Light-Year Distance Converter | km, Miles & AU

A light-year distance converter changes a distance given in light-years into other units, such as kilometers, miles, or astronomical units, and back again. A light-year is a unit of distance, not time, even though the name contains the word "year."

What is a light-year?

A light-year is the distance light travels through empty space in one Julian year, which lasts 365.25 days. Light moves at exactly 299,792,458 meters per second. Multiplying that speed by the number of seconds in a Julian year (31,557,600) gives the length of one light-year: 9,460,730,472,580,800 meters exactly.

One light-year equals:

• 9.4607 × 10¹² kilometers (about 9.46 trillion km)
• 5.8786 × 10¹² miles (about 5.88 trillion mi)
• 63,241.08 astronomical units (AU)

These figures follow from two fixed constants: the speed of light and the length of a Julian year. Neither one changes, so the light-year has an exact value in meters.

Light-year conversion formula

To convert a distance in light-years to another unit, multiply by the matching constant.

• Kilometers: d_km = d_ly × 9.4607 × 10¹²
• Miles: d_mi = d_ly × 5.8786 × 10¹²
• Astronomical units: d_AU = d_ly × 63,241.08

To convert the other way, divide by the same constant.

• d_ly = d_km ÷ (9.4607 × 10¹²)
• d_ly = d_mi ÷ (5.8786 × 10¹²)
• d_ly = d_AU ÷ 63,241.08

How to use the light-year converter

Enter a distance in light-years, then choose a target unit: kilometers, miles, or astronomical units. The converted value appears right away. To work in the other direction, type a value directly into the result field; the converter recalculates the matching number of light-years. A copy button lets you paste the result elsewhere. The converter does not accept negative numbers, because distance cannot be negative.

Very large results appear in scientific notation, such as 9.4607e+12 instead of 9,460,730,472,580.8. This keeps long numbers readable and avoids the display errors that can happen when a spreadsheet or text field truncates a 13-digit number.

Example: converting light-years to kilometers, miles, and AU

Proxima Centauri, the nearest known star to the Sun, is about 4.25 light-years away.

• Kilometers: 4.25 × 9.4607 × 10¹² ≈ 4.0208 × 10¹³ km
• Miles: 4.25 × 5.8786 × 10¹² ≈ 2.4984 × 10¹³ mi
• Astronomical units: 4.25 × 63,241.08 ≈ 268,774.58 AU

Even the nearest star outside the solar system sits roughly 40 trillion kilometers away.

Light-years compared with other distance units

Astronomical unit (AU)

One AU is Earth's average distance from the Sun, about 149.6 million kilometers. Astronomers use AU for distances within the solar system, where light-years produce awkward, tiny decimals. Neptune orbits at roughly 30 AU from the Sun, about 0.00047 light-years. Pluto's farthest point from the Sun, called its aphelion, is about 49.3 AU, or roughly 0.00078 light-years. Pluto's closest point, its perihelion, is about 29.7 AU. Its distance from the Sun therefore ranges from about 30 to 49 AU over one orbit.

Parsec

A parsec is about 3.26 light-years. It comes from parallax: the tiny apparent shift in a star's position when viewed from opposite sides of Earth's orbit. Astronomers often use parsecs in research because parallax measurements produce a distance in parsecs directly, with no conversion needed. Light-years are more common in writing aimed at a general audience, since most people find "4.25 light-years" easier to picture than "1.3 parsecs."

History of the light-year

Astronomers needed a workable unit for stellar distances once they could measure them. In 1838, Friedrich Bessel produced the first reliable distance to a star beyond the Sun, for 61 Cygni, using parallax. The term "light-year" came into common use later in the 19th century. It has stayed popular because it ties distance to something people already understand: the time light takes to travel.

Frequently asked questions

Is a light-year a unit of time or distance?

It measures distance, not time. It equals how far light travels in one year, in the same way "a day's drive" describes a distance rather than a length of time.

How fast does light travel?

Light travels at exactly 299,792,458 meters per second in a vacuum, about 186,282 miles per second.

How many kilometers are in a light-year?

One light-year equals 9.4607 × 10¹² kilometers, or about 9.46 trillion kilometers.

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

A light-year is based on how far light travels in a year. A parsec is based on stellar parallax and equals about 3.26 light-years.

Why does the converter reject negative numbers?

Distance cannot be negative, so a negative input produces an error instead of a converted value.

How precise are the conversions in this tool?

The tool uses the exact modern light-year definition: the speed of light, 299,792,458 meters per second, times one Julian year of 31,557,600 seconds, giving 9,460,730,472,580,800 meters exactly. Kilometer, mile, and AU results are calculated from that exact figure, then rounded for display.