Metric and imperial, briefly
The metric system scales by powers of ten, which is why converting millimetres to kilometres is just moving a decimal point. The imperial system grew out of body measurements and trade customs, which is why there are 12 inches in a foot, 3 feet in a yard and 1,760 yards in a mile — numbers that have no pattern because they were never designed as a system.
Three countries have not formally adopted metric as their primary system: the United States, Myanmar and Liberia. Even in the US, science, medicine and the military run on metric, and the imperial units themselves are legally defined in terms of metric ones — an inch is exactly 25.4 mm by international agreement since 1959.
Conversions worth memorising
The temperature conversions are the ones people get wrong, because unlike every other unit here, Celsius and Fahrenheit have different zero points. You cannot simply multiply — you must shift as well as scale. That is also why −40 °C equals −40 °F: it is the single point where the two scales cross.
Traps that have caused real damage
- US vs UK gallons. A UK gallon is about 20% larger. In 1983 an Air Canada 767 ran out of fuel at 41,000 feet after a pounds-versus-kilograms mix-up during refuelling — the aircraft glided to a landing and became known as the Gimli Glider.
- The Mars Climate Orbiter. Lost in 1999 because one team supplied thrust data in pound-force seconds while the navigation software expected newton-seconds. Cost: $327 million.
- KB vs KiB. Storage manufacturers use 1 KB = 1,000 bytes; operating systems often use 1,024. That is why a "1 TB" drive shows as roughly 931 GB — the drive is not lying, the two conventions simply differ, and the gap widens at every scale.
- Fluid ounces. A US fluid ounce is about 4% larger than a UK one, which matters in recipes and cocktails.