On an inch ruler, the fraction is the number of spaces past the last whole number over the number of spaces in one inch. A mark 7 spaces past the 2 on a scale of sixteenths reads 2 7/16 inches, and the longest mark between two whole numbers is the 1/2 mark. This guide is about reading the ruler; to convert a measurement to millimeters, use the fractional inches to mm converter, and to add measurements, see the guide to adding fractional inches.
Which mark matches which fraction
The longest mark between two whole numbers is 1/2. Next come 1/4 and 3/4, then the eighths, and the shortest marks are the sixteenths. Length helps you find your place, but the number of spaces sets the fraction, because not every ruler prints the same divisions.
| Spaces counted | Reduced fraction | Type of mark |
|---|---|---|
| 1/16 | 1/16 | Short (sixteenths) |
| 2/16 | 1/8 | Medium (eighths) |
| 3/16 | 3/16 | Short (sixteenths) |
| 4/16 | 1/4 | Long (quarters) |
| 5/16 | 5/16 | Short (sixteenths) |
| 6/16 | 3/8 | Medium (eighths) |
| 7/16 | 7/16 | Short (sixteenths) |
| 8/16 | 1/2 | Longest (half) |
| 9/16 | 9/16 | Short (sixteenths) |
| 10/16 | 5/8 | Medium (eighths) |
| 11/16 | 11/16 | Short (sixteenths) |
| 12/16 | 3/4 | Long (quarters) |
| 13/16 | 13/16 | Short (sixteenths) |
| 14/16 | 7/8 | Medium (eighths) |
| 15/16 | 15/16 | Short (sixteenths) |
The reduced fraction comes from dividing the numerator and denominator by their greatest common factor (GCF), and it changes only when the number of spaces is even. The marks in the first column with an odd numerator are already in lowest terms, and they are the shortest marks on the ruler. To reduce other readings, the simplify fractions tool finds the GCF for you.
Worked example: the blue mark in the figure
On a ruler marked in sixteenths, a mark sits 12 spaces past the 5, in the same position as the blue mark in the figure, which counts from 0. What is the measurement?
- Find the whole number. The last number before the mark is 5.
- Count the spaces in one inch. There are 16, so the denominator is 16.
- Count the spaces up to the mark. There are 12, so the fraction is 12/16.
- Reduce. Divide by the greatest common factor (GCF), the largest number that divides both: the GCF of 12 and 16 is 4, and 12/16 = 3/4.
Result: 5 3/4 inches.
Check: 3/4 lies between 1/2 and 1, and the blue mark is to the right of the long halfway line.
How much lies between two marks
If a piece has to fit between two marks, subtract the readings over the same denominator. With marks at 5/16 and 13/16 of the same inch, the difference is 13/16 − 5/16 = 8/16 = 1/2 inch. When the fractional part of the second number is smaller, borrow an inch: from 2 3/16 to 3 1/8, write 3 1/8 as 3 2/16 = 2 18/16, and then 2 18/16 − 2 3/16 = 15/16 inch.
Keep the unit until the end and do not round each reading separately, because the rounding errors add up. The fraction number line shows the same idea with subtraction as a distance, and the guide to adding fractional inches covers the case of several pieces.
A mark in millimeters
One inch is exactly 25.4 mm, so each mark converts by multiplying. For 5/8 inch: 5 × 25.4 = 127 and 127 ÷ 8 = 15.875 mm. Convert to millimeters only at the end, once you have read the fraction. The fractional inches to mm converter has the full table and the reverse direction.
When the edge falls between two marks
If the edge falls between two marks, the nearest reading is an estimate, not a printed mark. Write it down as one, or use a ruler with finer divisions. To choose the nearest mark in an orderly way, the round to a fraction tool lets you set the step, such as 1/16, and shows the error.
Practice
- A mark is 5 spaces past the 3 on a ruler marked in eighths. What is the reading?
- Reduce 10/16 inch.
- How many millimeters is 3 and 1/4 inches?
Show the answers
- 3 5/8 inches
- 5/8 inch (the GCF of 10 and 16 is 2)
- 82.55 mm, because 3 1/4 = 13/4 and 13/4 × 25.4 = 82.55
Sources
For the value of the inch, the U.S. National Institute of Standards and Technology page SI Units – Length states that the inch is exactly equivalent to 25.4 mm (accessed October 17, 2026). That is the constant used in every conversion in this guide. Use it to check by hand the results you get.