Convert Astronomical Unit to Light Year
Convert astronomical units to light years instantly. 1 astronomical unit = 1.581251e-5 light year — use the live calculator, the exact formula, a conversion table and worked examples. Also check the Light Year to Astronomical Unit converter for the reverse conversion.
Units explained
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
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 to Light Year conversion formula
The relationship between astronomical units and light years:
To convert astronomical units to light years, multiply the value in astronomical units by 1.581251e-5. To reverse, multiply light years by 63241.0770842663.
How to use this converter
Type a value into the calculator. The result in light years updates as you type. Tap a quick value, copy the result with one click, or use the swap arrow to jump straight to the Light Year to Astronomical Unit converter for the reverse direction.
Step-by-step: convert astronomical units to light years
- Write down the value in astronomical units (AU).
- Multiply that value by the factor 1.581251e-5.
- The product is the equivalent value in light years (ly).
- To reverse, multiply the light year value by 63241.0770842663.
Worked examples
Example 1 — Convert 1 AU to ly:
1 × 1.581251e-5 = 1.581251e-5 ly
Example 2 — Convert 100 AU to ly:
100 × 1.581251e-5 = 0.0015812507 ly
Astronomical Unit to Light Year conversion table
Standard reference values for converting astronomical units to light years:
| Astronomical Unit [AU] | Light Year [ly] |
|---|---|
| 0.01 | 1.581251e-7 |
| 0.1 | 1.581251e-6 |
| 1 | 1.581251e-5 |
| 2 | 3.162501e-5 |
| 3 | 4.743752e-5 |
| 4 | 6.325003e-5 |
| 5 | 7.906254e-5 |
| 10 | 0.0001581251 |
| 20 | 0.0003162501 |
| 30 | 0.0004743752 |
| 40 | 0.0006325003 |
| 50 | 0.0007906254 |
| 100 | 0.0015812507 |
| 500 | 0.0079062537 |
| 1000 | 0.0158125074 |
Frequently asked questions
How many light years is 1 astronomical unit?
How do I convert astronomical units to light years?
How do I convert light years back to astronomical units?
How many light years is 100 astronomical units?
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Sources & references
Conversion factor (1 AU = 1.581251e-5 ly) 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.