Light-Year Calculator

What is a light-year?

A light-year is a unit of distance, not of time: it is how far light travels through a vacuum in one year. Because light moves at about 299,792 kilometers per second, that distance is enormous — roughly 9.46 trillion kilometers. Astronomers use light-years to describe distances between stars, where kilometers or miles would produce unwieldy numbers.

This calculator converts any distance in light-years into kilometers, miles, astronomical units (AU), and parsecs, so you can move between everyday and astronomical distance units at a glance.

How does the calculator work?

One light-year equals a fixed number of each other unit, so the calculator simply multiplies your input by the appropriate constant:

• 1 light-year = 9.4607 × 10¹² kilometers
• 1 light-year = 5.8786 × 10¹² miles
• 1 light-year = 63,241.077 astronomical units
• 1 light-year = 0.3066014 parsecs

An astronomical unit is the average Earth–Sun distance, and a parsec is about 3.26 light-years — the distance at which one astronomical unit subtends an angle of one arcsecond.

Formula

$d_{\text{km}} = d_{\text{ly}} \times 9.4607 \times 10^{12}$

$d_{\text{mi}} = d_{\text{ly}} \times 5.8786 \times 10^{12}$

$d_{\text{AU}} = d_{\text{ly}} \times 63241.077$

$d_{\text{pc}} = d_{\text{ly}} \times 0.3066014$

where $d_{\text{ly}}$ is the distance in light-years.

Worked examples

Example 1 — one light-year. Setting $d_{\text{ly}} = 1$:

$d_{\text{AU}} = 1 \times 63241.077 = 63241.077\ \text{AU}$

$d_{\text{pc}} = 1 \times 0.3066014 = 0.30660\ \text{pc}$

Example 2 — the nearest star. Proxima Centauri lies about 4.2465 light-years away:

$d_{\text{pc}} = 4.2465 \times 0.3066014 \approx 1.3020\ \text{pc}$

So the closest star to the Sun is a little over one parsec away.

Conversion reference

Notes and practical uses

• Light-years and parsecs both measure distance; parsecs are preferred in professional astronomy, while light-years are more intuitive in popular science.
• For distances within the Solar System, astronomical units are the natural choice.
• To convert everyday lengths between metric and imperial units, see the length converter.

FAQ

Is a light-year a measure of time? No. Despite the word “year”, a light-year measures distance — the path length light covers in one year.

How many kilometers are in a light-year? About 9.4607 × 10¹² km, or roughly 9.46 trillion kilometers.

Which is bigger, a light-year or a parsec? A parsec is larger: one parsec equals about 3.26 light-years.