How to read a ruler (inches, centimeters and millimeters)
Most rulers have two scales: inches along one edge and centimeters along the other. They are read the same way — start at zero, count the big numbered lines, then count the small ones — but the small lines mean different things on each side, which is where people usually go wrong.
The metric side: centimeters and millimeters
Every numbered line is one centimeter. Between two numbers there are ten small spaces, and each of those is one millimeter. The line halfway between two numbers is a little longer than the others; it marks 5 mm, so you can find the middle without counting.
To read a length:
- Put the start of the object exactly on the 0 line, not on the end of the ruler. On many rulers the zero sits a few millimeters in from the edge.
- Find the last numbered line before the object ends. That’s your whole centimeters.
- Count the small lines after it. That’s your millimeters.
If a pencil ends two small lines past the 7, it is 7.2 cm, which is the same as 72 mm. Converting is just moving the decimal point: 1 cm = 10 mm.
The inch side
Every numbered line is one inch. The lines in between are fractions, and the longer the line, the bigger the fraction:
| Line length | Fraction | Decimal |
|---|---|---|
| Longest (middle) | ½ inch | 0.5 in |
| Next longest | ¼ and ¾ inch | 0.25, 0.75 in |
| Shorter | ⅛, ⅜, ⅝, ⅞ inch | 0.125, 0.375 … |
| Shortest (on most rulers) | 1/16 inch | 0.0625 in |
The trick is to read the fraction from the length of the line rather than counting from zero. If something ends on the second-longest line after the 3, that line is a quarter, so the length is 3¼ inches. If it ends one short line past the ½ mark, it’s ½ + ⅛ = ⅝ inch past the number.
Our ruler marks eighths of an inch. Its read-out gives inches as a decimal — 3.25 in rather than 3¼ in — because decimals are easier to add and compare. To turn a decimal into eighths, multiply the part after the point by 8: 0.375 × 8 = 3, so 0.375 in is ⅜ in.
Common mistakes
- Starting at the edge instead of zero. Wooden and plastic rulers often have a margin before the zero line. Start on the line.
- Looking at it from an angle. If your eye isn’t straight above the mark, the mark appears to move. This is called parallax, and it can easily cost you a millimeter.
- Mixing up the two edges. 5 on the inch side is 12.7 cm on the other side — always check which scale you’re reading.
Trying it on screen
The online ruler works the same way: drag marker A to where the object starts and marker B to where it ends, and the distance appears under the ruler. Switch between cm, mm and inches at the top. For an exact scale, calibrate it once with a bank card — here’s why that matters.