Light Year, Parsec and Astronomical Unit Converter
Light Year, Parsec and Astronomical Unit Converter
Every astronomical distance unit, both ways, from exact definitions rather than rounded textbook factors. The parsec is 648 000/π au exactly, the au is 149 597 870 700 m exactly and the light year is 9 460 730 472 580 800 m exactly — so 1 pc = 3.261 564 ly is a definition, not a measurement.
Astronomical distances, any unit to any unit
A star catalogue gives a distance of 1 parsec. Express it in light years, in astronomical units and in metres, and check the chain.
Three exact definitions, and everything else is division
- au
- astronomical unit. Roughly the Earth–Sun distance, but since IAU 2012 Resolution B2 it is a fixed conventional length, not a measurement of the orbit.
- ly
- light year: the distance light travels in one Julian year of 365.25 days. 299 792 458 × 31 557 600 metres, exactly.
- pc
- parsec: the distance at which 1 au subtends one arcsecond. The 648 000/π is simply the number of arcseconds in a radian.
- kpc, Mpc
- a thousand and a million parsecs. Galaxy-scale and cosmology-scale respectively; the Milky Way’s disc is about 30 kpc across.
- light-day
- a distance, not a time. 2.59 × 1013 m, or about 173 au — roughly four times the distance to Neptune.
Worked example
A star catalogue gives a distance of 1 parsec. Express it in light years, in astronomical units and in metres, and check the chain.
Parsec to au is a pure definition. 648 000/π = 206 264.806 247 au. No measurement is involved: that is the number of arcseconds in a radian, and the parsec is defined by it.
Parsec to metres. 206 264.806 247 × 149 597 870 700 m = 3.085 677 581 × 1016 m, or 30.857 trillion kilometres.
Parsec to light years. Divide by the light year: 3.085 677 581 × 1016 ÷ 9.460 730 473 × 1015 = 3.261 564 ly. The familiar “3.26” is this number rounded to three figures; using 3.26 instead introduces an error of 5 parts in 10,000, which matters once you are quoting four figures.
Sanity-check it the other way. One light year is 0.306 601 pc, and 1/3.261564 = 0.306601. Round trips in a conversion table are the cheapest error check there is.
And in au, as a feel for the scale. One light year is 63 241 au. So the nearest star, at 4.2465 ly, is about 268,000 times further away than the Sun — which is the single fact that makes interstellar travel hard.
The whole ladder, in metres, so you can see the gaps
| Unit | In metres | In au | In light years |
|---|---|---|---|
| 1 m | 1 | 6.685 × 10−12 | 1.057 × 10−16 |
| 1 km | 1000 | 6.685 × 10−9 | 1.057 × 10−13 |
| 1 light-second | 299 792 458 | 0.002 004 | 3.169 × 10−8 |
| 1 light-minute | 1.799 × 1010 | 0.120 234 | 1.901 × 10−6 |
| 1 au | 149 597 870 700 | 1 | 1.581 × 10−5 |
| 1 light-day | 2.590 × 1013 | 173.144 | 0.002 738 |
| 1 ly | 9.461 × 1015 | 63 241.077 | 1 |
| 1 pc | 3.086 × 1016 | 206 264.806 | 3.261 564 |
| 1 kpc | 3.086 × 1019 | 2.063 × 108 | 3 261.564 |
| 1 Mpc | 3.086 × 1022 | 2.063 × 1011 | 3 261 564 |
Some real distances, as a scale check
| Object | Distance | In au | In light years |
|---|---|---|---|
| Earth to the Moon | 384,400 km | 0.00257 | 0.0000000406 |
| Earth to the Sun | 1 au | 1 | 0.0000158 |
| Sun to Neptune | 30.1 au | 30.1 | 0.000476 |
| Voyager 1, 2026 | about 170 au | 170 | 0.00269 |
| Proxima Centauri | 1.3020 pc | 268,551 | 4.2465 |
| Sirius | 2.639 pc | 544,378 | 8.608 |
| Centre of the Milky Way | about 8 kpc | 1.65 × 109 | about 26,000 |
| Andromeda Galaxy | about 0.78 Mpc | 1.6 × 1011 | about 2.5 million |
Why there are four units for one quantity, and which one a professional uses
Astronomy uses four different units for distance because it works at four different scales, and each unit was chosen to make the numbers at its own scale readable. Inside the Solar System the astronomical unit puts every planet between 0.4 and 30. Between stars, the light year and the parsec put the neighbours between 1 and 100. Between galaxies, the megaparsec does the same job. Writing any of it in metres is correct and useless: the nearest star is 4 × 1016 m away, and nobody has an intuition for that.
