Length, Mass & Capacity Comparisons

Three different things get measured, and each has its own instrument. Length is how long something is, and you use a ruler or a tape. Capacity is how much a container holds, and you use a measuring jug. Mass is how heavy something is, and you use scales. A thermometer measures temperature, which is none of those three, and the test likes putting it in as a wrong answer. Length is in centimetres and metres, mass in grams and kilograms, and capacity in millilitres and litres.

Reading a jug is the same habit as reading a number line. Do not guess from how full it looks — work out what one mark is worth first. Take two labels, see how far apart they are, and count the marks between them. Here two labels are two hundred apart with two marks between, so each mark is a hundred millilitres. Then count the marks up to the water line. This jug has six hundred millilitres in it.

Scales work exactly the same way. Find what one mark is worth, then count round to the needle. Here the marks go up in hundreds of grams and the needle points at seven hundred, so the mass is seven hundred grams. A thousand grams is the same as one kilogram, so this is a bit under a kilo. And if the needle sits between two marks, your answer sits between those two numbers — which is often enough to rule out three of the four options.

Some questions say about, and they want an estimate rather than an exact reading. A bucket holds ten litres when full and the water is about halfway up, so there is about five litres in it. Half of ten is five. One litre would be far too little, and twenty litres could not fit in the bucket at all, so those options go straight out. That is the method for estimating questions: work out what a half or a quarter would be, then throw out any option bigger than the container.

To compare two measurements you must have them in the same unit, so change one of them first. Is ninety centimetres longer than a metre? A metre is a hundred centimetres, so no, the metre is longer. Is nine hundred grams heavier than a kilogram? A kilogram is a thousand grams, so no. Does two litres hold more than fifteen hundred millilitres? Two litres is two thousand millilitres, so yes. Remember the three: a hundred centimetres in a metre, a thousand grams in a kilogram, a thousand millilitres in a litre. Quiz and exercises now.