Convert Light Year to Millimeter
Convert light years to millimeters instantly. 1 light year = 9.46073e+18 millimeter — use the live calculator, the exact formula, a conversion table and worked examples. Also check the Millimeter 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.
Millimeter
A millimeter is a metric unit of length equal to one thousandth of a meter (1/1000 m). It is the standard small-scale unit in the SI system and is widely used wherever sub-centimeter precision matters.
The millimeter was defined alongside the meter when France adopted the metric system in 1795, using the standard SI prefix milli- (from Latin mille, "thousand") to denote one-thousandth.
Millimeters are used in engineering drawings, machining, manufacturing, paper sizes, rainfall measurement, and any context where dimensions below one centimeter need to be specified clearly. Most rulers worldwide are marked in millimeters.
Adopted in 1795 in France as part of the original metric system; standardized internationally by the Metre Convention in 1875 and confirmed in the SI in 1960.
Light Year to Millimeter conversion formula
The relationship between light years and millimeters:
To convert light years to millimeters, multiply the value in light years by 9.46073e+18. To reverse, multiply millimeters by 1.057001e-19.
How to use this converter
Type a value into the calculator. The result in millimeters updates as you type. Tap a quick value, copy the result with one click, or use the swap arrow to jump straight to the Millimeter to Light Year converter for the reverse direction.
Step-by-step: convert light years to millimeters
- Write down the value in light years (ly).
- Multiply that value by the factor 9.46073e+18.
- The product is the equivalent value in millimeters (mm).
- To reverse, multiply the millimeter value by 1.057001e-19.
Worked examples
Example 1 — Convert 1 ly to mm:
1 × 9.46073e+18 = 9.46073e+18 mm
Example 2 — Convert 100 ly to mm:
100 × 9.46073e+18 = 9.46073e+20 mm
Light Year to Millimeter conversion table
Standard reference values for converting light years to millimeters:
| Light Year [ly] | Millimeter [mm] |
|---|---|
| 0.01 | 9.46073e+16 |
| 0.1 | 9.46073e+17 |
| 1 | 9.46073e+18 |
| 2 | 1.892146e+19 |
| 3 | 2.838219e+19 |
| 4 | 3.784292e+19 |
| 5 | 4.730365e+19 |
| 10 | 9.46073e+19 |
| 20 | 1.892146e+20 |
| 30 | 2.838219e+20 |
| 40 | 3.784292e+20 |
| 50 | 4.730365e+20 |
| 100 | 9.46073e+20 |
| 500 | 4.730365e+21 |
| 1000 | 9.46073e+21 |
Frequently asked questions
How many millimeters is 1 light year?
How do I convert light years to millimeters?
How do I convert millimeters back to light years?
How many millimeters is 100 light years?
Popular length unit conversions
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Metric / SI (4 units)
Imperial / US Customary (4 units)
Astronomical (8 units)
Sources & references
Conversion factor (1 ly = 9.46073e+18 mm) 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.