mm to Inches Converter

The formula, both directions

NIST Special Publication 811 lists the inch as 2.54 × 10⁻² m and the mil as 2.54 × 10⁻⁵ m, both flagged as exact. The most frequent slip is dividing millimeters by 2.54: that is the centimeter factor and gives an answer ten times too large (50 mm ÷ 2.54 = 19.7, not 1.97). The second is rounding 25.4 down to 25, which inflates every result by 1.6 percent: 100 mm ÷ 25 = 4.000 in instead of 3.937 in, an error of 1.6 mm.

Converting mm to a fraction of an inch

Tape measures and drill sets use binary fractions, so a decimal like 0.3937 in has to be rounded to a 16th, 32nd or 64th. The method:

• Divide millimeters by 25.4 to get decimal inches.
• Multiply by the denominator you want (16, 32 or 64).
• Round to the nearest whole number; that is the numerator.
• Reduce the fraction (8/16 becomes 1/2).

For 10 mm: 0.3937 × 16 = 6.30, so 6/16 = 3/8 in; 0.3937 × 64 = 25.2, so 25/64 in. The table shows the leftover error in millimeters, which tells you whether the fraction is good enough for the job.

A plus sign means the fraction is larger than the metric size. A 1/16-inch graduation is 1.5875 mm wide, so rounding to sixteenths can leave you up to about 0.8 mm off; 64ths cut that to at most 0.2 mm.

Fractional inches to millimeters

Going the other way is a single multiplication and needs no rounding beyond the display, because every binary fraction of an inch has a finite millimeter value.

Close pairs: when a metric size can stand in for an inch size

Hardware stores in the US sell both systems side by side, and some pairs are nearly identical while others only look close. The gap in the table above is what decides it:

• 8 mm and 5/16 in differ by 0.06 mm. For a clearance hole, either drill works.
• 19 mm and 3/4 in differ by 0.05 mm, below what most hand measurements can detect.
• 6 mm and 1/4 in differ by 0.35 mm. A 6 mm hole is too tight for a 1/4-inch dowel or bolt; a 1/4-inch hole is loose on a 6 mm pin.
• 12 mm and 1/2 in differ by 0.70 mm, too much for anything that should fit snugly.

Whether a gap matters depends on the fit. A clearance hole tolerates a few tenths of a millimeter, a press fit or a bearing seat tolerates far less, and a wrench on a hex head that is even slightly oversized can round the corners off. When in doubt, measure the actual part with calipers instead of trusting a nominal size: lumber, pipe and tubing sizes in the US are trade names, not measurements of the piece.

How many decimals to keep

NIST's guidance on unit conversion is to keep full precision through the calculation and round only the final result. A practical rule is to match the precision of the original measurement: a dimension given as 12 mm (implied ±0.5 mm) does not justify 0.4724 in; 0.47 in already carries that information. Machinists usually work in thousandths of an inch (0.001 in = 0.0254 mm), so a millimeter value with two decimals converts sensibly to three decimals in inches. The calculator shows four decimals so you can decide; it does not round to fractions for you, which is why the table above is here.

Where mm-to-inch conversions come up

• Drills and fasteners: metric bolts (M6, M8, M10) are named by nominal major diameter in millimeters, while US drill indexes run in 64ths.
• Screens and devices: phone thickness and bezel sizes are quoted in millimeters, screen diagonals in inches.
• Jewelry and beads: bead and stone sizes are sold in millimeters; a 4 mm bead is 0.157 in, just over 5/32 in.
• Rainfall and snow: 25.4 mm of rain is exactly one inch.
• 3D printing and CAD: models are usually built in millimeters, so a part meant to fit an inch-sized component needs the exact 25.4 factor, not 25.