How many meters are in a lightyear?

Answer 1

A light-year is exactly 9,460,730,472,580,800 meters.

This is about 9.5 × 1015 meters, 9.5 Pm (petametres), or about 9.5 quadrillion meters.

It is defined as the distance that light travels in a vacuum in one Julian year. As a Julian year is defined as exactly 365.25 days, and the speed of light in a vacuum is defined as exactly 299,792,458 meters/second, a light year is

365.25 days × (24 hours/day) × (60 minutes/hour) × (60 seconds/minute) × 299,792,458 meters/second = 9,460,730,472,580,800 meters.

This is not an approximation but an exact number due to the definitions of a Julian year and the speed of light.