Every sixty-fourth of an inch, converted to millimetres — the complete machinist reference, computed from the exact definition of the inch. If you work with US drill bits, sockets, sheet stock or lumber and metric manuals, printers or parts, this chart is the bridge: find the fraction, read the millimetres. The Conversion Calculator handles any value in between; this page prints the fractions themselves.

The 64ths chart

Sixty-fourths are the finest division a common steel rule shows, and the working precision of tap-drill charts. Read the landmarks first: 1/16 = 1.5875 mm, 1/8 = 3.175 mm, 1/4 = 6.35 mm, 3/8 = 9.525 mm, 1/2 = 12.7 mm, 5/8 = 15.875 mm, 3/4 = 19.05 mm, and the full inch = 25.4 mm exactly. Every other row is one 0.396875 mm step from its neighbours — the sixty-fourth part of 25.4 — which is what makes the chart so regular: learn the step and any row can be rebuilt from the landmark above it.

The two lookups that bring people here

1/16″ to mm is the chart’s first landmark: 0.0625 × 25.4 = 1.5875 mm. It is the increment of fine drill sets and thin sheet stock, and the smallest fraction most rulers engrave. And what is 5/8 in mm? 0.625 × 25.4 = 15.875 mm — the 5/8″ in mm answer that socket and tubing work asks for weekly (a 5/8 in wrench and a 16 mm wrench differ by 0.125 mm, close enough to start a bolt and far enough to round one). Both answers are exact, because 25.4 is exact: the inch has been defined as 25.4 millimeters since 1959.

Why this conversion exists at all

Two honest paragraphs, as promised. The fractional inch survives because American workshops were tooled in it for a century: drill indexes, saw blades, plumbing and lumber all carry fractional sizes, and the installed base does not retire. The millimetre rules everywhere else because it belongs to the decimal system — drawings from Europe, Japan and increasingly from US engineering firms dimension in mm, where tolerances stack by moving a decimal point instead of by adding sixty-fourths. The conversion exists because both worlds are real and they meet at the workbench: an imported machine with mm fasteners under a fractional toolbox, a US plan reprinted in a metric manual.

The meeting points have names worth knowing. Drill bits come in three overlapping systems — fractional (this chart), numbered and lettered gauges (finer steps for tap drills), and metric (round mm sizes) — and a machinist converts between them constantly, because the right drill for a tap is rarely sold in the system the tap chart uses. Sheet metal adds its own caution: gauge numbers are not fractions at all (a 16-gauge sheet is about 0.0595 in, or 1.51 mm, in steel — and a different thickness in aluminium), so gauge converts through its own table, never through this one. What this chart owns is the honest core: fractional inches, in sixty-fourths, in millimetres.

Decimal to mm, and mm back to fractions

Callipers and micrometers read decimals, not fractions, and decimal to mm is the same single multiplication with the fraction already divided out: decimal inches × 25.4. A calliper reading of 0.4375 in is 11.1125 mm (it is also 7/16 — the chart’s row 28 — because decimal and fractional are two spellings of one length). Decimal feet convert the same way through 304.8 mm per foot: 2.5 ft = 762 mm exactly. The return journey, mm to the nearest fraction, runs in three steps: divide by 25.4 to get decimal inches, take the fractional part, and multiply by 64 to find the nearest sixty-fourth. A 10 mm dowel: 10 ÷ 25.4 = 0.393701 in; 0.393701 × 64 = 25.197, so the nearest 64th is 25/64 = 9.921875 mm — just under — and 26/64 (13/32) = 10.31875 mm is just over. Which side of a 10 mm hole a 13/32 in pin belongs on is a fit question; the chart’s job is to show both candidates with their true sizes, which it now has.

