Add half and a bit

About this video

Metric units convert by moving the decimal point, but miles and inches don't go in tens, so you need another trick. This animation shows two rough rules for converting in your head. Miles to kilometres: take the number, add half of it, then add a little more. A road sign saying 10 miles is 10 + 5 + 1, about 16 km. The exact answer is 16.09 km, close enough to plan a drive.

Inches to centimetres: double the number, then add half the original number. A 50-inch screen is 100 + 25, about 125 cm (exactly 127 cm, since an inch is 2.54 cm). Rough answers like these are fine for everyday life. For building a bridge or sending a spacecraft, engineers use exact numbers, and everyone has to use the same units.

Transcript

1. 0:00The sign says 10 miles, but your map counts in kilometres.
2. 0:04Miles don't go in tens, so moving the point won't help.
3. 0:09A rough rule: the number, plus half, plus a bit. Ten, plus five, plus one: about 16.
4. 0:16The exact answer is 16.09. Close enough to drive.
5. 0:20Inches to centimetres: double it, then add half. A 50-inch screen: 100, plus 25, about 125 centimetres.
6. 0:26Building a bridge or a spacecraft? Use exact numbers.
7. 0:30But close enough is often enough.