How far can light travel in one year?

light can travel 5,874,601,670,040 in a year around the earth

That depends on what it's traveling IN. The speed of light is different in every substance.

It's fastest in NO substance ... in vacuum, like in outer space. The speed of light in vacuum is 299,792,458 meters per second.

That's

9,454,254.955 million kilometers / 5,874,601.672 million miles

in 365.0 days.