All three of the defining constants are now exact, which is newer than most textbooks. The astronomical unit used to be defined by the dynamics of the Solar System and had a measured value that improved with every spacecraft; in 2012 the IAU simply fixed it at 149 597 870 700 m and moved on. The parsec followed in 2015, fixed at exactly 648 000/π au. The light year has always been exact once the metre was defined by the speed of light, because it is c multiplied by a Julian year of exactly 365.25 days. So every conversion on this page is a definition, and if you see one quoted with an uncertainty, the uncertainty belongs to something else.
The parsec beats the light year for working astronomers for one reason: d = 1/p. A parallax of 0.05 arcseconds is 20 parsecs, immediately, with no constant to look up. In light years the same measurement is 65.2, which is a number you have to compute. Since parallax is the measurement that underpins all the others, the unit that makes parallax trivial wins. The light year survives in public writing because it carries a meaning the parsec does not: look at an object 100 light years away and you are seeing it as it was in 1926.
A light year is a distance. A light-day and a light-minute are distances too. This sounds obvious written down and still costs marks every year, because the unit is named after a time. The Sun is 8.3 light-minutes away, meaning 8.3 × the distance light covers in a minute; it is not 8.3 minutes away in any sense that makes dimensional sense. The useful consequence is real, though: light-travel time and distance are numerically the same in these units, so an object 2.5 million light years away is also seen 2.5 million years in the past.
Where the rounding bites. The familiar factor 3.26 for parsecs to light years is the exact 3.261 563 777 rounded to three figures, an error of about 0.05%. That is invisible in a two-figure answer and visible in a four-figure one. Worse is 9.46 × 1015 m for the light year, which many books give: the exact value is 9.460 730 472 580 800 m, so 9.46 is good to four figures but 9.5 is not. The factors in this converter are carried to twelve or more figures, which is far more than any astronomical measurement deserves and exactly right for a unit conversion — the conversion should never be the limiting error in your answer. One caveat in the other direction: the metre row of the converter is limited by the arithmetic, not by the definition. A parsec is exactly 30 856 775 814 913 673 m, but the calculator works in double-precision floating point, which carries about sixteen significant figures, so it prints the last two or three digits of that row wrongly. Read the metre row to four or five figures and take the exact value from the definition if you need all seventeen.
One warning about the light year specifically. It is defined with the Julian year of 365.25 days, not the tropical year of about 365.2422 days and not a calendar year of 365. The difference is 1 part in 15,000, which is larger than the precision of several of the digits people happily quote. If you recompute the light year yourself from c and “a year”, use 31 557 600 seconds and you will match the IAU value exactly; use 31 536 000 and you will not.
Which unit to use, in one line each. Inside the Solar System, the astronomical unit: every planet is a one- or two-digit number and the inverse-square law is trivial to apply. For spacecraft telemetry and radio links, light-minutes and light-hours, because the signal delay is then the same number. Between stars, the parsec if you are doing astronomy and the light year if you are explaining it to somebody. Across the Galaxy, kiloparsecs — the Sun is about 8 kpc from the centre. Between galaxies and in all of cosmology, megaparsecs, because that is the unit the Hubble constant is quoted per. Metres only when a formula demands SI, which Cambridge International’s syllabus explicitly does for Hubble’s law.
The commonest mistake here is not an arithmetic one. It is converting in the wrong direction, and it is easy to do because both of the obvious factors are close to 3. Parsecs to light years is a multiplication by 3.2616, because a parsec is bigger; light years to parsecs is a division by the same number, or equivalently a multiplication by 0.3066. Getting it backwards is a factor of 10.6 out — big enough to notice if you have a feel for the answer and completely invisible if you do not. The check that costs nothing: a star you can see with your eyes is a few parsecs to a few hundred parsecs away, never a few thousandths and never a few million.
The diagram above exists to make the gaps between these units visible rather than nominal. On a logarithmic axis the units are not evenly spaced: metre to kilometre is three decades, kilometre to au is eight, au to parsec is five, and parsec to megaparsec is six. Those gaps are the reason a single unit cannot serve the whole subject, and they are also why the blue row in the diagram — your number, read in each unit — spreads across most of the axis for any value you type. A number is meaningless until the unit is attached, and at these scales a missing unit is not a small error.
