The astronomical distance converter above uses exact definitional values, not the rounded approximations that circulate everywhere. That distinction matters more than it sounds. The astronomical unit stopped being a measured quantity in 2012 and became a defined number of metres. The parsec was given an exact definition in terms of the astronomical unit in 2015. The light-year has always been exact once the speed of light was fixed, provided you agree on which year. Every conversion on this page therefore has a right answer to as many digits as you care to carry.
Arb Digital publishes this as a separate tool from the length converter for a simple reason: that page handles millimetres through kilometres and miles, and stops there. It has no astronomical unit, no light-year and no parsec, because those units are useless below planetary scales and awkward to combine with everyday ones. This page picks up where it leaves off.
What This Astronomical Distance Converter Does
You give it a number and a unit. It converts to whatever unit you select in the second dropdown, and simultaneously reports the same distance in the four units astronomers actually use for scale: astronomical units for the Solar System, light-years for popular writing, parsecs for professional work, and kilometres so you have something to anchor against.
The subtitle under the headline gives the light travel time, which is the most intuitive translation of any astronomical distance. A light-year is a distance, but the year buried in it is a genuine travel time, and seeing that a number in parsecs corresponds to some number of years of light travel makes the scale land in a way that raw digits do not.
The note underneath the grid changes with the size of the distance, naming a familiar object at roughly the same scale. That is a sanity check as much as a curiosity: if you meant to type the distance to Mars and the tool says you are somewhere beyond the Local Group, a unit has gone astray.
How to Use It
- Pick the unit you have, then the unit you want. The two dropdowns are independent, so any pair works, including converting a unit to itself as a check.
- Use exponent notation for extreme values. Typing 2.5e6 for two and a half million is easier and less error-prone than counting zeros.
- Read the grid rather than only the headline. The four standard units are always shown, so you get the scale from several directions without changing the dropdown.
- Use light travel time when explaining a distance. Saying a galaxy is 2.5 million light-years away is the same statement as saying its light left 2.5 million years ago, and the second version usually lands harder.
- Check the scale note. If the object it names is wildly different from what you had in mind, you probably have the wrong unit selected.
The Exact Definitions This Converter Uses
The astronomical unit is defined as exactly 149,597,870,700 metres. That value was adopted by the International Astronomical Union in Resolution B2 of its 2012 General Assembly, which converted the AU from a quantity derived from the Gaussian gravitational constant into a fixed conventional length. Before that resolution the AU was tied to the dynamics of the Solar System and shifted slightly as measurements improved; since it, it is simply a number. The IAU 2012 resolutions document carries the wording.
The light-year is the distance light travels in vacuum in one Julian year of exactly 365.25 days. Since the second is defined and the speed of light is fixed by NIST and CODATA at exactly 299,792,458 m/s, the light-year is exact too: 365.25 × 86,400 × 299,792,458 = 9,460,730,472,580,800 metres. The Julian year matters. Using a tropical or a calendar year instead changes the figure in the fifth significant digit, which is why the IAU specifies the Julian year explicitly.
The parsec is defined as exactly 648,000 / π astronomical units, adopted in IAU Resolution B2 of 2015. That works out to 30,856,775,814,913,673 metres, or 3.26156 light-years, or 206,264.806 astronomical units. The odd-looking constant is not arbitrary: 648,000 is the number of arcseconds in a half-turn, and dividing by π converts the small-angle definition into a plain ratio.
Work through the default. One parsec is 206,264.806 AU by definition. Multiplying by 149,597,870,700 m gives 3.0857×10¹⁶ m, which is 3.0857×10¹³ km. Dividing by the light-year gives 3.2616 light-years, and light takes 3.26 years to cross it.
Why the Parsec Exists at All
The light-year is the unit everyone outside astronomy uses, and the parsec is the one working astronomers actually publish in. That looks like professional contrariness until you see where the parsec comes from.
The oldest reliable way to measure a stellar distance is trigonometric parallax. Observe a star six months apart, from opposite ends of Earth's orbit, and it appears to shift slightly against the far background. Half that shift is the parallax angle. A parsec is defined as the distance at which that angle is exactly one arcsecond — the name is a contraction of "parallax of one second".
The payoff is that the conversion from measurement to distance becomes a reciprocal. A star with a parallax of 0.1 arcseconds is 10 parsecs away; one at 0.02 arcseconds is 50 parsecs away. No constants, no unit factors, just one over the angle. In an era of hand computation that was decisive, and the convention outlived the reason. It remains genuinely convenient today, because the raw output of an astrometric survey is an angle and the parsec keeps it that way.
