Measure with Units and Tools

Measurement describes how long, heavy, or full something is using a number and a unit. A number without a unit can be confusing: “The table is 5” does not tell us whether it is five centimetres, feet, or something else. In this lesson, you will choose a unit and tool, estimate an amount, and measure carefully.

Choose the thing and the unit

First decide what you are measuring: length, mass, or capacity. Length tells how far from one end to another. Mass describes how much matter an object has. Capacity describes how much a container can hold. Each kind of measurement uses suitable units and tools.

Match the tool to the job

Use a ruler or tape measure for length, a scale for mass, and a measuring cup or marked container for capacity. A ruler can measure a pencil, but it does not tell how much water a jug can hold. A kitchen scale can compare the mass of apples, but it does not tell how wide the fruit bowl is.

Choose a unit that fits the object. A centimetre is useful for a small classroom object. A metre may be better for the length of a desk or room. A kilometre is useful for a longer trip. For a small amount of juice, millilitres may be handy; for a large bottle, litres may be easier.

Estimate before measuring

An estimate is a thoughtful guess close to the actual measurement. Before using a ruler, you might estimate that a notebook is about 25 centimetres long. Then line up the zero mark with one end and read the mark at the other end. If you start at the ruler’s edge instead of zero, your measurement may be off.

Estimating helps you notice mistakes. If a pencil seems to measure 200 metres, the number or unit probably does not fit. If a cup holds 250 millilitres, that is a more reasonable amount than 250 litres. Ask yourself whether the answer makes sense for the object.

Measure carefully and report clearly

Use the same starting point

When measuring length, place the zero mark at one end, keep the ruler straight, and read the mark at the other end. If the object starts between numbered marks, count from zero rather than from the ruler’s edge. For a curved object, use a flexible tape or measure around it only if the question asks for distance around.

For mass and capacity, follow the markings on the scale or container. Keep the container level when reading a liquid mark. A measurement is easier to compare when everyone uses the same unit and method.

Metric units are used in many places, and customary units such as inches, feet, pounds, and cups are used in some countries, including the United States. Local classroom materials may use different systems. It is fine to use either system when the task allows it, but do not mix units in one calculation unless you convert them first.

Key takeaways

• A measurement includes a number and a unit.
• Choose the tool and unit that fit length, mass, or capacity.
• Estimate first to check whether the final answer makes sense.
• Use the zero point and read marked tools carefully.

Practice

Choose a useful tool and unit to measure the length of a pencil, the mass of an orange, and the capacity of a water bottle. Sample answers: A ruler and centimetres for the pencil; a scale and grams for the orange; a marked measuring cup and millilitres for the bottle. Other sensible unit choices are possible.