. It cuts 0.175 mm oversize, which is proportionally the largest error in the table and enough to leave a 3 mm dowel loose.

Workshop and 3D Printing Questions

What is 10 mm in inches?

0.3937 in. On a fractional index that sits between 25/64" (0.3906 in) and 13/32" (0.4063 in), with the 25/64" the closer of the two at 0.078 mm under.

Which imperial bit is nearest to 8 mm?

5/16", at 7.938 mm. It is only 0.062 mm undersize, which for most timber, plastic and mild-steel work is indistinguishable from the right bit — and it errs in the safe direction, since a hole can always be opened up.

How close does a clearance hole need to be?

Deliberately loose, not exact. The standard close fit for an M6 bolt is a 6.4 mm hole, the normal fit is 6.6 mm and the loose fit is 7 mm. Drilling exactly 6 mm gives you an interference fit and a bolt that will not go through.

Why does my 3D printer insist on millimetres?

Because the G-code standard the firmware runs defaults to millimetres, and every calibration figure — steps per millimetre, retraction distance, layer height — is expressed that way. A model exported in inches arrives 25.4 times too small, which is the classic symptom of the wrong export unit.

How thick is 16 gauge sheet in millimetres?

For uncoated steel, 0.0598 in — about 1.52 mm. Gauge is not a unit but a lookup table, and it differs by material: 16 gauge aluminium is 0.0508 in, or 1.29 mm. Always convert from the thickness figure rather than the gauge number.