1 light-year = roughly 5,874,601,700,000 miles. (rounded)
To work it out, take the speed of light, and change units a few times:
(186,282.397 miles/second)
times
(86,400 second/day)
times
(365 day/year)
= (186,282.397 x 86,400 x 365) mile/year .
You can whip that out on your calculator in no time.
