# Light-Year and Astronomical Unit Converter Convert between light-years (ly), parsecs (pc), astronomical units (AU), kilometers and meters using IAU-defined constants, with a parallax-angle → distance helper. > Canonical page: https://elysiatools.com/en/tools/light-year-and-astronomical-unit-converter - **Category:** Astronomy - **Keywords:** light-year, parsec, astronomical unit, distance, parallax, cosmic, space, converter, astronomy, au, ly, pc ## Overview Convert cosmic distance scales and compute distance from stellar parallax. Units: ly, pc, AU, km, m. Parallax helper: d(pc) = 1 / π(arcsec). ## Inputs - **Value** (number): The distance value to convert. - **From unit** (select): The unit of the value you entered. - **Decimal places** (number): Number of digits after the decimal point in the result. - **Scientific notation** (checkbox): Display results in scientific (exponential) notation. - **Parallax angle (arcsec) — optional** (text): Optional: a stellar parallax angle in arcseconds. Returns distance in pc/ly/AU/km via d(pc) = 1/π(″). ## When to use - When translating astronomical distances in research papers between light-years, parsecs, and astronomical units. - When calculating the distance of a star in parsecs or light-years using its measured stellar parallax angle in arcseconds. - When converting massive cosmic scales into standard metric units like kilometers or meters for educational comparisons. ## How it works - Enter the distance value you want to convert and select its starting unit from the dropdown menu. - Optionally, enter a stellar parallax angle in arcseconds to calculate the distance using the formula d(pc) = 1 / parallax. - Configure the output format by setting the preferred number of decimal places or enabling scientific notation. - The tool instantly computes and displays the equivalent values across all supported astronomical and metric units. ## Use cases - Converting exoplanet orbital distances from astronomical units (AU) to kilometers for public science communication. - Computing the distance to nearby stars like Proxima Centauri using parallax measurements from space telescopes. - Standardizing diverse distance units in astrophysics homework assignments or research papers. ## Frequently asked questions ### What constants are used for the conversions? The converter uses standard IAU-defined constants, where 1 AU is exactly 149,597,870,700 meters, and a light-year is the distance light travels in one Julian year. ### How does the parallax helper calculate distance? It uses the small-angle relationship where distance in parsecs (pc) equals 1 divided by the parallax angle in arcseconds (″). ### Can I convert very large values using scientific notation? Yes, you can check the scientific notation option to display results in exponential format. ### What is the relationship between a parsec and a light-year? One parsec (pc) is approximately equal to 3.26156 light-years (ly). ### Does this tool support conversions back to meters and kilometers? Yes, it converts any input unit into meters and kilometers alongside astronomical units. ## Related tools - [Codon to Amino Acid Translator](https://elysiatools.com/en/tools/codon-amino-acid-translator): Translate a DNA or mRNA sequence into protein using the standard genetic code. 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