mm to Inches Converter

The international inch has been defined as exactly 25.4 millimeters since the 1959 International Yard and Pound Agreement. Before that, the American inch and the British inch differed by a couple of parts per million — enough to matter in precision engineering, which is why the agreement was signed by the standards bodies of six countries at once. The modern factor is exact, so conversions introduce no error beyond your chosen display precision.

Inverting, one millimeter is 0.0393701 inches. A quick mental anchor: 25 mm is 0.984 inches, so a millimeter measurement divided by 25 is within 1.6% of the inch figure. For anything looser than carpentry, that shortcut is enough; for anything tighter, use the exact factor.

Technical drawings worldwide favour millimeters over centimeters or meters for one blunt reason: millimeters remove decimal points. A dimension written 2400 cannot be misread the way 2.4 can when a decimal point is lost in a photocopy or a fax. ISO drawing standards assume millimeters unless a unit is explicitly stated, which is why European drawings often carry no unit symbols at all.

The imperial equivalent of that precision habit is the thou, or mil: one thousandth of an inch, 0.0254 mm. Machinists speak in thou the way metric machinists speak in hundredths of a millimeter. A tolerance of ±0.005 in is ±5 thou, which is ±0.127 mm.

Imperial construction and woodworking measure in binary fractions: halves, quarters, eighths, sixteenths, and thirty-seconds. Metric measurement is decimal. The two do not map cleanly, because most fractions produce endless decimals in millimeters and most round millimeter values produce ugly fractions in inches.

The clean cases are worth knowing. 1/16 in = 1.5875 mm, 1/8 in = 3.175 mm, 1/4 in = 6.35 mm, 1/2 in = 12.7 mm, 3/4 in = 19.05 mm, and 1 in = 25.4 mm. Going the other way: 1 mm ≈ 3/64 in, 5 mm ≈ 13/64 in, 10 mm ≈ 25/64 in, 20 mm ≈ 25/32 in.

Because of this mismatch, plywood and lumber sold in metric markets are near-misses rather than equivalents. A '12 mm' sheet stands in for 1/2 inch (12.7 mm) and is 0.7 mm thinner; an '18 mm' sheet substitutes for 3/4 inch (19.05 mm) and is a full millimeter thinner. Stacked across a cabinet carcass, those shortfalls become a visible gap.

This calculator shows the nearest 1/16 and 1/32 fraction alongside the decimal so you can see how much you are rounding. If the nearest 1/32 is more than a few thou away from the true value, the fraction is not a safe substitute for the metric dimension and the part should be machined to the decimal figure.

Metric fasteners are named by their nominal shank diameter in millimeters: an M6 bolt is 6 mm across the threads. Imperial fasteners are named by fractional diameter or by a gauge number below 1/4 inch, so a #10 screw is 0.190 in, which is 4.83 mm. There is no clean crossover — M5 (5 mm) sits between #10 and 1/4 in, and neither will thread into the other.

Wrench and socket sizes measure across the flats of the hex, not the bolt diameter. The classic near-misses are 13 mm versus 1/2 in (12.7 mm) and 19 mm versus 3/4 in (19.05 mm). Using an imperial socket on a metric bolt gives about 0.3 mm of slop, which is enough to round the corners off a tight fastener.

Drill bits multiply the confusion because three systems coexist: fractional (1/64 increments), number and letter gauges, and metric. A 1/4 in bit is 6.35 mm, while the nearest metric bit is 6.5 mm — 0.15 mm oversize, which is the difference between a press fit and a loose hole. Tapping charts always specify the exact drill for the thread rather than the nearest convenient bit.

Pipe threads are the worst offender: NPT and BSP sizes are nominal labels that match neither their inch nor their millimeter measurement. A '1/2 inch' NPT fitting measures about 21.3 mm outside diameter. Converting the label is meaningless; only the thread standard tells you what fits.

A metric drawing calls for a 12 mm hole positioned 63.5 mm from a datum edge, with a tolerance of ±0.05 mm. The shop runs imperial equipment. Converting: 12 mm = 0.4724 in, 63.5 mm = 2.5000 in exactly, and the tolerance is ±0.002 in, or ±2 thou.

The 63.5 mm dimension converting to exactly 2.5 inches is a strong hint that the drawing was originally imperial and converted to metric, a common history in aerospace and automotive supply chains. When you find dimensions that convert to round imperial numbers, ask for the original drawing rather than reconverting — every round trip adds rounding.

The 12 mm hole has no fractional equivalent. The nearest fractional bits are 15/32 in (11.906 mm) and 1/2 in (12.7 mm), which are 0.094 mm undersize and 0.7 mm oversize respectively — both outside the ±0.05 mm tolerance. The hole must be reamed or bored to size rather than drilled with a stock imperial bit.

The general rule this illustrates: convert once, at the start, and keep the tolerance in the same system as the dimension. Converting a nominal dimension while leaving the tolerance in the other system is how parts get made 'to print' and still fail inspection.

Millimeters to inches — 1 mm = 0.0394 in · 5 mm = 0.1969 in · 10 mm = 0.3937 in · 25 mm = 0.9843 in · 50 mm = 1.9685 in · 100 mm = 3.9370 in · 250 mm = 9.8425 in · 1,000 mm = 39.370 in.

Inches to millimeters — 1/16 in = 1.588 mm · 1/8 in = 3.175 mm · 1/4 in = 6.35 mm · 3/8 in = 9.525 mm · 1/2 in = 12.7 mm · 3/4 in = 19.05 mm · 1 in = 25.4 mm · 12 in = 304.8 mm.

Common hardware — M5 = 0.197 in · M6 = 0.236 in · M8 = 0.315 in · M10 = 0.394 in · #8 screw = 4.17 mm · #10 screw = 4.83 mm · 1/4-20 bolt = 6.35 mm.

Precision units — 1 thou (mil) = 0.0254 mm · 0.01 mm = 0.39 thou · 1 micron = 0.001 mm = 0.039 thou · human hair ≈ 70 microns ≈ 2.8 thou.

Display sizes are quoted as a diagonal in inches worldwide, including in fully metric countries — a 27-inch monitor is 685.8 mm diagonally. Panel manufacturers, however, specify the active area in millimeters, so a 27-inch 16:9 panel is 597.7 × 336.2 mm. Mounting and bezel design work in millimeters even though marketing works in inches.

Paper is the reverse. ISO A-series sizes are metric by construction: A4 is 210 × 297 mm, which is 8.27 × 11.69 inches. US Letter is 8.5 × 11 inches, or 215.9 × 279.4 mm — wider and shorter than A4. A document laid out for one and printed on the other will either clip or leave an uneven margin, which is why print shops ask before scaling.

Photography keeps both: sensor and film formats are metric (35 mm, 24 × 36 mm, APS-C at 22.2 × 14.8 mm) while print sizes in the United States are in inches (4×6, 5×7, 8×10). The aspect ratios do not match, so a 3:2 frame cropped to an 8×10 print loses about 20% of the image.

Tire sizing mixes systems in a single code. A 225/45R17 tire has a 225 mm section width, a sidewall 45% of that width (101 mm), and a 17-inch wheel diameter. Two units in one label is unusual, but it is the global standard and unlikely to change.