How fast is the speed of light?

Light in vacuum has a speed with no error bar. Since 1983 the metre has been defined as the distance light travels in vacuum in 1/299,792,458 of a second. The second comes from a caesium frequency. Once both are fixed, the speed of light is exact: 299,792,458 metres per second. The International Bureau of Weights and Measures states that definition in the SI Brochure.

An older classroom habit was to treat the speed as something you measured and then rounded to 300,000 kilometres per second, or 186,000 miles per second. Those are reminders. They are not the definition. The exact kilometre-per-hour figure is the metre figure times 3.6, which is 1,079,252,848.8 km/h. The international mile is exactly 1,609.344 metres, so the same speed is 186,282.397 miles per second, or about 670,616,629 miles per hour. This site converts at read time from the exact SI value. It does not store a second, rounded measurement.

Nothing with mass reaches that speed. The rankings on this site are full of things that are fast for what they are — a cheetah, a jet, a re-entering spacecraft — and all of them are a long way under light. Parker Solar Probe's cited perihelion speed is enormous next to a car and still a small fraction of c. Saying “percent of light speed” is a comparison, and it belongs next to the entry, not instead of the source unit.

In fibre, glass and air, light is slower than it is in vacuum. A refractive index is why. The exact SI number is the vacuum speed. A network ping is not a measurement of c, and a game that lets a character “go light speed” has not repealed the metre.