Frequently asked questions
Which is bigger, a light year or a parsec?
A parsec, by a factor of 3.261 564. One parsec is 3.26 light years, and one light year is 0.3066 parsecs. The memorable version is “a parsec is a bit over three light years”, and the direction is worth fixing in your head because getting it backwards produces answers wrong by a factor of 10.6.
Why is it 648 000 divided by pi?
Because that is how many arcseconds there are in one radian: 180 × 3600 ÷ π. The parsec is the distance at which 1 au subtends one arcsecond, and for a small angle the distance is the baseline divided by the angle in radians. So d = 1 au ÷ (1 arcsecond in radians) = 1 au × 648 000/π. The π is a unit conversion, not a circle.
Is the astronomical unit still the Earth–Sun distance?
Approximately, but not by definition any more. Since IAU 2012 Resolution B2 the au is a fixed conventional length of exactly 149 597 870 700 m. Earth’s actual distance from the Sun varies through the year by about 1.7% either side of that and changes slowly over long periods, so tying a unit to it was never satisfactory. The definition now stands on its own.
What is a light-day, and is it a time?
It is a distance: the distance light travels in one day, 2.59 × 1013 m or about 173 au. Every “light-” unit works this way — light-second, light-minute, light-day, light year — and all of them are lengths. The naming is genuinely confusing and the dimensional check is the cure: if the answer to a distance question comes out in seconds, something has gone wrong.
Why does the diagram show my number in every unit at once?
Because it has to, and the result is better than the alternative. The engine behind these pages hands the diagram the number you typed but not the unit you chose, because the unit is a text label and the diagram is drawn purely from numbers. Rather than guess a unit and risk drawing something that contradicts the answer, the diagram draws one row per unit: whichever unit you picked, the arrow in that row is correct and labelled. The side effect is useful — you can see at a glance that the same number means five wildly different distances.
Why are all five arrows in the diagram the same length?
Because the axis is logarithmic and multiplying by your number is the same number of decades whatever unit you start from. Ten of something is one decade to the right of one of that something, always. That is the property that makes a log scale worth drawing at all here: the gaps between the units carry the conversion factors, and your own number is a single rigid shift applied to all of them.
Should I write AU, au or A.U.?
The IAU’s recommended symbol, set in the 2012 resolution that fixed the value, is the lower-case au. You will meet AU, A.U. and ua in older literature — ua was the French-derived form used in earlier SI brochures — and none of them is wrong enough to lose a mark. For the others: ly for the light year, pc, kpc and Mpc for the parsec and its multiples, all without a full stop and all unchanged in the plural.
How do I convert a redshift or a parallax into these units?
Neither is a length, so neither belongs in this converter. A parallax in arcseconds becomes a distance in parsecs by taking the reciprocal, which the parallax page does; a redshift becomes a rough distance through Hubble’s law, which the Hubble’s law page does. Bring the answer here afterwards if you want it in light years.
Related calculators
References
- IAU 2012 General Assembly, Resolution B2, on the re-definition of the astronomical unit of length. The astronomical unit is now a conventional unit of length equal to exactly 149 597 870 700 m, with the recommended symbol
au. - IAU 2015 General Assembly, Resolution B2 on recommended zero points for the absolute and apparent bolometric magnitude scales (E. Mamajek et al., arXiv:1510.06262). Fixes Mbol = 0 at a luminosity of exactly L0 = 3.0128 × 1028 W and mbol = 0 at an irradiance of exactly 2.518 021 002 × 10−8 W m−2. The same resolution makes the parsec exactly 648 000/π astronomical units.
- BIPM, The International System of Units (SI), 9th edition (2019). The speed of light in vacuum is exactly 299 792 458 m s−1 by the definition of the metre; the astronomical unit appears in Table 8 as a non-SI unit accepted for use with the SI.
- Cambridge Assessment International Education, Cambridge International AS & A Level Physics 9702 syllabus for examination in 2025, 2026 and 2027, Topic 25 “Astronomy and cosmology”. 25.1 gives the radiant flux inverse-square law F = L/4πd² and the standard-candle idea; 25.3 gives Δλ/λ ≈ v/c and Hubble’s law v ≈ H0d, marked “SI units only”.