Light-years, parsecs and astronomical units, all of them exact by definition — and the light-travel time, which is the part that makes the numbers mean something.
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1 light-year
1.0000 light-years
Light takes 1.0000 years to cross it. Comparable to the distance to proxima centauri.
The same distance in every unit
Ordered by size — every one of these is exact
Unit
Value
What it is
kilometre
9.4607e+12
Useful out to about the orbit of Mars, and unwieldy beyond it.
light-second
3.1558e+7
How far light travels in a second. The Moon is about 1.28 of these away.
light-minute
525,960.00
The Sun is 8.32 light-minutes away, which is the delay on everything you see of it.
astronomical unit
63,241.08
Exactly 149 597 870 700 m since 2012 — a defined length rather than a measurement of the Earth’s orbit.
light-hour
8,766.00
Roughly the scale of the outer solar system.
light-day
365.2500
Still well inside the Oort cloud.
light-year
1.0000
A DISTANCE, using the Julian year of exactly 365.25 days. Exact, because c and the year both are.
parsec
0.306601
The distance at which one au subtends one arcsecond — exactly 648000/π au. What professional astronomy actually uses.
kiloparsec
0.000307
Galactic scales. The centre of the Milky Way is about 8.2 of these away.
megaparsec
3.0660e-7
Intergalactic scales, and the unit the Hubble constant is quoted against.
Note where the astronomical unit sits: between the light-minute and the light-hour, because the Sun is 8.32 light-minutes away. And every figure here is exact. The metre is defined from the speed of light, the astronomical unit was fixed at exactly 149 597 870 700 m in 2012, and the Julian year is a definition — so a light-year is exactly 9 460 730 472 580 800 m rather than a measurement.
Which year is in a light-year
A light-year with a 365-day year9.4543e+12 km-0.0684% against the Julian year
The standard light-year9.4607e+12 km365.25 days, exactly
Difference6.4755e+9 kmover a single light-year
The year in a light-year is the Julian year of exactly 365.25 days — a defined quantity, not the calendar. Using 365 gives an answer 0.068% short, which sounds like nothing and is about six and a half billion kilometres over one light-year. It is also the reason two sources can quote slightly different figures and both be reporting honestly.
Where the parsec comes from
Distance1.3012 pc4.2441 light-years
One parsec is206,264.8062 auexactly 648000 ÷ π
A parsec is the distance at which one astronomical unit subtends one arcsecond — so distance in parsecs is exactly one over the parallax in arcseconds, which is the whole reason the unit was invented. The familiar 3.26 light-years is derived from that geometry rather than measured, and it is why professional astronomy uses parsecs while popular writing uses light-years.
Something to measure it against
Reference distances, across thirteen decades
Object
Light-years
Parsecs
Light takes
The Moon
4.0631e-8
1.2458e-8
1.282 s
The Sun
1.5813e-5
4.8481e-6
8.317 min
Neptune
0.000475
0.000146
4.168 h
Voyager 1
0.002609
0.000800
22.871 h
Proxima Centauri
4.2465
1.3020
4.2465 years
Sirius
8.6110
2.6401
8.6110 years
The galactic centre
26,670.00
8,177.06
26,670.00 years
The Andromeda galaxy
2,537,000.00
777,847.74
2,537,000.00 years
The last column is the useful one. Everything in the sky is seen as it was when the light left it — the Sun eight minutes ago, Proxima four years ago, Andromeda two and a half million years ago. That is the sense in which a light-year is about time, and it is a consequence of the distance rather than the meaning of the unit.
What this converter covers
Ten units ordered by size, from the kilometre to the megaparsec, with parallax and a ladder of real objects to measure against.
Kilometres, astronomical units, light-seconds through light-years, parsecs to megaparsecs
Light-travel time for any distance
What using a 365-day year instead of the Julian one costs
Distance from parallax, which is the parsec’s definition
Reference distances from the Moon to Andromeda
A distance, not a time Exact by definition Parallax and parsecs Ten units, ordered
Free, no signup — exact by definition, not an estimate.
Updated 8 September 2026
At a glance
Formula shown
1 ly = c × 365.25 × 86400 m · 1 pc = 648000/π au · distance in pc = 1 ÷ parallax in arcsec
Scenario support
1 pc is 3.261564 ly · a 365-day year is 6.5 billion km short over one light-year
Educational estimate
Planning support from the values you enter — not professional advice.
A light-year is a distance
It is the commonest misconception in astronomy and it is entirely about the word. “Year” is not naming a duration here — it is doing the work of “how far light travels in”.
A light-year is 9 460 730 472 580 800 metres, or about 9.46 trillion kilometres. It belongs in the same list as the kilometre and the mile, and the calculator above puts it there.
