Convert Light Year to Astronomical Unit
Convert light years to astronomical units instantly. 1 light year = 63241.0770842663 astronomical unit — use the live calculator, the exact formula, a conversion table and worked examples. Also check the Astronomical Unit to Light Year converter for the reverse conversion.
Units explained
Light Year
A light year is the distance light travels in one Julian year in vacuum — exactly 9,460,730,472,580,800 meters (about 9.46×10¹⁵ m). It is the most common unit for expressing inter-stellar distances in non-technical astronomy.
Defined as the speed of light (299,792,458 m/s, exact) multiplied by one Julian year (365.25 days × 86400 seconds/day). Both factors are exact, making the light year exact by definition.
Light years are used in popular astronomy, science journalism, and stellar distance descriptions. The nearest star (Proxima Centauri) is 4.24 ly away; the Milky Way is about 100,000 ly across.
Concept of distance-as-time used since the 19th century; the modern exact definition follows from the 1983 SI definition of the meter via the speed of light.
Astronomical Unit
The astronomical unit (AU) is a unit of length equal to exactly 149,597,870,700 meters, approximately the average distance from Earth to the Sun. It is the fundamental unit for distance within the solar system.
The AU was originally defined as the semi-major axis of Earth's orbit. In 2012, IAU Resolution B2 redefined the AU as exactly 149,597,870,700 meters, severing its dependence on the actual fluctuating Earth–Sun distance.
AUs are used to express distances within the solar system (Mars: 1.5 AU, Pluto: ~40 AU at perihelion), exoplanet orbital radii, and other planetary-scale distances. Standard in planetary science and astronomy.
Original definition tied to Earth's orbit (Newton, 17th century); fixed at exactly 149,597,870,700 m by IAU Resolution B2 in 2012.
Light Year to Astronomical Unit conversion formula
The relationship between light years and astronomical units:
To convert light years to astronomical units, multiply the value in light years by 63241.0770842663. To reverse, multiply astronomical units by 1.581251e-5.
How to use this converter
Type a value into the calculator. The result in astronomical units updates as you type. Tap a quick value, copy the result with one click, or use the swap arrow to jump straight to the Astronomical Unit to Light Year converter for the reverse direction.
Step-by-step: convert light years to astronomical units
- Write down the value in light years (ly).
- Multiply that value by the factor 63241.0770842663.
- The product is the equivalent value in astronomical units (AU).
- To reverse, multiply the astronomical unit value by 1.581251e-5.
Worked examples
Example 1 — Convert 1 ly to AU:
1 × 63241.0770842663 = 63241.0770842663 AU
Example 2 — Convert 100 ly to AU:
100 × 63241.0770842663 = 6324107.7084266283 AU
Light Year to Astronomical Unit conversion table
Standard reference values for converting light years to astronomical units:
| Light Year [ly] | Astronomical Unit [AU] |
|---|---|
| 0.01 | 632.4107708427 |
| 0.1 | 6324.1077084266 |
| 1 | 63241.0770842663 |
| 2 | 126482.1541685326 |
| 3 | 189723.2312527988 |
| 4 | 252964.3083370651 |
| 5 | 316205.3854213314 |
| 10 | 632410.7708426628 |
| 20 | 1264821.5416853256 |
| 30 | 1897232.3125279886 |
| 40 | 2529643.0833706511 |
| 50 | 3162053.8542133141 |
| 100 | 6324107.7084266283 |
| 500 | 3.162054e+7 |
| 1000 | 6.324108e+7 |
Frequently asked questions
How many astronomical units is 1 light year?
How do I convert light years to astronomical units?
How do I convert astronomical units back to light years?
How many astronomical units is 100 light years?
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Astronomical (8 units)
Sources & references
Conversion factor (1 ly = 63241.0770842663 AU) verified against the following authoritative sources:
- BIPM — The International System of Units (SI Brochure 9th ed.)
Official BIPM publication defining the seven SI base units (including the meter) and the rules for their use. The global authority on units of measurement.
- NIST — Guide to the SI
US National Institute of Standards and Technology reference covering the SI base and derived units with definitions and usage rules for US technical practice.
- NIST Special Publication 811 — Guide for the Use of the International System of Units
Detailed NIST guide covering exact conversion factors between SI and US customary units along with formatting and rounding conventions.
- NIST — Refinement of values for the yard and pound (Federal Register 1959)
The treaty (signed by US
- International Astronomical Union — System of Astronomical Constants
The IAU defines astronomical units including the AU (149597870700 m exactly) light-year and parsec used in astronomy and astrophysics.