How fast does light travel, in kilometres per second?

How to get close to this number

The memorable handle is that light circles the Earth about seven and a half times a second. If you can recall the circumference of the planet — and most people can, at least to the right order of magnitude — you can multiply your way to the speed of light without having memorised it. That is the general move: keep one ratio and one familiar length, and derive the rest.

The frequent error is confusing the speed with the unit of distance it defines. A light year measures distance, not time, and mixing the two is the single most common mistake in any question about astronomical scale. If a guess comes out wildly wrong here, check first that it is answering in distance per second rather than in something per year.

The deepest thing about this figure is that it is not a measurement at all any more. Since the metre was redefined in terms of light, the speed of light is a fixed constant by definition, and experiments that once measured it now calibrate the metre instead. So unlike almost every other number on this site, it cannot drift, cannot be revised by a better instrument, and will carry the same value in every future edition of the source.