How this inch fraction converter works
Inches can be written two ways: as a decimal (1.375 in) or as a fraction (1 3/8 in). Tape measures and rulers are marked in fractions, while calculators, drawings and metric tools speak in decimals and millimeters. This tool bridges all three so you can read any value the way your tools expect.
To convert a decimal inch to a fraction, the calculator multiplies the decimal by your chosen denominator — 16, 32 or 64 — and rounds to the nearest whole number. That whole number over the denominator is the raw fraction, which is then reduced using the greatest common divisor so 6/16 becomes 3/8. Whole inches are split off first, so 1.375 in reads as 1 3/8 in rather than 22/16. Millimeters are handled with the exact relationship 1 inch = 25.4 mm: a millimeter entry is divided by 25.4 to get decimal inches before snapping to a fraction.
Common fraction ↔ decimal ↔ mm equivalents
| Fraction (in) | Decimal (in) | Millimeters (mm) |
|---|---|---|
| 1/16 | 0.0625 | 1.5875 |
| 1/8 | 0.125 | 3.175 |
| 3/16 | 0.1875 | 4.7625 |
| 1/4 | 0.25 | 6.35 |
| 3/8 | 0.375 | 9.525 |
| 1/2 | 0.5 | 12.7 |
| 5/8 | 0.625 | 15.875 |
| 3/4 | 0.75 | 19.05 |
| 1 | 1.0 | 25.4 |
Reference note: 1 inch = 25.4 mm exactly. Most decimals and mm values do not fall exactly on a marked fraction, so results snap to the nearest one at your chosen precision and the rounding error is shown.
Frequently asked questions
- How do I convert a decimal to a fraction of an inch?
- Pick a precision such as 1/16 or 1/64, multiply the decimal by that denominator, and round to the nearest whole number. That whole number over the denominator is your fraction, which you then reduce. For example, 0.375 × 16 = 6, so 0.375 in is 6/16, which reduces to 3/8 in.
- How many mm in an inch?
- One inch equals exactly 25.4 millimeters. So to convert inches to mm you multiply by 25.4, and to convert mm to inches you divide by 25.4. For example, 2 inches is 50.8 mm.
- What is 0.5 inches as a fraction?
- 0.5 inches is exactly 1/2 inch, and it equals 12.7 mm. At any precision the math gives 8/16, 16/32 or 32/64, all of which reduce to 1/2.
- How do I convert mm to inches?
- Divide the millimeter value by 25.4 to get decimal inches, then round that decimal to your chosen fraction precision. For example, 10 mm ÷ 25.4 ≈ 0.3937 in, which rounds to about 25/64 in.
- What precision should I use?
- Use the precision that matches your tools and material. Rulers and tape measures usually mark 1/16 in, fine woodworking and machining often use 1/32 or 1/64 in. A finer denominator gives a closer match but a busier fraction.
- Why is my fraction approximate?
- Most decimals and millimeter values do not land exactly on a sixteenth or sixty-fourth of an inch, so the tool snaps to the nearest fraction and shows the rounding error. Choosing a finer precision such as 1/64 reduces that error.