Measurement guide

How to read a ruler in inches, cm, and mm

Reading a ruler is mostly about matching the end of an object to the nearest tick mark. Metric rulers use a decimal pattern of centimeters and millimeters, while inch rulers usually divide each inch into fractions.

Open the rulerCalibrate first

How to read an inch ruler

An inch ruler counts whole inches first, then the fractional marks after the last whole number. The longer the mark, the larger the fraction.

MarkHow to spot itExample
1/2 inchLongest mark between two whole inch numbersA mark halfway between 2 and 3 inches is 2 1/2 inches.
1/4 inchNext-longest mark between the whole inch and half-inch marksOne quarter past 2 inches is 2 1/4 inches.
1/8 inchShorter marks that divide each quarter inch in halfThree eighths past 2 inches is 2 3/8 inches.
1/16 inchSmallest common marks on many rulersSeven small marks past 2 inches is 2 7/16 inches.

How to read centimeters and millimeters

Find the centimeter number

The large numbered metric marks are centimeters. Each centimeter equals 10 millimeters.

Count the millimeter ticks

The small marks between centimeter numbers are millimeters. The middle mark is usually 5 mm, or half a centimeter.

Combine the values

If an object ends at 4 cm plus 6 small ticks, the length is 4.6 cm or 46 mm.

Common ruler reading mistakes

Starting at the ruler edge instead of the zero mark when the printed zero is inset.

Mixing inch fractions with decimal centimeters in the same measurement.

Rounding too early instead of recording the closest visible tick mark.

Using an uncalibrated screen ruler for actual-size measurements.

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