How to measure length correctly

Choose the right device

First choose a device that suits the length:

• a 15-cm scale for small things, such as a pencil or an eraser;
• a metre scale or a measuring tape for the height of a room or the length of a table;
• a long measuring tape for a playground, such as when marking a kabaddi court;
• a flexible tape, like a tailor’s tape, for curved things, such as the girth of a tree or the size of your chest.

A rigid metre scale cannot wrap around a tree trunk or a waist, which is why some measuring devices are made of flexible materials.

Place the scale correctly

Place the scale in contact with the object, along its length, so that one end of the object is at the zero mark. A scale held at a slant, or away from the object, gives a wrong reading.

Keep your eye directly above the mark

While reading the scale, the position of your eye should be directly above the end of the object. If you look from one side or the other, the end seems to line up with a different mark, and your reading is wrong. In the book’s picture, the correct position is B, straight above the tip of the pencil.

Measuring with a broken scale

If the zero end of a scale is broken or its zero mark is unclear, you can still use it. Put one end of the object at any other full mark, say 1.0 cm, read the mark at the other end, say 10.4 cm, and subtract: 10.4 cm − 1.0 cm = 9.4 cm.

Visually challenged students use scales with raised markings that can be felt by touch.

Measuring a curved line

To find the length of string lights needed for curved arches, or of any curved line, use a flexible measuring tape, or lay a thread carefully along the curve, mark where it starts and ends, straighten the thread and measure it with a metre scale.

Why do friends get slightly different answers?

When classmates measure the same pencil, their answers may differ a little. The reasons can be that the scale was not exactly along the object, the eye was not directly above the mark, the starting point was not exactly at zero, or the reading fell between two millimetre marks and was rounded differently. Careful measurement keeps these differences small.

Very small and very long lengths

To find the thickness of one page, measure a stack of many sheets pressed together and divide by the number of sheets: if 100 sheets measure 1 cm (10 mm), one sheet is about 0.1 mm thick. For long distances along a road, you can count the turns of a bicycle wheel and multiply by the length of its outer edge, measured with a string.

Write the result properly

Every result has a number and a unit, with a space between them: 9.4 cm, 1.42 m. Unit symbols are lowercase and never take an ‘s’ or a full stop.

For the units themselves, read standard units of length. For the textbook’s questions, see the Chapter 5 NCERT solutions; for the whole chapter, see the Chapter 5 notes.