Twelve, three, seventeen sixty

About this video

Screens are sold in inches, planes fly at 35,000 feet, and road signs in the United States and the UK count miles. These old units of length began with the human body: an inch was roughly the width of a thumb and a foot was a foot. Unlike the metric system, they don't go in tens. There are 12 inches in a foot, 3 feet in a yard and 1,760 yards in a mile, so every step has its own number to remember.

This animation lines up the old units against metric ones and gives the rough rules that carry you between them. An inch is about 2.5 cm (exactly 2.54), which is why many rulers have an inch edge and a centimetre edge. A foot is about 30 cm, and a mile is about 1.6 km, so a sign saying 20 miles means about 32 km.

Transcript

1. 0:00A 55-inch TV. Flying at 35,000 feet. 20 miles to town.
2. 0:04Old body-based units, still used in the US and UK.
3. 0:08Inch to foot: 12. Foot to yard: 3. Yard to mile: 1,760.
4. 0:13Metric units go in tens: 10, 100, 1,000.
5. 0:17With old units, every step has a new number to remember.
6. 0:21Many rulers show both. An inch is about 2.5 centimetres.
7. 0:25A foot is about 30 centimetres; a mile, about 1.6 kilometres.
8. 0:29So a sign saying 20 miles means about 32 kilometres.
9. 0:33Odd steps, but rough rules carry you across.