Astronomical Distance Converter
Astronomical distances span an enormous range, from 150 million km to the Sun to 93 billion light-years to the observable universe boundary. Different fields of astronomy use different units: planetary scientists use AU, stellar astronomers use parsecs, and popular science favors light-years. This converter accepts a value in any of five units and simultaneously displays it in all others, using exact or best-known values from the IAU and NIST.
Conversion factors (exact or best-known values)
1 AU = 149,597,870,700 m (IAU 2012, exact)
1 ly = 9,460,730,472,580,800 m (c * Julian year)
1 pc = 3.085677581491367e16 m (180*3600/pi * AU)
1 pc = 3.26156 ly = 206,264.806 AU
All conversions go through meters as the base unit. The AU is exact by IAU definition. The light-year uses the exact speed of light (c = 299,792,458 m/s) and a Julian year of exactly 365.25 * 86,400 = 31,557,600 seconds. The parsec is derived from the AU and pi.
Distance reference landmarks
Moon: 0.00257 AU (384,400 km). Sun: 1 AU. Mars (mean): 1.52 AU. Neptune: 30.07 AU. Proxima Centauri: 1.30 pc (4.24 ly, 268,770 AU). Galactic center: about 8,178 pc (26,673 ly). Andromeda Galaxy: about 770,000 pc (2.51 million ly). Observable universe edge: about 14.25 Gpc.
Astronomical distances: frequently asked questions
What is an astronomical unit (AU)?
The astronomical unit is a unit of length defined by the IAU 2012 Resolution B2 as exactly 149,597,870,700 meters. It was originally defined as the average Earth-Sun distance, but the modern definition is a fixed exact value for consistency in orbital mechanics calculations.
How long is a light-year in meters?
One light-year is the distance light travels in one Julian year (365.25 days). Using c = 299,792,458 m/s and 31,557,600 seconds per Julian year: 1 ly = 9,460,730,472,580,800 meters, or approximately 9.461 x 10 to the 15th meters.
What is a parsec?
A parsec is the distance at which 1 AU subtends an angle of 1 arcsecond. One parsec = 180 * 3600 / pi AU = approximately 206,264.806 AU, or approximately 3.0857 x 10 to the 16th meters, or approximately 3.2616 light-years.
How do parsecs, light-years, and AU compare?
1 parsec = 3.26156 light-years = 206,265 AU. 1 light-year = 0.30660 parsecs = 63,241 AU. 1 AU = 149,597,870.7 km = 1.581 x 10 to the -5th light-years. Parsecs are preferred in observational astronomy; light-years in popular science.
Why are there different units for astronomical distances?
Different units suit different scales. AU is natural for solar system distances; light-years for interstellar distances up to tens of thousands of ly; parsecs for stellar and galactic astronomy. Megaparsecs (Mpc) are used for galaxy clusters and cosmological distances.
Official sources
- IAU 2012 Resolution B2 (AU definition): iau.org.
- NIST speed of light (c = 299,792,458 m/s): physics.nist.gov.
Reviewed by the CalculatorHub team, edited by James Graham, 15 June 2026. See our methodology.