Scales Worth Carrying in Your Head
The three units are not interchangeable in practice, because each fits a particular regime and looks absurd outside it. The astronomical unit fits the Solar System: Earth is 1 AU from the Sun, Jupiter is 5.2, Neptune is 30, and the Voyager probes are somewhere past 150. Quoting any of those in light-years produces numbers with five leading zeros.
The light-year and the parsec fit stellar distances. Proxima Centauri, the nearest star, is 4.25 light-years or 1.30 parsecs away. The centre of our galaxy is about 8 kiloparsecs, or 26,000 light-years. Andromeda is roughly 780 kiloparsecs, or 2.5 million light-years. Beyond the Local Group, megaparsecs take over, and cosmological distances are quoted in hundreds or thousands of them.
One consequence is worth stating because it is genuinely strange. Looking at anything in the sky is looking into the past, by exactly the light travel time this tool reports. The Sun you see is eight minutes old. Proxima Centauri is four years old. Andromeda is two and a half million years old, which is older than our species. There is no way to see any of them as they are now, and the delay is not a limitation of instruments but of causality.
Where This Sits Next to Our Other Converters
The boundaries are clean. The length converter covers metric and imperial units from millimetres to miles and has no astronomical units in it. This page covers astronomical units and includes kilometres, miles and metres so you can cross between the two worlds without a second step. The unit converter is the general-purpose entry point when you are not sure which specific tool you need.
For the physics rather than the units, the Kepler's third law calculator relates orbital period to semi-major axis in astronomical units, the escape velocity calculator and the gravitational force calculator handle the dynamics, and the angular resolution calculator tells you what a telescope can actually separate at those distances. The scientific notation converter is useful when the exponents get unwieldy. Everything is indexed on the free online tools hub.
Arb Digital builds free tools like this one because useful pages earn attention. If you want tools, calculators or content built for your own audience, we can help.
Browse All Free Tools Talk to Arb DigitalCommon Mistakes to Avoid
- Treating the light-year as a time — it is a distance. The confusion is built into the name and survives into a surprising amount of published writing.
- Using a calendar year for the light-year — the definition specifies a Julian year of exactly 365.25 days. A 365-day year changes the answer in the fifth significant figure.
- Rounding the parsec to 3.26 light-years and then chaining conversions — the exact ratio is 3.261563777, and rounding early compounds through multi-step calculations.
- Quoting Solar System distances in light-years — the numbers are so small they lose all meaning. Use astronomical units below the scale of the nearest star.
- Mixing up kiloparsecs and megaparsecs — a factor of a thousand separates galactic from intergalactic scales, and the abbreviations differ by one letter.
Related Free Tools From Arb Digital
For terrestrial distances use the length converter, and for anything else the general unit converter. The Kepler's third law calculator works directly in astronomical units, while the escape velocity calculator and gravitational force calculator cover orbital dynamics. Pair this page with the angular resolution calculator to see what detail a telescope could resolve at the distance you converted, and use the scientific notation converter when the exponents get long. Everything is on the free online tools hub.
Frequently Asked Questions
Exactly 149,597,870.7 kilometres, or 149,597,870,700 metres. That value was fixed by definition in International Astronomical Union Resolution B2 of 2012, replacing an earlier definition derived from Solar System dynamics.
9,460,730,472,580,800 metres. It comes from a Julian year of exactly 365.25 days, which is 31,557,600 seconds, multiplied by the defined speed of light of 299,792,458 metres per second. Because both inputs are exact, the light-year is exact too.
A parsec is the distance at which a star shows a trigonometric parallax of one arcsecond over a baseline of one astronomical unit. The International Astronomical Union defined it in 2015 as exactly 648,000 divided by pi astronomical units, which works out to 206,264.806 astronomical units, or 3.261563777 light-years.
Because distance in parsecs is simply the reciprocal of the measured parallax in arcseconds, with no conversion constant. A star with a parallax of 0.1 arcseconds is 10 parsecs away. That relationship makes the parsec the natural unit for astrometric measurement, and the convention has stuck.
A distance. It is how far light travels in one Julian year in vacuum. The confusion is understandable given the name, but a light-year is a length in exactly the same sense that a kilometre is.
Because the length converter covers everyday metric and imperial units from millimetres to miles, where astronomical units would produce meaningless numbers. Keeping them apart lets each tool use a sensible list of options rather than one list that suits nobody.
Proxima Centauri is about 4.25 light-years away, which is 1.30 parsecs or roughly 268,000 astronomical units. Light from it left more than four years ago, so you are always seeing it as it was rather than as it is.
This tool is provided for educational and reference use. It converts between defined units exactly, but the distances to real astronomical objects are measured quantities with their own uncertainties, so treat any figure you enter for a real object as carrying that measurement error with it.
