What this calculator does
Astronomical distances span so many orders of magnitude that no single unit works across them. Kilometres suit the solar system poorly and interstellar space not at all. The astronomical unit handles planetary distances, light-years and parsecs handle stellar ones, and each is defined exactly rather than measured.
The parsec is the one that sounds arbitrary and is not. It is the distance at which one astronomical unit subtends one arcsecond, which makes it 648,000/π astronomical units and about 3.26 light-years. It exists because it falls straight out of parallax measurement, which is how stellar distances were first determined.
The formula
Every unit has an exact factor in kilometres. The astronomical unit is the IAU-defined 149,597,870.7 km. The light-year is the distance light travels in a Julian year of 365.25 days at exactly 299,792,458 m/s. The parsec follows from the arcsecond definition. Converting multiplies by the source factor and divides by the target.
| Term | Meaning |
|---|---|
| Astronomical unit (au) | Exactly 149,597,870.7 km, roughly the Earth to Sun distance. |
| Light-year (ly) | The distance light covers in a Julian year, about 9.46 trillion km. |
| Parsec (pc) | The distance at which 1 au subtends 1 arcsecond, about 3.26 light-years. |
| Light-minute | The distance light covers in a minute, about 18 million km. Useful inside the solar system. |
The inputs explained
| Field | What to enter |
|---|---|
| Value | The distance value to convert. |
| From | The unit it is currently in. |
| To | The unit you want the answer in. |
When to use it
Making a distance intelligible
A figure in kilometres becomes meaningless past a certain scale. Converting to light-minutes or light-years replaces an unreadable number with one that also tells you how old the light is.
Reading a research paper or catalogue
Professional astronomy works in parsecs, kiloparsecs and megaparsecs while popular writing uses light-years. Converting is needed to move between the two.
Working through an astronomy problem
Parallax gives distance in parsecs directly, and most other steps want astronomical units or metres, so the conversion sits in the middle of the working.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
What is one of each astronomical unit in kilometres?
One unit of each kind, converted to kilometres to show the scale between them.
| From | In kilometres |
|---|---|
| Light-minute | 17,987,547.48 km |
| Astronomical unit | 149,597,870.7 km |
| Light-year | 9,460,730,473,000 km |
| Parsec | 30,856,775,810,000 km |
Converting 1 light-year to the other units
A single light-year expressed in each of the other astronomical units.
| To | Result |
|---|---|
| Kilometres | 9,460,730,473,000 km |
| Astronomical units | 63,241.07708 au |
| Parsecs | 0.30660139 pc |
| Light-minutes | 525,960 lmin |
Questions
How far is a light-year?
About 9.46 trillion kilometres, or 63,241 astronomical units. It is a distance rather than a time despite the name, being how far light travels in one Julian year of 365.25 days.
What is a parsec and why is it used?
The distance at which one astronomical unit subtends an angle of one arcsecond, about 3.26 light-years. It is used because it comes directly out of parallax measurement: a star with a parallax of one arcsecond is one parsec away, with no conversion needed, and the relationship is simply distance in parsecs equals one over the parallax in arcseconds.
Which year does a light-year use?
The Julian year of exactly 365.25 days, which is the IAU convention. Using a calendar year of 365 days would give a figure about 0.07% smaller. The difference rarely matters but the definition is fixed, so the light-year is an exact quantity rather than an approximate one.
How long does light take to reach us from the Sun?
About 8.3 minutes, since the Earth is one astronomical unit away and an astronomical unit is roughly 8.3 light-minutes. Entering 1 au and converting to light-minutes gives the figure directly.
For light travel time rather than distance, see the time converter. For terrestrial lengths, see the length converter.