At the bench: drills, plywood and printers

Three places the chart earns its keep. Tap drills: an M6 × 1.0 metric thread wants a 5.0 mm drill before tapping — 5.0 ÷ 25.4 = 0.19685 in, which sits between 3/16 (0.1875) and 13/64 (0.203125) on this chart, equal to neither. That gap is the entire reason metric drill sets exist: the correct drill for a metric tap is a metric drill, and the chart’s job is to show you the size of the compromise if you reach for a fractional one instead. Plywood: a sheet sold as 3/4 in is nominally 19.05 mm by this chart, but much modern sheet stock is manufactured to a metric 18 mm and merely labelled 3/4 — measure the sheet, not the label, before a dado is cut to either figure. 3D printing: the trade runs in millimetres end to end — a 0.4 mm nozzle, 0.2 mm layers (0.007874 in) — while the brass fittings and lead screws on imported kits may still be imperial; the chart is where a 1/8 in pipe thread (10.287 mm across its threads) meets the 9.728 mm BSP thread it is so often confused with, and the half-millimetre between them stops being a surprise.

One more reading skill: metric drawings dimension in whole millimetres with no unit marks and no decimals needed, so a drawing’s “1250” is 1,250 mm — 1.25 m, or 49.212598 in by this chart’s method (1250 ÷ 25.4). Fractional drawings dimension in inches with the fraction doing the decimal’s job. Converting a whole drawing is therefore a change of language, not just of numbers, and it is done dimension by dimension through the chart — never by scaling the printed sheet, which would convert the paper and not the part.

Nominal sizes: where the chart meets the label

The chart converts actual inches, and US lumber labels are not actual inches — the last trap in this territory, and it catches metric readers hardest because the metric system has no equivalent custom. A “two-by-four” is 1.5 × 3.5 actual inches — 38.1 × 88.9 mm by this chart — the missing half-inch having been planed away in finishing and frozen into the name a century ago. A “1-inch” board is 3/4 in thick, 19.05 mm. Plywood repeats the joke in the other direction, with sheets manufactured at a metric 18 mm and labelled 3/4 in (19.05 mm by the chart), two-tenths and a full millimetre apart from their fractional twins. The rule that survives all of it: convert the measurement, never the name. A calliper does not care what the shelf edge claims, and a digital calliper — reading to 0.001 in, or 0.0254 mm — settles in one squeeze what the label argues about. Fractional sizes are a language; the chart translates the language, but the part itself is the only text that counts.

It is worth seeing how the working machinist holds this chart in memory: not as sixty-four rows but as eight landmarks and a step. The landmarks are the eighths — 3.175, 6.35, 9.525, 12.7, 15.875, 19.05, 22.225 and 25.4 — and between any two of them the sixteenths halve the gap and the thirty-seconds and sixty-fourths quarter and eighth it. A measurement of 0.578 in is recognised, not computed: it sits between 9/16 (0.5625, 14.2875 mm) and 5/8 (0.625, 15.875 mm), nearer the first, and the chart confirms 37/64 = 14.684375 mm when the fit turns out to care. Fluency with fractions is mostly this — a small set of exact landmarks, a known step, and a chart for the rest.

A note on decimal feet, the fraction that is not one

One neighbouring dialect deserves a warning, because it borrows this chart’s clothes. American surveyors and civil engineers often measure in decimal feet — tenths and hundredths of a foot, written 5.78 ft — where the hundredth of a foot (0.01 ft) is 0.12 in, or 3.048 mm, close to but not an eighth of an inch. A dimension of 5.78 ft is therefore 5 ft 9.36 in — 5 ft 9 23/64 in by this chart (0.36 in × 64 = 23.04) — and not 5 ft 7.8 in, the reading that has mis-set more than one form. The conversion is the same × 25.4 once the decimal foot has been honestly turned into inches; the chart covers the fractional end of that journey as it covers every other.

Thirty-seconds or sixty-fourths?

The hub page prints the 32nds table, because thirty-seconds are what joinery, framing and general workshop measuring actually use — a step of 0.79375 mm is finer than a pencil line. Sixty-fourths halve that step to 0.396875 mm and belong to machining, where a tap drill is chosen to leave a specific thread engagement and half a sixty-fourth is a real quantity of metal. Neither table is more correct; they are the same multiplication at two resolutions. If your ruler, your drill index and your drawing disagree about which resolution a job needs, the drawing wins — and this chart is where its fractions come to be measured.