What is a light year?

A light year is the distance light travels in one Julian year (365.25 days) through a vacuum. The International Astronomical Union (IAU) defines it as exactly 9.4607304725808 × 10¹⁵ meters, or equivalently 9.4607304725808 × 10²⁰ millimeters.

Why are the numbers so large?

A single light year is roughly 9.46 × 10¹⁵ meters, and a millimeter is 10⁻³ meters. Combining those factors gives numbers with 20 or more digits, which is why scientific notation is essential for these values.

How accurate is the conversion?

The conversion constant (9.4607304725808 × 10²⁰ mm per light year) is exact by definition. The tool uses JavaScript's IEEE 754 double-precision floating-point, which gives about 15–17 significant digits — more than enough for any practical astronomy calculation.

Can I enter negative values or zero?

Zero is accepted and returns zero. Negative values are rejected with an inline error message because a negative distance has no physical meaning in this context.

Does this tool work offline?

Yes. All computation runs entirely in your browser. Once the page has loaded no network connection is required.

Light Years to Millimeters Converter

Name: Light Years to Millimeters Converter
Availability: InStock
Author: Nham Vu

Enter a distance in light years and instantly get the equivalent in millimeters using the exact IAU conversion factor.

Light Years to Millimeters

Distance in Light Years

Accepts decimals and scientific notation (e.g. 2.6e3).

Quick Examples

Result

Enter a value above to see the result.

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Summary

Enter a distance in light years and instantly get the equivalent in millimeters using the exact IAU conversion factor.

How it works

Enter any number of light years in the input field (decimals and scientific notation are accepted).
The converter multiplies your value by the IAU-defined constant 9.4607304725808 × 10²⁰.
The result is displayed both in standard decimal notation and in normalized scientific notation.
Copy the result with one click for use in other applications.

Use cases

Astronomy homework and exam problems requiring unit conversions.
Double-checking textbook or research paper figures involving stellar distances.
Engineering or physics calculations that need SI base units (mm).
Science communication — illustrating the sheer scale of interstellar distances.
Curriculum development for astronomy and physics courses.
Quick sanity-checking of simulation parameters in astrophysics software.

Frequently Asked Questions

Last updated: 2026-06-10 · Reviewed by Nham Vu