Gauge Charts
Fraction to Decimal Chart
Inch fractions on a tape measure divide down in halves: 1/2, 1/4, 1/8, 1/16, 1/32 and 1/64. The table below lists all 64 positions with the decimal and millimetre equivalent, which is what you need when a drawing is dimensioned in decimals and your tape is not.
Inch fractions, decimals and millimetres
| Fraction | Decimal (in) | mm |
|---|---|---|
| 1/64 | 0.0156 | 0.397 |
| 1/32 | 0.0313 | 0.794 |
| 3/64 | 0.0469 | 1.191 |
| 1/16 | 0.0625 | 1.587 |
| 5/64 | 0.0781 | 1.984 |
| 3/32 | 0.0938 | 2.381 |
| 7/64 | 0.1094 | 2.778 |
| 1/8 | 0.1250 | 3.175 |
| 9/64 | 0.1406 | 3.572 |
| 5/32 | 0.1563 | 3.969 |
| 11/64 | 0.1719 | 4.366 |
| 3/16 | 0.1875 | 4.762 |
| 13/64 | 0.2031 | 5.159 |
| 7/32 | 0.2188 | 5.556 |
| 15/64 | 0.2344 | 5.953 |
| 1/4 | 0.2500 | 6.350 |
| 17/64 | 0.2656 | 6.747 |
| 9/32 | 0.2813 | 7.144 |
| 19/64 | 0.2969 | 7.541 |
| 5/16 | 0.3125 | 7.938 |
| 21/64 | 0.3281 | 8.334 |
| 11/32 | 0.3438 | 8.731 |
| 23/64 | 0.3594 | 9.128 |
| 3/8 | 0.3750 | 9.525 |
| 25/64 | 0.3906 | 9.922 |
| 13/32 | 0.4063 | 10.319 |
| 27/64 | 0.4219 | 10.716 |
| 7/16 | 0.4375 | 11.112 |
| 29/64 | 0.4531 | 11.509 |
| 15/32 | 0.4688 | 11.906 |
| 31/64 | 0.4844 | 12.303 |
| 1/2 | 0.5000 | 12.700 |
| 33/64 | 0.5156 | 13.097 |
| 17/32 | 0.5313 | 13.494 |
| 35/64 | 0.5469 | 13.891 |
| 9/16 | 0.5625 | 14.287 |
| 37/64 | 0.5781 | 14.684 |
| 19/32 | 0.5938 | 15.081 |
| 39/64 | 0.6094 | 15.478 |
| 5/8 | 0.6250 | 15.875 |
| 41/64 | 0.6406 | 16.272 |
| 21/32 | 0.6563 | 16.669 |
| 43/64 | 0.6719 | 17.066 |
| 11/16 | 0.6875 | 17.462 |
| 45/64 | 0.7031 | 17.859 |
| 23/32 | 0.7188 | 18.256 |
| 47/64 | 0.7344 | 18.653 |
| 3/4 | 0.7500 | 19.050 |
| 49/64 | 0.7656 | 19.447 |
| 25/32 | 0.7813 | 19.844 |
| 51/64 | 0.7969 | 20.241 |
| 13/16 | 0.8125 | 20.637 |
| 53/64 | 0.8281 | 21.034 |
| 27/32 | 0.8438 | 21.431 |
| 55/64 | 0.8594 | 21.828 |
| 7/8 | 0.8750 | 22.225 |
| 57/64 | 0.8906 | 22.622 |
| 29/32 | 0.9063 | 23.019 |
| 59/64 | 0.9219 | 23.416 |
| 15/16 | 0.9375 | 23.813 |
| 61/64 | 0.9531 | 24.209 |
| 31/32 | 0.9688 | 24.606 |
| 63/64 | 0.9844 | 25.003 |
| 1/1 | 1.0000 | 25.400 |
Reading a tape measure without counting marks
- The longest mark between the inch numbers is the half. The next longest is the quarter, then the eighth, then the sixteenth. Most tapes stop at sixteenths; machinist rules go to 32nds or 64ths.
- Count by the smallest division on your tape and reduce afterwards. Twelve sixteenths is easier to read off the marks than three quarters, and they are the same distance.
- A fraction reduces when the numerator and denominator share a factor. 8/64 is 1/8 because both divide by eight, which is why the table below shows each position in lowest terms.
- To go from a decimal on a drawing to the nearest tape mark, multiply by 64 and round. 0.412 times 64 is 26.4, so the nearest 64th is 26/64, which reduces to 13/32.
Converting between fractions, decimals and millimetres
All three are the same length written three ways, and each conversion is exact rather than approximate.
- Fraction to decimal: divide the top by the bottom. 5/16 is 5 divided by 16, which is 0.3125 inches exactly.
- Decimal to millimetres: multiply by 25.4. The inch has been defined as exactly 25.4 millimetres since 1959, so this conversion loses nothing.
- Millimetres to the nearest 64th: divide by 25.4, multiply by 64, then round. 10 mm is 0.3937 inches, which is 25.2 sixty-fourths, so 25/64 is the nearest mark and it is 0.003 inches under.
- Fractions never repeat as decimals here because every denominator is a power of two, so each of the 64 positions terminates exactly.
Common questions
What is 1/8 as a decimal?
- 0.125 inches, which is 3.175 millimetres. One eighth is 8/64, so it sits at the eighth mark on a 64th scale and the fourth on a 32nd scale.
What is 3/4 as a decimal?
- 0.75 inches, which is 19.050 millimetres. Three quarters is 48/64, so it is the third of the four long quarter marks between inch numbers.
What is 0.5 inches as a fraction?
- One half exactly, which is 32/64 and 12.700 millimetres. Any decimal that is a sum of halves, quarters, eighths, sixteenths, thirty-seconds or sixty-fourths converts exactly; anything else lands between marks.
How many 64ths are in an inch?
- Sixty-four. A 64th is 0.015625 inches, or 0.397 millimetres, which is about the thickness of a stack of four sheets of printer paper.
Is 25.4 mm per inch exact or rounded?
- Exact. The international yard and pound agreement of 1959 defined the inch as precisely 25.4 millimetres, so every millimetre figure in the table is a conversion rather than a measurement.
Common questions
How do I convert a fraction to a decimal?
- Divide the top number by the bottom number. Three-eighths is 3 divided by 8, which is 0.375 inches. To go to millimetres, multiply the decimal by 25.4, so 0.375 inches is 9.525 mm.
What is 0.0625 as a fraction?
- One sixteenth. Multiply by 64 to get the number of 64ths, so 0.0625 times 64 is 4, giving 4/64, which reduces to 1/16.
Printable fraction to decimal chart
All 64 fractions of an inch as decimals and millimetres, on a single sheet.
- Fraction to decimal chart (PDF): Every 64th with its decimal and exact millimetre equivalent. Free to print and share with attribution.