Convert a fractional or mixed-number inch measurement to millimeters using the exact 25.4 mm per inch relationship. See the fraction and the metric length on the same ruler.
Open the fraction to mm calculator · More Physics and Math videos
The calculation
Millimeters = (whole inches + numerator ÷ denominator) × 25.4
Worked example
3/8 inch = 0.375 inch; 0.375 × 25.4 = 9.525 mm. For 1 3/8 inches, 1 + 3/8 = 1.375 inches, which gives 34.925 mm. Rounded to hundredths these are 9.53 and 34.93 mm.
The input fraction must represent inches. An exact unit conversion does not imply that a real measurement has unlimited precision; preserve meaningful tolerances.
Video chapters
- 0:00 — A fraction needs a unit
- 0:16 — Divide the inch into equal pieces
- 0:31 — Convert the fraction to decimal inches
- 0:47 — Multiply by 25.4 millimeters per inch
- 1:07 — Include the whole part of a mixed number
- 1:23 — Reverse the units to check
- 1:42 — Round to the measurement’s needs
- 2:00 — Divide the fraction, add the whole, multiply
Read the full transcript
What is three eighths in millimeters? First confirm that the fraction means three eighths of an inch. A fraction by itself is only a number. Once the inch unit is attached, we can use the exact relationship: one inch equals twenty-five point four millimeters.
The denominator eight divides one inch into eight equal pieces. The numerator three selects three of those pieces. Each eighth has a length of twenty-five point four divided by eight, or three point one seven five millimeters. Three selected pieces will make our required length.
You can also divide the numerator by the denominator first. Three divided by eight is zero point three seven five inches. This decimal describes the same three highlighted pieces. It is not three point eight inches; the fraction bar means division, not a decimal point.
Multiply zero point three seven five inches by twenty-five point four millimeters per inch. Inches cancel, leaving nine point five two five millimeters. The other route agrees: three times three point one seven five also equals nine point five two five. Two calculations, one unchanged physical length.
For one and three eighths inches, include the whole inch before converting. One plus three eighths is one point three seven five inches. Multiply by twenty-five point four to get thirty-four point nine two five millimeters. Ignoring the whole inch would lose exactly twenty-five point four millimeters.
Divide thirty-four point nine two five millimeters by twenty-five point four. You get one point three seven five inches. Subtract the whole one, and the fractional part is zero point three seven five. Multiplying that part by eight gives three, confirming one and three eighths inches.
The conversion factor is exact, but a physical measurement can still have uncertainty. If hundredths of a millimeter are appropriate, nine point five two five rounds to nine point five three, and thirty-four point nine two five rounds to thirty-four point nine three. Do not confuse more displayed digits with a more accurate measurement.
Identify the inch measurement, divide numerator by denominator, include any whole inches, and multiply by twenty-five point four. Keep full precision until the final rounding. Three eighths of an inch is nine point five two five millimeters; one and three eighths is thirty-four point nine two five millimeters.