How to Read a Metric Ruler Scale

Updated February 21, 2017

Science students quickly discover that the metric system is, in many regards, a superior system of measurement compared to the English system that predominates in the United States. The metric system was designed as a base-10 measurement system, meaning that differences between metric units simply involve moving a decimal place. Although measurements can be taken in English units and later converted to metric units, this process is tedious, requires access to unit conversion tables and adds an unnecessary step to the measurement process.

Place the ruler across an object to be measured. As an example, consider the width of a piece of regular paper. Place the ruler near the top or bottom edge to help ensure that the ruler is straight.

Adjust the ruler so that the mark labelled 0 (zero) sits exactly on the left edge of the paper. All measurements with a ruler should start from zero.

Note the graduations on the ruler. Most rulers feature long marks with nine shorter marks between each long mark. The long marks will be numbered and represent centimetres. The nine shorter marks represent one-tenth of a centimetre, or a single millimetre each.

Hold the ruler firmly in place and note where the ruler meets the right edge of the piece of paper. For a standard 8.5- by 11-inch piece of paper, this should fall between the long marks numbered 21 and 22. This means that the width of the paper falls between 21 and 22cm.

Locate the small millimetre mark on the ruler that is nearest to the edge of the paper and count the number of smaller marks (from left to right) between 21 and 22cm. For a regular sheet of paper, this should be about 6 marks. Thus the paper is 21cm plus 6mm. Because a millimetre is one-tenth of a centimetre, this can be represented as 21.6cm.

Convert the measurement to the desired metric unit by moving the decimal the appropriate number of places to the left or right. If, for example, the desired unit is millimetres, then

(21.6cm) x (10mm/cm) = 216mm.

Or, if the desired unit is meters (m), then

(21.6cm) x (1m / 100cm) = 0.216m.

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