This chart lists fractions alongside their decimal and percent values, so you can convert at a glance without dividing. You choose which fractions to show — the common ones, inch fractions from halves to 64ths with their millimeter equivalents, every denominator from 2 to 64, or a single denominator — how many decimal places to display and how to sort the rows.
How to use the chart
- Choose which fractions to list. Inch fractions add a millimeters column.
- For a single denominator, enter it (2 to 64).
- Pick the number of decimal places; percents are shown with two fewer.
- Sort by value to see the fractions in order from smallest to largest, or by denominator to group them.
- Leave lowest terms only checked to hide duplicates such as 2/4; uncheck it for a complete grid.
The tape counts how many fractions are listed and how many have terminating versus repeating decimals. The Exact decimal column shows repeating digits in parentheses — 0.(3) means 0.333… — and very long repeating blocks are summarized by their length.
How the values are calculated
The 25.4 factor is exact: since 1959 the international inch has been defined as exactly 25.4 mm, a definition NIST uses for US customary units.
Common fractions
| Fraction | Decimal | Percent | Fraction | Decimal | Percent |
|---|---|---|---|---|---|
| 1/2 | 0.5 | 50% | 1/3 | 0.333… | 33.33% |
| 1/4 | 0.25 | 25% | 2/3 | 0.666… | 66.67% |
| 3/4 | 0.75 | 75% | 1/6 | 0.1666… | 16.67% |
| 1/5 | 0.2 | 20% | 1/8 | 0.125 | 12.5% |
| 2/5 | 0.4 | 40% | 3/8 | 0.375 | 37.5% |
| 1/10 | 0.1 | 10% | 5/8 | 0.625 | 62.5% |
Inch fractions
Tape measures in the US are marked in halves, quarters, eighths and sixteenths of an inch; machinists go to 32nds and 64ths. Every one of these terminates because the denominators are powers of 2:
| Inches | Decimal | Millimeters |
|---|---|---|
| 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 |
Each 64th of an inch adds 0.015625 in (about 0.397 mm). To convert other lengths, see the mm to inches converter.
Patterns worth knowing
- Ninths repeat their numerator: 1/9 = 0.111…, 4/9 = 0.444…, 7/9 = 0.777….
- Elevenths repeat a two-digit multiple of 9: 1/11 = 0.0909…, 2/11 = 0.1818…, 3/11 = 0.2727….
- Sevenths cycle through the same six digits: 1/7 = 0.142857…, 2/7 = 0.285714…, 3/7 = 0.428571….
- Halving doubles the denominator and halves the decimal: 1/4 = 0.25, 1/8 = 0.125, 1/16 = 0.0625.
Worked example: reading the chart
Suppose a drill bit is labeled 5/16 inch and you need the decimal size. Choose inch fractions, sort by value, and find 5/16: 0.3125 in, which is 7.9375 mm. To confirm, divide 5 by 16 = 0.3125, and multiply by 25.4 = 7.9375.
For fractions not in the chart, the fraction to decimal calculator divides any fraction and shows the long division; the decimal to fraction calculator goes the other way. To see these fractions as positions, use fractions on a number line.
Frequently asked questions
What is 3/8 as a decimal?
3/8 = 0.375, or 37.5%. In the chart, choose common fractions or inch fractions and find 3/8; its millimeter equivalent is 9.525 mm.
Which fractions have repeating decimals?
Any fraction in lowest terms whose denominator has a prime factor other than 2 or 5. Thirds, sixths, sevenths, ninths, elevenths and twelfths repeat; halves, quarters, fifths, eighths, tenths and sixteenths terminate.
How is the inch fraction chart converted to millimeters?
One inch is exactly 25.4 millimeters, so each fraction of an inch is multiplied by 25.4. For example 1/16 in = 25.4 ÷ 16 = 1.5875 mm.
Why does the chart show fewer fractions with lowest terms only?
Fractions like 2/4, 3/6 and 4/8 equal 1/2, so they are listed once in lowest terms. Uncheck the box to see every numerator for every denominator, which is useful when you are reading a ruler marked in a single unit.