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Converting measurements

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# Converting between metric and imperial units

Here are some examples of metric and imperial measures of length, mass and capacity:

MetricImperial
Lengthmm, cm, m, kminch, foot, yard, mile
Massmg, g, kgounce (oz), pound (lb), stone
Capacityml, cl, lpint, gallon

You will be expected to know some common conversions between metric and imperial units. Some of these are shown below, but check with your teacher which ones you need to learn.

• 1 km = 5/8 mile
• 1 m = 39.37 inches
• 1 foot = 30.5 cm
• 1 inch = 2.54 cm
• 1 kg = 2.2 lb
• 1 gallon = 4.5 litres
• 1 litre = 1 3/4 pints
Question

A cup has a circumference of 10 inches. What is it in cm?

We know that 1 inch = 2.54 cm.

We are converting from inches to cm, so we multiply by 2.54

10 × 2.54 = 25.4 cm.

It is easy to get confused over whether to multiply or divide, so here is a way to check your answers:

After doing the conversion, always think about whether your answer should be bigger or smaller than the original number.

In this question, which involves converting inches into cms, we know that cms are smaller than inches, so you should end up with a bigger number than the one you started with. You therefore need to multiply.

Question

Use this method to say which of the following conversions are definitely incorrect?

• 61 cm = 1860.5 feet
• 5 gallons = 22.5 litres
• 14 lb = 30.8 kg

Remember cm are smaller than feet, so 61 cm = 1860.5 feet must be wrong.

And pounds are smaller than kg, so 14lb = 30.8 kg must be wrong too.

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