Astronomical Unit Converter

Convert astronomical distances between AU, light-years, parsecs, kilometres, miles, and light-seconds with solar-system and interstellar reference context.

Last updated 7 March 2026

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Astronomy Distance

Astronomical unit converter: AU, light-years, parsecs, and light-seconds explained

An astronomical unit converter is useful when a distance moves between solar-system and interstellar scales. AU works well for planetary orbits, while light-years and parsecs are clearer once the span reaches nearby stars and beyond. A good converter keeps those scales visible together so the number stays intuitive instead of abstract.

About this calculator

Reviewed March 2026

Methodology

The converter maps every supported unit into metres first, then derives AU, light-years, parsecs, kilometres, miles, and light-seconds from that shared base using exact or standard astronomical definitions.

Limitations

This tool converts distance units only and does not model changing orbital positions, redshift, signal delay through media, or observational geometry.
Displayed values are rounded for readability even though the internal conversion factors are fixed or standard astronomical constants.

Disclaimer

Use this page as an educational and planning reference for astronomy distances. Research, navigation, and observational work should still use the source standards or specialist software appropriate to the task.

Sources

IAU Resolution B2 (2012) — astronomical unit re-definition — Primary IAU source defining the astronomical unit as an exact number of metres.
NASA Space Place — What is a light-year? — Accessible NASA explainer for light-year meaning and light-travel interpretation.
ESA — Measuring cosmic distances — ESA reference covering astronomical distance ladders including parallax and parsec-scale interpretation.

See our calculation methodology and editorial standards for how this estimate is produced.

Why AU and light-years are used for different scales

Astronomical units were designed around the solar system. One AU is the defined nominal distance from Earth to the Sun, so it gives an immediately readable scale for planetary orbits, comet paths, and inner-system geometry.

Light-years and parsecs are more practical once the distance becomes interstellar. At that scale, a value measured in kilometres or miles becomes too large to read comfortably, and AU counts quickly turn into long strings of digits.

How the conversion stays internally consistent

This page converts every input through metres first. From there it derives AU, light-years, parsecs, kilometres, miles, and light-seconds from one shared physical distance instead of chaining rounded shortcuts together.

The astronomical unit is defined by the International Astronomical Union as exactly 149,597,870,700 metres. The light-second follows directly from the defined speed of light, and the light-year is the distance light travels in one Julian year. Parsec values then follow from the standard angular definition used in astronomy.

1 AU = 149,597,870,700 m (exact)

IAU-defined astronomical unit used for solar-system distance work.

1 ls = 299,792,458 m (exact)

Light-second derived from the defined speed of light in vacuum.

1 ly ≈ 63,241.077 AU

Connects an interstellar light-travel unit back to the AU scale.

When to use each distance label

Use AU for orbital radii, asteroid distances, and solar-system comparisons where the Sun-Earth baseline is a useful mental anchor. Use light-years when the audience benefits from a light-travel interpretation, especially for nearby stars or broad public-facing astronomy content.

Use parsecs in more technical astronomical contexts because they connect directly to angular measurement and stellar parallax. Kilometres and miles are still helpful as supporting values when you need an Earth-scale sanity check or a bridge for non-specialist readers.

Frequently asked questions

Is one astronomical unit exactly the Earth-Sun distance today?

Not exactly in the orbital sense. Earth's real distance from the Sun changes over the year, but 1 AU is a fixed defined value used as a standard reference distance in astronomy.

Why does the converter show light-seconds too?

Because light-seconds make shorter astronomical distances easier to interpret. They provide a direct sense of signal-travel time and help bridge between Earth-scale and solar-system scales.

What is the difference between a light-year and a parsec?

A light-year is based on light travel over one Julian year. A parsec is based on stellar parallax geometry. Both describe the same kind of distance, but parsecs are more common in technical astronomy.

Can this converter replace an ephemeris or orbital model?

No. It converts defined distance units accurately, but it does not calculate changing orbital positions, relativistic effects, or observation geometry for a real astronomical event.

Astronomical Unit Converter

Astronomical unit converter: AU, light-years, parsecs, and light-seconds explained

Why AU and light-years are used for different scales

How the conversion stays internally consistent

When to use each distance label

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