Parsecs to light-years
A parsec is the distance at which one astronomical unit subtends an angle of one arcsecond — mathematically 648000/π AU. That equals about 3.26156 light-years, or roughly 30.857 trillion kilometres. Both units measure the vast distances between stars, so converting between them is a routine step in astronomy and astrophysics.
How it works
The converter uses the exact constant 1 parsec = 3.26156377716743 light-years and works in both directions:
light-years = parsecs × 3.26156
parsecs = light-years ÷ 3.26156Type a value in either field and the other updates instantly. Results are formatted with more decimals for small numbers and scientific notation for very large or very small values.
Example
Convert 1.3 parsecs (about the distance to Proxima Centauri):
- 1.3 × 3.26156 ≈ 4.240 light-years
And the reverse, 10 light-years:
- 10 ÷ 3.26156 ≈ 3.066 parsecs
| Parsecs | Light-years |
|---|---|
| 1 | 3.26156 |
| 1.3 | 4.240 |
| 10 | 32.6156 |
| 1000 | 3261.56 |
The conversion runs entirely in your browser — nothing is uploaded.
Why two units exist for the same thing
The light-year is intuitive — it is the distance light covers in one year and maps naturally onto human timescales. When you say “that star is 4 light-years away” you are also saying “the light you see left 4 years ago,” which has a pleasing physical interpretation.
The parsec exists because it falls directly out of the parallax method of measuring stellar distances. When the Earth moves from one side of its orbit to the other (a baseline of 2 AU), nearby stars appear to shift very slightly against the background of more distant stars. The angle of that shift, called parallax, is measured in arcseconds. A star whose parallax angle is exactly 1 arcsecond is, by definition, 1 parsec away. Because stellar parallax is how distances were first measured observationally, parsecs became the working unit of professional astronomy — they convert directly to a measurement without needing to know the speed of light.
Modern catalogues like the Hipparcos and Gaia star databases publish distances in parsecs or milliparsecs (mas parallax), so anyone working with those catalogues will encounter the unit constantly.
Scale context — putting parsecs and light-years in perspective
| Object | Distance (approx.) |
|---|---|
| Proxima Centauri (nearest star) | 1.30 pc / 4.24 ly |
| Sirius | 2.64 pc / 8.60 ly |
| The Orion Nebula | ~400 pc / ~1,300 ly |
| Galactic centre | ~8,000 pc (8 kpc) / ~26,000 ly |
| Andromeda Galaxy | ~780,000 pc (0.78 Mpc) / ~2.5 million ly |
At cosmological scales, megaparsecs (Mpc) become the natural unit — galaxy clusters and voids are measured in tens to hundreds of Mpc, while the observable universe spans about 14,000 Mpc. In everyday astronomical writing, nearby stars get parsecs or light-years interchangeably, but anything beyond our galaxy almost always appears in megaparsecs in the research literature.
Practical notes for converters
When reading star catalogues or exoplanet papers, you may see distances quoted as a parallax in milliarcseconds (mas). To convert: distance in parsecs = 1000 ÷ parallax(mas). For example, a parallax of 769 mas gives roughly 1.30 pc — consistent with Proxima Centauri. Multiply that by 3.26156 to get light-years.