Converting between metric and imperial lengths

0:00Metric units, like centimetres, metres and kilometres, go up in powers of ten.

0:07Imperial units, like inches, feet and miles, are still used in some places.

0:13These facts link them. One foot is 12 inches. One inch is exactly 2.54 centimetres. One mile is about 1.609 kilometres.

0:26A person is 6 feet 2 inches tall. How tall is that in centimetres?

0:32First, change it all to inches. 6 feet is 6 times 12, which is 72 inches.

0:40Add the other 2 inches to get 74 inches.

0:44As a fraction, 2.54 centimetres over 1 inch is a conversion factor. It equals 1, because the top and bottom are the same length.

0:54Multiplying by 1 keeps the length the same, and the inches cancel, leaving centimetres.

1:0074 times 2.54 is 187.96 centimetres, so the person is about 188 centimetres tall.

1:11Check: a centimetre is shorter than an inch, so there should be more centimetres. 188 is more than 74, so it makes sense.

1:22Now the other way: how many miles is a 10 kilometre run?

1:27We want miles, so miles go on top: 1 mile over 1.609 kilometres.

1:34The kilometres cancel, so we divide: 10 divided by 1.609 is about 6.2 miles.

1:43A common mistake is to multiply instead: 10 times 1.609 is 16.09 miles.

1:52But a mile is longer than a kilometre, so there should be fewer miles. 16.09 is too big, and 6.2 makes sense.

2:02To convert, multiply by a fraction with the unit you want on top and the unit you have below. Then check: bigger or smaller?