Mercury is the closet planet to the Sun at a distance of 36 million miles. The closest star to the Sun is Proxima Centauri, which is 4.24 light years away. A light year is 5.9 * 1012 miles.

So, if you are trying to convert lightyears to miles, you'll do so with this formula:

mi = (ly) * 5.9e^12, or 59,000,000,000,000. so it would be:

miles = (4.24) (5.9e^12) = roughly 2.5e^13 so Proxima Centauri is 250,000,000,000,000 miles from the Sun

To convert miles to lightyears, you'll just switch them around:

lightyears = (3.6e^6) / (5.9e^12)

Mercury is roughly 0.000006123880 lightyears from the Sun