The speed of light in vacuo – “c” – is one of the fundamental physical constants. In the units we usually use, miles per second or kilometres per second, the exact value, which is 299,792.458km/s, looks complicated and may be hard to remember. Wouldn’t it be nice if there were more familiar units in which the value, or a close approximation to it, was a really simple number?
Well, there is indeed a reasonably close approximation: 1 foot/nanosecond.
Yes, light goes almost 1 foot in 1 nanosecond – surely that’s easy to recall! And the difference from the exact value is only 1.67%, which is slight enough to be ignored for a general understanding.
That’s not accurate enough for you? Then look at your foot ruler to remind yourself that 300mm is pretty nearly a foot, then state the speed of light as 300mm/nanosecond. That is a much closer approximation: the difference is down to 0.07%.
(The calculations are mine, so any factual errors are mine alone)