Light Year Calculator

Background.

A light-year is a distance, not a time. It is exactly the distance a photon covers in a vacuum during one Julian year — 365.25 days of 86,400 SI seconds each — and because both the speed of light and the Julian year are fixed by definition rather than measured, the light-year has an exact value in metres: 9,460,730,472,580,800 m, which the International Astronomical Union publishes as 9,460,730,472,580.8 km. This calculator converts any distance you type into light-years, parsecs, astronomical units, kilometres, international miles, metres and light-minutes simultaneously, so you never have to chain two conversions and hope the rounding survives.

Every conversion factor used here is exact by definition, and that is the point of the page. The speed of light in vacuum has been the defining constant c = 299,792,458 m/s since the 2019 revision of the SI, so it carries no uncertainty at all. The astronomical unit was fixed by IAU 2012 Resolution B2 at exactly 149,597,870,700 m, ending decades in which it was a measured quantity tied to the Gaussian gravitational constant. The parsec was defined by IAU 2015 Resolution B2 as exactly (648000/π) astronomical units, which is the same thing as the distance at which one astronomical unit subtends an angle of one arcsecond, because there are exactly 648000/π arcseconds in a radian. The international mile has been exactly 1609.344 m since 1959. Nothing on this page is an estimate; the only irrational number involved is π, which the engine carries to 45 significant digits.

That matters because "light-year" is one of the few astronomical terms where different definitions genuinely circulate. If you use the tropical year (365.24219 days) instead of the Julian year, you get 9,460,528,404,879,000 m — about 200 billion metres smaller. Use the Gregorian calendar year (365.2425 days) and you get a third answer. The IAU settles this: its published definition is the distance travelled by light in one Julian year in a vacuum, and that is the only definition this calculator implements. If a figure you are checking against differs from ours in the fifth significant digit, the year definition is almost always the reason.

The three astronomical units of length answer different questions, and it is worth knowing which one a source is using. The astronomical unit is the natural ruler for the Solar System: Neptune's orbit is tens of au across, and spacecraft navigation is done in au. The light-year is the natural ruler for public communication, because it states a distance and a light-travel time in the same breath — light from an object 100 light-years away left it 100 years ago. The parsec is the natural ruler for research, because it falls straight out of the parallax measurement: a star with a parallax of one arcsecond sits at one parsec, so distances in parsecs are simply the reciprocal of parallaxes in arcseconds. One parsec is 3.2615637772 light-years, so professional catalogue distances always look about 3.26 times smaller than the same distance quoted in the popular press.

On precision and reading the results: all arithmetic runs at 40 significant digits and is rounded only once, at the point of output, to ten decimal places. That means the calculator will happily print far more digits than your input justifies. If you enter a parallax-derived distance good to four significant figures, only the first four digits of the answer are meaningful — the rest are exact arithmetic on an inexact input. Distances here are unsigned positive magnitudes; a zero or negative entry is rejected rather than silently converted, because neither has a physical reading as a separation between two objects.

What is light year calculator?

The light-year (symbol ly) is a unit of length equal to the distance electromagnetic radiation travels through a vacuum in one Julian year. Its value follows directly from two defined constants: the speed of light in vacuum, c = 299,792,458 m/s exactly, and the Julian year, 31,557,600 SI seconds exactly (365.25 × 86,400). Multiplying them gives 9,460,730,472,580,800 m — an exact integer number of metres, with no experimental uncertainty whatsoever.

The parsec (pc) is the unit professional astronomy actually uses. IAU 2015 Resolution B2 defines it as exactly (648000/π) astronomical units, equivalent to the geometric statement that a parsec is the distance at which one astronomical unit subtends one arcsecond of angle. That gives 1 pc = 3.0856775814913673 × 10^16 m = 3.2615637772 ly. The astronomical unit (au), fixed by IAU 2012 Resolution B2 at exactly 149,597,870,700 m, is a conventional unit chosen to be close to the mean Earth–Sun distance but no longer defined by it.

All three, together with the light-minute and light-second used for Solar-System travel times, are lengths with dimension L. None is a time, an angle or a speed. A light-year per year, if you ever meet it, is a speed equal to c. The page rejects zero and negative inputs because a distance is a positive magnitude — direction, where it matters, is carried by a separate vector, not by the sign of a separation.

How to use this calculator.