The units, in order of size
Unit
In kilometres
Scale
Light-second
299,792
Earth to Moon is 1.28
Light-minute
1.799 × 10⁷
Inner solar system
Astronomical unit
1.496 × 10⁸
Earth to Sun, by definition
Light-hour
1.079 × 10⁹
Outer solar system
Light-year
9.461 × 10¹²
Nearest stars are a few
Parsec
3.086 × 10¹³
3.26 light-years
Notice where the astronomical unit falls: between the light-minute and the light-hour, because the Sun is 8.32 light-minutes away. Ordering the units by size answers “is a parsec bigger than a light-year” without anyone having to ask.
Which year, exactly
A second and quieter question follows immediately: which year? The calendar year is 365 days, or 366 in a leap year. The tropical year is 365.2422. The light-year uses neither — it uses the Julian year of exactly 365.25 days.
That is a definition rather than an approximation of anything, chosen precisely so the unit stops depending on which year you happen to be in.
What the choice of year costs
Year used
Light-year (km)
Against the standard
365 days
9.4542 × 10¹²
−0.068%
365.25 days — Julian
9.4607 × 10¹²
—
366 days
9.4801 × 10¹²
+0.205%
Using 365 gives an answer 0.068% short. That sounds like nothing until it is written out: about six and a half billion kilometres, over a single light-year. Over the 26,670 light-years to the galactic centre it is larger than the entire solar system many times over.
It is also why two otherwise careful sources can quote slightly different figures for the same distance. The disagreement is usually about which year, not about the astronomy.
The parsec is geometry
A parsec is not defined as 3.26 light-years. It is defined by an angle, and the 3.26 falls out.
As the Earth moves around its orbit, a nearby star appears to shift against the distant background. Half the total shift over a year is the star’s parallax. A parsec is the distance at which that parallax is exactly one arcsecond — one 3600th of a degree.
The name is the definition contracted: a parallax of one arcsecond. And it makes the arithmetic of the observation trivial: distance in parsecs is one over the parallax in arcseconds, with nothing else in it. That is the entire reason the unit exists, and the reason professional astronomy uses it while popular writing uses light-years.
The geometry gives the exact value directly. There are 648000 arcseconds in half a turn, and the small-angle relation divides by π — so a parsec is exactly 648000/π astronomical units, or 206264.806. Multiply that by the astronomical unit and divide by the light-year and 3.261564 comes out.
One consequence worth knowing about real data: parallax gets harder with distance, because the angle shrinks. A star at 100 parsecs shows a parallax of 0.01 arcseconds, and at 1000 parsecs it is 0.001 — which is why space astrometry missions are built specifically to measure very small angles, and why distances beyond a few thousand parsecs come from other methods entirely.
Everything here is exact
Unusually for astronomy, none of the conversions on this page carries any measurement uncertainty at all. Three separate definitions are why.
The metre is defined from a fixed speed of light of exactly 299 792 458 m/s — so the speed of light is not measured, it is a definition, and measuring it more accurately just refines the metre.
The astronomical unit was fixed by the IAU in 2012 at exactly 149 597 870 700 m. Before that it was tied to the Earth’s orbit and the mass of the Sun, both of which are measured and both of which drift. Now it is simply a number.
The Julian year is 365.25 days of 86400 seconds, by definition. So a light-year is an exact integer of metres, and a parsec is exact up to π.
What is not exact is any distance to anything real. The conversions are definitional; the distances to Proxima Centauri or Andromeda are measurements with real uncertainties that grow with distance. Treating a converted figure as though it inherited the exactness of the conversion is the last mistake this page can help with.
Looking backwards in time
There is a sense in which the light-year is about time, and it is worth separating from the misconception, because it is the more interesting fact.
Nothing in the sky is seen as it is. Everything is seen as it was when the light left it, and the delay is exactly the distance in light-time.
How old the view is
Object
Distance
Light left it
The Moon
384,400 km
1.28 seconds ago
The Sun
1 au
8.32 minutes ago
Neptune
30.07 au
4.17 hours ago
Proxima Centauri
4.25 ly
4.25 years ago
The galactic centre
26,670 ly
26,670 years ago
Andromeda
2.54 million ly
2.54 million years ago
The Andromeda galaxy as it appears tonight is Andromeda before there were people to look at it. That is a consequence of the distance rather than the meaning of the unit — and it is why the light-year, for all the confusion it causes, is a genuinely useful thing to think in.
It also has a practical edge. A radio signal to a lunar mission takes 1.28 seconds each way, which is why conversation with the Moon has a noticeable pause and conversation with Mars — four to twenty-four light-minutes depending on the orbits — is not conversation at all.
