The Fibonacci Sequence is made up of numbers that are the sum of the previous two numbers in the sequence, starting with 0 and 1.
It goes like this: 0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144, et cetera.
Observe how 1 equals 0+1; whereas 2 equals 1+1; 3 is 1+12; 5 is 2+3; and 8 is 3+5; and so on. The number after 144 is 233, or 89+144.
The Fibonacci number describes the golden spiral, an ideal form much beloved by artists, architects and designers through the ages. Interestingly, it also neatly matches the relationship between kilometers and miles.
Observe that three miles is roughly five kilometers, five miles is roughly eight kilometers, eight miles is roughly 13 kilometers. It’s not absolutely perfect because eight miles is actually 12.875 kilometers, but as top-off-the-head conversions go, it is immensely accurate.
“Thus the metric system did not really catch on in the States, unless you count the increasing popularity of the nine-millimeter bullet.”
– Dave Barry
If you need to convert a number that’s not on the Fibonacci sequence, you can just break out the Fibonacci numbers, convert, and add the answers. For instance, 100 can be broken down into 89 + 8 + 3, all Fibonacci numbers. The next numbers are 144, 13, and 5, which add up to 162. Note that 100 miles is actually equal to 160.934. Again, close enough.