1. 01Type the distance you have into the Distance box as a positive number.
2. 02Choose which unit that number is in — light-years, parsecs, au, km, miles, metres, light-minutes or light-seconds.
3. 03Read the same distance expressed in every other unit at once. The primary result is in light-years.
4. 04Use the parsec figure when quoting to a catalogue or a research paper, and the light-year figure for public writing.
5. 05Check the scale band underneath: it tells you which defined-unit regime the distance sits in, from sub-light-second to extragalactic.

The formula.

Every result is a single multiplication followed by a single division. The engine first converts your input to metres by multiplying by the exact number of metres in your chosen unit, then divides that metre value by the exact number of metres in each target unit. There is no chained conversion, so no intermediate rounding error can accumulate.

The defining chain is: 1 ly = c × 31,557,600 s = 299,792,458 × 31,557,600 = 9,460,730,472,580,800 m. 1 au = 149,597,870,700 m by fiat (IAU 2012 B2). 1 pc = (648000/π) × 1 au, which evaluates to 3.0856775814913673 × 10^16 m. 1 mi = 1609.344 m. 1 light-minute = 60c = 17,987,547,480 m. Dividing the light-year by the light-minute gives 525,960 exactly, which is simply 365.25 days expressed in minutes — a useful arithmetic check that the Julian-year convention is the one in force.

Rounding stage: none of these steps rounds. All arithmetic is carried at 40 significant digits in a private high-precision decimal context, and rounding is applied exactly once, at the return boundary, to ten decimal places. Because π is irrational, the parsec conversion is the only one that is not exactly representable; carrying π to 45 digits makes its contribution to the rounded answer smaller than one part in 10^30, far below double-precision resolution.

Significant figures are your responsibility, not the calculator's. It prints ten decimal places regardless of how well your input is known. A distance derived from a parallax with a 0.4% uncertainty is meaningful to about three significant figures; the calculator will still print thirteen. Invalid-domain behaviour is explicit rather than silent: a distance of zero or below raises a field error and no result is produced, and a unit outside the supported list is rejected rather than defaulted.

A worked example.

Frequently asked questions.

References& sources.

1. [1]International Astronomical Union, "Measuring the Universe: the IAU and astronomical units" (IAU public archive, retrieved 2026-07-29). States that the astronomical unit is "a conventional unit of length equal to 149 597 870 700 m exactly" per IAU 2012 Resolution B2; that the light-year "is equal to the distance traveled by light in one Julian year in a vacuum", equal to 9,460,730,472,580.8 km or 63,241 au; and that the parsec is "the distance at which one Astronomical Unit subtends an angle of one arcsecond", about 30.857×10¹² km or about 206,000 au. Independent, free to access.
2. [2]Mamajek, E. E., et al. (2015), "IAU 2015 Resolution B2 on Recommended Zero Points for the Absolute and Apparent Bolometric Magnitude Scales", arXiv:1510.06262 (retrieved 2026-07-29). Resolution text states the parsec is "exactly (648 000/π) au", giving 3.085 677 581 × 10¹⁶ m. Peer-reviewed preprint of an adopted IAU resolution; open access.
3. [3]NIST/CODATA 2022, "Speed of light in vacuum": c = 299 792 458 m s⁻¹, exact (no standard uncertainty). Retrieved 2026-07-29. Independent standards body; open access.
4. [4]NIST Special Publication 330 (US edition of the BIPM SI Brochure, 9th ed.), Section 4, Table 8 — non-SI units accepted for use with the SI. Lists "1 au = 149 597 870 700 m" citing the 2012 IAU resolution, and notes that the light-year and parsec are not included in Table 8. Retrieved 2026-07-29. Independent standards body; open access.
5. [5]European Space Agency, Science & Technology education resource, "Stellar Distances" (retrieved 2026-07-29). States the parsec is "the distance at which an object has a parallax of one arcsecond", that d(parsec) = 1/p(arcsecond), and that "1 parsec = 3.26 light years". Independent of the IAU documents above; open access.
6. [6]SIMBAD astronomical database (CDS, Strasbourg), object "Proxima Centauri": parallax 768.0665 ± 0.0499 mas, reference 2020yCat.1350....0G (Gaia Early Data Release 3). Retrieved 2026-07-29. Source of the worked example's distance. Independent data centre; open access.