Related calculators
Other distance, physics and notation tools:
LengthMillimetres to miles on the exact 1959 factors, with the mil kept clearly apart from the millimetre — they differ 25-fold.
Speedmph, km/h, m/s, ft/s and knots on exact factors, with the nautical mile behind the knot explained rather than assumed.
Scientific NotationScientific, engineering and decimal forms with significant figures counted — and ambiguous inputs flagged rather than silently resolved.
AngleDegrees, radians, gradians, arcminutes and arcseconds — and why a spreadsheet’s SIN(90) returns 0.894 rather than 1.
Frequency to WavelengthFrequency, wavelength, photon energy and wavenumber — in vacuum and in the medium, including coax velocity factors.
Gray to SievertGy, Sv, rad, rem, Bq and Ci grouped by what they measure — with every cross-quantity conversion refused, because two of them share a dimension and are not the same thing.
Everything on this page is exact, and three separate definitions are why. The metre comes from a fixed speed of light; the astronomical unit was fixed at exactly 149 597 870 700 m by the IAU in 2012, detaching it from any measurement of the Earth’s orbit; and the Julian year is a definition. The parsec then follows from geometry alone.
IAU 2012 Resolution B2 — the astronomical unitInternational Astronomical Union · verified 2026-09-08 · That the astronomical unit is a conventional unit of length equal to exactly 149 597 870 700 m, redefined in 2012 so that it no longer depends on a measurement of the Earth’s orbit or on the mass of the Sun
SI Brochure, 9th edition — the metre and the speed of lightBureau International des Poids et Mesures · verified 2026-09-08 · That the metre is defined from a fixed speed of light of exactly 299 792 458 m/s, which is what makes the light-year an exact length once the Julian year is fixed
IAU Style Manual — units and the definition of the parsecInternational Astronomical Union · verified 2026-09-08 · The Julian year of exactly 365.25 days as the year used in the light-year, and the parsec as exactly 648000/π astronomical units — the distance at which one astronomical unit subtends one arcsecond
Conversion note
The unit conversions here are exact; the distances to real objects are not. Stellar distances come from parallax measurements with their own uncertainties, and those uncertainties grow with distance — a nearby star is known to a fraction of a per cent while a galaxy's distance may carry several per cent or more, and different methods disagree. The figures for the reference objects are representative published values as of writing and are periodically revised, sometimes substantially. Distances to galaxies also depend on the cosmological model at large scales, where 'distance' itself splits into several distinct quantities that agree only nearby. Nothing here accounts for the expansion of the universe, which matters beyond the local group. Use the conversions with confidence and the object distances as approximations.
Published the Light Year Converter: kilometre, light-second, light-minute, astronomical unit, light-hour, light-day, light-year, parsec, kiloparsec and megaparsec, listed in ascending order of size so the ladder itself answers which unit is larger.
Places the astronomical unit between the light-minute and the light-hour rather than in a separate group, because the Sun is 8.32 light-minutes away and hiding that ordering is what makes people guess wrong about a parsec.
States the thing the name obscures: a light-year is a DISTANCE, 9,460,730,472,580,800 metres exactly, and the word year is doing the work of 'how far light travels in'.
Makes the choice of year explicit and prices it. The light-year uses the Julian year of exactly 365.25 days; a 365-day year gives an answer 0.068 per cent short, which is about 6.5 billion kilometres over a single light-year, and is why two careful sources can quote different figures for the same star.
Derives the parsec from its definition rather than from the familiar 3.26: it is exactly 648000/pi astronomical units, the distance at which one astronomical unit subtends one arcsecond, so distance in parsecs is simply one over the parallax in arcseconds.
Converts a measured parallax to a distance and back, and shows why the method runs out: a star at 1000 parsecs shifts by a thousandth of an arcsecond, which is why distances beyond a few thousand parsecs come from other methods entirely.
Records that every conversion here is EXACT, and says which three definitions make that true -- a fixed speed of light, the IAU's 2012 fixing of the astronomical unit at exactly 149,597,870,700 m, and the Julian year -- while warning that no distance to a real object inherits that exactness.
Reports the light-travel time for any distance, and gives a ladder of real objects from the Moon at 1.28 light-seconds to Andromeda at 2.54 million light-years, so a converted number has something to mean.
Verified by 47 automated cases, asserting the unit ladder ascends, that a parsec is 3.261564 light-years and 206264.806 astronomical units, that the light-year is an exact integer of metres, that parallax and distance round-trip, and that a non-positive parallax is refused rather than returned as a negative distance.
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