Online Ruler

Published September 5, 2026

How to read a ruler - cm, mm, and inches explained

Reading a ruler is a two-minute skill: the long lines are whole units, the short lines are fractions, and the two scales tell you which unit you are looking at. A ruler has a metric edge (centimeters and millimeters) and, on most rulers, an imperial edge (inches). The online ruler at Online Ruler shows both at once: centimeters and millimeters on top, inches on the bottom.

The metric edge: centimeters and millimeters

The metric scale works in tens:

  • The longest lines are centimeters, numbered 0, 1, 2, 3 and so on.
  • The shortest lines are millimeters - 10 of them fit between two centimeter marks.
  • The line halfway between two centimeter marks is a half-centimeter mark (5 mm), usually slightly longer than the millimeter ticks.

To measure something in metric, align the object with the 0 mark and read the last full centimeter, then count the small ticks. For example, an object ending 7 ticks past the 5 cm line is 5.7 cm, which is 57 mm.

The imperial edge: inches and fractions

The inch edge divides each inch into fractions:

  • 1 inch - the numbered lines, 1 inch apart.
  • 1/2 inch - the longest intermediate line, exactly halfway.
  • 1/4 inch - the next-longest lines, between the inch and half-inch marks.
  • 1/8 and 1/16 inch - progressively shorter lines (only on rulers with those graduations).

So a measurement of “5 and a half inches plus one quarter” is written 5 3/4 inches. On an online ruler you can switch the graduation between 1/8, 1/16, and 1/32 to see finer ticks.

The one conversion you need

1 inch = 2.54 cm, exactly. That means:

  • 12 inches (1 foot) = 30.48 cm
  • 6 inches = 15.24 cm
  • 1 cm = 0.3937 inches

You rarely need it, because a dual-scale ruler already shows both units - which is the whole point of using a ruler online.

Quick checklist

  1. Always start from the 0 mark, not the edge of the ruler.
  2. Read the long numbered lines first, then count small ticks.
  3. Metric: 10 mm per cm. Imperial: count fraction lines (1/2, 1/4, 1/8, 1/16).
  4. For on-screen rulers, calibrate first - an uncalibrated screen ruler is only as accurate as your display’s pixel density.

Ready to measure? Open the online ruler, calibrate it once against a credit card, and start measuring in cm, mm, or inches.

FAQ

How do I read centimeters on a ruler?

On the metric edge, the long numbered lines are centimeters (10 mm apart). The short lines between them are millimeters. Count the small ticks from the numbered line to find fractions of a centimeter - for example, 7 small ticks past the 5 means 5.7 cm.

How do I read inches on a ruler?

The inch edge has a numbered line for every full inch. Between them, the longest intermediate line marks the half-inch, the next-longest lines mark quarter inches, and shorter lines mark eighths or sixteenths, depending on the graduation of the ruler.

How many millimeters are in a centimeter?

There are exactly 10 millimeters in 1 centimeter. A millimeter is one tenth of a centimeter and is the smallest tick on a standard metric ruler.

How many centimeters are in 12 inches?

There are exactly 30.48 centimeters in 12 inches, because 1 inch equals 2.54 cm. A 12-inch ruler is therefore the same length as a 30 cm ruler to within half a centimeter.