Last updated: July 16, 2026
Astronomical Unit Calculator
Creators
Dharmendra SinghReviewers
Creators
Dharmendra SinghReviewers
Quick Answer
The astronomical unit (AU) is the standard unit for measuring distances within our solar system, defined exactly as 149,597,870,700 metres by the IAU in 2012. This calculator converts AU to and from kilometres, miles, light years, parsecs, solar radii, and metric prefixed units using the exact two-step bridge formula: result = (value / fromFactor) × toFactor.
One astronomical unit equals exactly 149,597,870,700 metres, or approximately 149.6 million kilometres and 92.96 million miles — the average distance from the Earth to the Sun.
Key Takeaways
- 1 astronomical unit (AU) is exactly 149,597,870,700 metres by the IAU 2012 definition.
- The AU is the natural unit for describing solar system distances; Earth is 1 AU from the Sun by definition.
- 1 AU equals approximately 149.6 million km, 92.96 million miles, or 8 light-minutes.
- Larger astronomical distances are measured in light years (1 ly ≈ 63,241 AU) or parsecs (1 pc ≈ 206,265 AU).
- Conversion uses a two-step bridge: input → AU → output, using exact IAU factors.
Creators
Dharmendra SinghReviewers
Creators
Dharmendra SinghReviewers
Formula
result = (value / FACTOR_from) × FACTOR_to
Where:
- 1 AU=1 Astronomical Unit (IAU 2012)(m)
- F=Conversion Factor (units per 1 AU)(varies)
- value=Input value to be converted(varies)
Worked Examples
Earth–Sun Average Distance
Convert 1 astronomical unit to kilometers — the defining distance from Earth to the Sun.
- 1Look up the IAU 2012 conversion factor: 1 au = 149,597,870,700 m = 149,597,870.7 km
- 2Divide input by fromUnit factor to get AU: 1 ÷ 1 = 1 AU
- 3Multiply by toUnit factor: 1 × 149,597,870.7 = 149,597,870.7 km
Mars Average Distance in Kilometers
Mars orbits the Sun at an average distance of 1.524 AU. Convert to kilometers.
- 1Mars semi-major axis = 1.524 AU (IAU planetary fact sheet)
- 2Convert to AU bridge: 1.524 ÷ 1 = 1.524 AU
- 3Apply km factor: 1.524 × 149,597,870.7 = 227,987,154.9 km
One Parsec to Astronomical Units
A parsec is the parallax-defined unit; 1 pc equals approximately 206,265 AU.
- 1Parsec factor: 1 au = 4.848137 × 10⁻⁶ pc, so 1 pc = 1/4.848137e-6 AU
- 2Calculate: 1 ÷ 4.848137e-6 = 206,264.8 AU
- 3Result: 1 parsec ≈ 206,265 astronomical units
Introduction
An astronomical unit (au) is the mean distance between the Earth and the Sun, formally defined by the International Astronomical Union in 2012 as exactly 149,597,870,700 metres. It is the natural yardstick of the solar system, making distances to planets, asteroids, and comets immediately intuitive. This calculator converts any value between the au and 13 other length units — from millimetres to light years — using the exact IAU 2012 definition.
What Is an Astronomical Unit?
The astronomical unit (symbol: au) is a unit of length equal to exactly 149,597,870,700 metres (about 149.6 million kilometres or 92.96 million miles). It was historically defined as the semi-major axis of Earth's orbit around the Sun, but the IAU voted in 2012 to fix it as an exact SI-derived constant so that planetary ephemerides could be expressed in a unit independent of the Gaussian gravitational constant. Before that resolution, the au was determined empirically each time better measurements of the solar system became available. Today, radar ranging of planetary surfaces and spacecraft transponder tracking make the au one of the most precisely known length standards. For context, light takes approximately 8 minutes and 20 seconds to travel 1 au, and the full round trip for a radar signal to Venus at closest approach takes about 5 minutes. You can compare distances with our distance converter or explore further with the length converter.
IAU 2012 Exact Definition
Resolution B2 of the 28th General Assembly of the International Astronomical Union (Beijing, 2012) defined: 1 au = 149,597,870,700 m (exact). This value replaced the earlier dynamical definition and is now part of the SI system of units at the level of a conventional constant. The precision of this value — 12 significant figures — comes from decades of radar measurements and laser ranging. For comparison, the previous best dynamical estimate was 149,597,870,691 ± 6 m, a difference of only 9 metres. The IAU definition is maintained by the Bureau International des Poids et Mesures (BIPM) and is consistent with modern relativistic celestial mechanics. See the full resolution at https://www.iau.org/administration/resolutions/general_assemblies/ and the NIST value at https://physics.nist.gov/cgi-bin/cuu/Value?au.
Complete Conversion Factor Table
All conversions use the two-step AU bridge: convert the input to AU, then from AU to the target unit. The conversion factors relative to 1 AU are: kilometres — 149,597,870.7 km; miles — 92,955,807.267 mi; metres — 149,597,870,700 m; light years — 1.5812507 × 10⁻⁵ ly; parsecs — 4.848137 × 10⁻⁶ pc; solar radii — 214.94 R☉; megameters — 149.5978707 Mm; gigameters — 0.1495978707 Gm. The light year value is based on the IAU 2012 value of the speed of light (299,792,458 m/s) and the Julian year (365.25 days). The parsec value derives from the definition 1 pc = 648,000/π au ≈ 206,264.806 au. See https://ssd.jpl.nasa.gov/astro_par.html for NASA/JPL's authoritative parameter set.
