Light Year Calculator

Background.

A light-year is 9,460,730,472,580.8 kilometres, about 5.88 trillion miles, and unlike most numbers in astronomy that figure is not a measurement. It is the distance light covers in a vacuum in one Julian year, and both ingredients are fixed by definition. The speed of light has been exactly 299,792,458 metres per second since the metre was redefined in terms of it in 1983, and a Julian year is exactly 365.25 days of 86,400 seconds. Multiply the two and you get a whole number of metres: 9,460,730,472,580,800.

The other units offered here are just as exact. The astronomical unit, the ruler for the Solar System, was fixed by the International Astronomical Union in 2012 at 149,597,870,700 m. The parsec, the unit professional astronomers use for stars and galaxies, was defined in 2015 as 648,000/π astronomical units, about 3.2616 light-years. The international mile has been 1,609.344 m since 1959. Converting between them is therefore pure arithmetic, done in one step: your distance becomes metres, and the metres are divided by the length of each unit.

What changes from unit to unit is the question it answers best. Light-minutes and light-years turn distance into delay: sunlight is 8.3167 minutes old when it reaches Earth, and the light now arriving from Proxima Centauri, the nearest star beyond the Sun, left about 4.25 years ago. Parsecs fall straight out of the way stellar distances are measured, by parallax, which is why catalogues and research papers use them. Astronomical units keep Solar-System numbers small enough to compare at a glance.

Because every factor is exact, the only uncertainty in an answer is the uncertainty in the number you type. A distance known to four figures remains a four-figure distance, however many digits appear on the screen.

What is light year calculator?

A light-year (ly) is a unit of length: the distance light travels through a vacuum in one Julian year of 31,557,600 seconds. With c = 299,792,458 m/s that comes to 9,460,730,472,580,800 m exactly, which the IAU publishes as 9,460,730,472,580.8 km. Despite the name it is not a unit of time, and a speed of one light-year per year would simply be the speed of light.

The parsec (pc) comes from geometry. As Earth orbits the Sun, a nearby star appears to shift against much more distant ones, and half of that yearly swing is the star's parallax angle. A star with a parallax of one arcsecond, 1/3,600 of a degree, is one parsec away, which is the distance at which one astronomical unit spans exactly that angle. A radian contains 648,000/π arcseconds, so 1 pc = 648,000/π au = 3.0857 × 10¹⁶ m, or 3.2616 ly. Larger distances are given in kiloparsecs and megaparsecs; a megaparsec is about 3.26 million light-years, and the expansion rate of the universe, the Hubble constant, is quoted in kilometres per second per megaparsec.

The astronomical unit (au) is 149,597,870,700 m, a conventional value close to the average Earth–Sun distance but no longer defined by it. The light-minute, 17,987,547,480 m, and the light-second, 299,792,458 m, are convenient inside the Solar System and for radio delays to spacecraft. None of these is an SI unit. The astronomical unit appears in the SI Brochure's list of non-SI units accepted for use with the SI; the light-year and the parsec do not, although both are standard in astronomy.

How to use this calculator.

1. 01Type the distance as a positive number in Distance. Zero and negative values are not accepted, and the largest entry is 10¹² of whichever unit you choose.
2. 02Choose its unit under 'Unit of the distance you entered': light-years, parsecs, astronomical units, kilometres, international miles, metres, light-minutes or light-seconds. Star catalogues normally give parsecs; news reports normally give light-years.
3. 03Read the light-year figure first and every other unit beneath it. Large values switch to scientific notation, so a metres line reading 4.017 × 10¹⁶ means about 40 quadrillion metres.
4. 04For distances inside the Solar System, rely on the kilometre, au and light-minute lines: the light-year and parsec values are then tiny fractions and keep only their leading digits.
5. 05Check the scale line, which places the distance between fixed unit boundaries: one light-second, one light-minute, 1 au, 1,000 au, one light-year, one parsec, a kiloparsec and a megaparsec.

The formula.

Every conversion passes through metres. The distance d you enter must be greater than zero; it is multiplied by the number of metres in its unit and then divided by the number of metres in each output unit: d(target) = d × m(from) ÷ m(target). Nothing is converted from one astronomical unit to another and then onward, so no error can build up along a chain.

The metre values are definitions. 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, from IAU 2012 Resolution B2. 1 pc = (648,000/π) × 149,597,870,700 m = 3.0857 × 10¹⁶ m, from IAU 2015 Resolution B2; π is the only irrational number involved and is carried to far more digits than any result shows. A light-minute is 60c = 17,987,547,480 m, and a mile is 1,609.344 m.

The 648,000/π in the parsec comes from small-angle geometry. A radian is 180/π degrees and each degree is 3,600 arcseconds, so a radian holds 180 × 3,600/π = 648,000/π, about 206,264.8 arcseconds. At a distance where a baseline of 1 au spans one arcsecond, the distance is the baseline divided by that angle in radians, which is 206,264.8 au.

Two ratios make useful checks. A light-year divided by a light-minute is exactly 525,960, the number of minutes in 365.25 days, so a figure built on a different year length fails it. A parsec divided by a light-year is 3.2616, not a round number, because one unit is defined by an angle and the other by a time.

Results are given to ten decimal places. That is ample from kilometres to megaparsecs but trims very small fractions: one light-second comes out as 3.17 × 10⁻⁸ ly, against 3.169 × 10⁻⁸ in full, and a millimetre shows as 0 ly. For distances that small, the kilometre and au lines hold the precision.

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.