Reading an inch ruler
Each inch on a standard ruler is split into 16 equal parts. The length of each line tells you what it stands for:
- Longest line, with a number: a whole inch.
- Second longest, halfway between numbers: 1/2 inch.
- Third: the quarters, 1/4 and 3/4 inch.
- Fourth: the eighths, 1/8, 3/8, 5/8 and 7/8.
- Shortest: the sixteenths, 1/16, 3/16 and so on.
Step by step
- Put the 0 end of the ruler (the first line, not the edge of the ruler) at one end of the object.
- Note the last whole number before the other end. That is the whole inches.
- Count the small marks past that number to the end of the object. Each one is 1/16".
- Simplify the fraction: 8/16 = 1/2, 4/16 = 1/4, 6/16 = 3/8, 12/16 = 3/4. So 3 inches and 10 marks is 3 10/16 = 3 5/8".
Reading a centimetre and millimetre ruler
Metric rulers are simpler. The numbered lines are centimetres. Each centimetre has 10 small lines, the millimetres, and the slightly longer line in the middle is 5 mm. A reading of 7 numbered lines plus 4 small lines is 7.4 cm, which is 74 mm. Write measurements in mm when precision matters, since mm avoids decimals.
Common mistakes
- Starting at the ruler's edge instead of the 0 line. Many rulers have a few millimetres before 0.
- Reading at an angle. Look straight down at the mark to avoid parallax, and keep the ruler flat against the object.
- Mixing up eighths and sixteenths. Count the marks, then simplify.
- Using a printout that was scaled. Check the scale bars on our printable rulers before you trust a paper ruler.
Fractions to decimals cheat sheet
| Fraction | Decimal | mm | Fraction | Decimal | mm |
|---|---|---|---|---|---|
| 1/16" | 0.0625 | 1.59 | 9/16" | 0.5625 | 14.29 |
| 1/8" | 0.1250 | 3.17 | 5/8" | 0.6250 | 15.88 |
| 3/16" | 0.1875 | 4.76 | 11/16" | 0.6875 | 17.46 |
| 1/4" | 0.2500 | 6.35 | 3/4" | 0.7500 | 19.05 |
| 5/16" | 0.3125 | 7.94 | 13/16" | 0.8125 | 20.64 |
| 3/8" | 0.3750 | 9.52 | 7/8" | 0.8750 | 22.22 |
| 7/16" | 0.4375 | 11.11 | 15/16" | 0.9375 | 23.81 |
| 1/2" | 0.5000 | 12.70 | 1" | 1.0000 | 25.40 |
To convert any reading, use the inch fraction converter.
Frequently asked questions
What are the small lines on a ruler called?
They are graduations or tick marks. On an inch ruler the shortest are usually sixteenths of an inch. On a metric ruler they are millimetres.
How many sixteenths are in an inch?
16. So 8/16 is half an inch and 4/16 is a quarter.
Is the edge of the ruler zero?
Not always. Many rulers have a small gap before the 0 line. Line up the 0 mark, not the edge, unless your ruler is a zero-edge ruler.
Last reviewed 2026-10-07.