Solar System Distances in AU
The AU is perfectly scaled for inner-solar-system distances. Mercury orbits at 0.387 AU (57.9 million km); Venus at 0.723 AU (108.2 million km); Earth at 1.000 AU by definition; Mars at 1.524 AU (227.9 million km). The asteroid belt ranges from about 2.2 to 3.2 AU. The giant planets are farther: Jupiter at 5.2 AU, Saturn at 9.5 AU, Uranus at 19.2 AU, and Neptune at 30.1 AU. Pluto's highly elliptical orbit averages 39.5 AU, while the hypothetical Oort Cloud extends from roughly 2,000 to 100,000 AU. The Voyager 1 spacecraft, launched in 1977, is now beyond 160 AU — the farthest human-made object from the Sun. Convert any of these distances to kilometres or miles with our kilometers to miles calculator.
Historical Development of the AU
The concept of measuring Earth's distance from the Sun dates to ancient Greece. Aristarchus of Samos (~310–230 BC) made the first known geometric estimate, obtaining a ratio of about 20:1 for the Sun-to-Moon distance. Johannes Kepler's third law (1619) enabled relative orbital distances but not the absolute AU value. The first serious measurement came from Giovanni Cassini's parallax measurement of Mars in 1672, yielding about 140 million km. The transit-of-Venus method (Halley, 1716) was used in the expeditions of 1761 and 1769 — including Captain Cook's voyage — to obtain the au to within 1%. By the mid-20th century, radar measurements of Venus (Lincoln Laboratory, 1958; Jet Propulsion Laboratory, 1961) raised precision to parts per million. The 2012 IAU definition fixed the au forever. A thorough historical account is available at https://www.britannica.com/science/astronomical-unit.
Practical Applications of AU Conversions
Astronomers use AU daily for orbit calculations, mission planning, and communicating distances intuitively. NASA's Horizons system outputs ephemerides in AU and AU/day. Mission designers at JPL convert AU to kilometres for trajectory calculations, using 1 au = 149,597,870.7 km as the working value. Spacecraft communications engineers use light-travel time (1 au ≈ 499.004 s) to compute round-trip signal delays. Educators use the AU to introduce scale: if the Sun were a basketball (24 cm), the Earth would be a peppercorn 26 metres away. Our meters to feet converter can help bridge metric–imperial gaps in these scale models. For engineering-scale distance work, the length converter handles all SI prefixes.
Quick Reference Card
Astronomical Unit Quick-Reference
Quick reference • Astronomical Unit Calculator
result = (value / FACTOR_from) × FACTOR_toValid range: Any positive distance value; AU factors span from 10⁻⁶ (parsec) to 10¹⁴ (millimetre)
Common Values
⚠ Watch Out
- •Do not confuse the AU with the 'gold unit' (sometimes also abbreviated AU in chemistry contexts).
- •Light-year and parsec values are very small fractions of 1 AU; use scientific notation to avoid losing precision.
- •The solar radius factor (214.94) is an average — the Sun's equatorial radius differs slightly from its polar radius.
- •For distances beyond a few thousand AU, relativistic corrections to light travel time may matter for precision ephemerides.
Pro Tips
- →Use AU for any measurement within the solar system; switch to light-years for nearby stars.
- →For mission design, NASA JPL provides DE series ephemerides with AU-precision positions for all major solar system bodies.
- →Memorise: 1 AU ≈ 8 light-minutes, 1 ly ≈ 63,241 AU, 1 pc ≈ 3.26 ly ≈ 206,265 AU.
- →When converting AU to metres, use 1.496 × 10¹¹ m for back-of-envelope calculations.
FAQs
What is 1 astronomical unit in kilometers?
Exactly 149,597,870.7 kilometres, based on the IAU 2012 definition of 1 au = 149,597,870,700 metres.
How far is 1 AU in miles?
1 AU equals approximately 92,955,807 miles (about 93 million miles). This is the average Earth–Sun distance converted using 1 mile = 1,609.344 metres.
How many light years is 1 AU?
1 AU is approximately 1.5813 × 10⁻⁵ light years. Conversely, 1 light year equals about 63,241 AU. Light crosses 1 AU in roughly 8 minutes 20 seconds.
What is the difference between an AU and a parsec?
A parsec (pc) is defined as the distance at which 1 AU subtends a parallax angle of 1 arcsecond. Therefore, 1 pc = 206,264.806 AU ≈ 3.086 × 10¹³ km. While the AU is used for intra-solar-system distances, parsecs are used for stellar and galactic distances.
Why did the IAU redefine the AU in 2012?
The pre-2012 definition tied the AU to the Gaussian gravitational constant, which is not an SI unit and changed as measurement precision improved. The 2012 redefinition fixed 1 au = 149,597,870,700 m exactly, making it a conventional constant independent of gravitational theory and consistent with General Relativity-based ephemerides.
How many AU is it to the nearest star?
Proxima Centauri, the nearest known star to the Sun, is approximately 1.295 parsecs or about 268,000 AU (roughly 4.22 light years) away. Alpha Centauri A and B are slightly farther at about 277,000 AU.