What is a Light Year?

The light-year is the standard astronomical distance unit, equal to 9.461 trillion kilometers traveled in one year.

Overview

The light-year equals approximately 9,460,730,472,580,800 meters (about 9.461 trillion km) and is the standard astronomical unit for stellar distances. Despite its name, a light-year is a unit of distance, not time — it represents how far light travels in vacuum during one Julian year (365.25 days) at the speed of light (299,792,458 m/s). The nearest star to our Sun, Proxima Centauri, is 4.24 light-years away. The Milky Way galaxy is about 100,000 light-years across, and the observable universe extends roughly 93 billion light-years in diameter. Astronomers more often use the parsec (3.26 light-years) for technical work, but the light-year remains popular in education and science communication because it intuitively conveys both distance and the time light needs to travel that far — which is why we see distant galaxies as they were millions of years ago.

Symbol

Convert Light Year to all units

Live result

Meter9,460,700,000,000,000 m Kilometer9,460,700,000,000 km Centimeter946,070,000,000,000,000 cm Millimeter9,460,700,000,000,000,000 mm Mile5,878,606,400,000 mi Yard10,346,347,331,583,552 yd Foot31,039,041,994,750,656 ft Inch372,468,503,937,007,900 in Nautical Mile5,108,369,300,000 nmi Micrometer9.461 × 10^21 µm Nanometer9.461 × 10^24 nm Furlong47,028,852,000,000 fur Fathom5,173,173,665,791,776 ftm Angstrom9.461 × 10^25 Å

Relationship to Other Length Units

1 ly equals

Visual reference for how the light year relates to other length units. Each row links to the full converter for that pair.

1 ly=9,460,700,000,000,000 m 1 ly=9,460,700,000,000 km 1 ly=946,070,000,000,000,000 cm 1 ly=9,460,700,000,000,000,000 mm 1 ly=5,878,606,400,000 mi 1 ly=10,346,347,331,583,552 yd 1 ly=31,039,041,994,750,656 ft 1 ly=372,468,503,937,007,900 in 1 ly=5,108,369,300,000 nmi 1 ly=9.461 × 10^21 µm 1 ly=9.461 × 10^24 nm 1 ly=47,028,852,000,000 fur

See all 15 length units in matrix view →

When Is the Light Year Used?

Interstellar and galactic distances in astronomy
Popular-science descriptions of the observable universe
Exoplanet distance reporting in the media

Real-world examples

Proxima Centauri, the nearest star beyond the Sun, is 4.24 ly away. The Milky Way is about 100,000 ly across.

Tips for Using the Light Year

The light year is a unit of distance, not time, despite the name.
Professional astronomers usually prefer parsecs (1 pc ≈ 3.26 ly) for precision work.
Light from the Andromeda galaxy takes 2.5 million years to reach us — we see it as it was 2.5 Myr ago.

Common Mistakes

Confusing light year with light second — light second ≈ 300,000 km.
Treating light years as time rather than distance in casual conversation.
Using light years for solar-system distances — use astronomical units (AU) instead.

Convert Light Year to Other Length Units

Light Year to Meter

1 ly = 9,460,700,000,000,000 m

Light Year to Kilometer

1 ly = 9,460,700,000,000 km

Light Year to Centimeter

1 ly = 946,070,000,000,000,000 cm

Light Year to Millimeter

1 ly = 9,460,700,000,000,000,000 mm

Light Year to Mile

1 ly = 5,878,606,400,000 mi

Light Year to Yard

1 ly = 10,346,347,331,583,552 yd

Light Year to Foot

1 ly = 31,039,041,994,750,656 ft

Light Year to Inch

1 ly = 372,468,503,937,007,900 in

Light Year to Nautical Mile

1 ly = 5,108,369,300,000 nmi

Light Year to Micrometer

1 ly = 9.461 × 10^21 µm

Light Year to Nanometer

1 ly = 9.461 × 10^24 nm

Light Year to Furlong

1 ly = 47,028,852,000,000 fur

Light Year to Fathom

1 ly = 5,173,173,665,791,776 ftm

Light Year to Angstrom

1 ly = 9.461 × 10^25 Å

Convert Other Units to Light Year

Meter to Light Year

1 m = 1.057 × 10^-16 ly

Kilometer to Light Year

1 km = 1.057 × 10^-13 ly

Centimeter to Light Year

1 cm = 1.057 × 10^-18 ly

Millimeter to Light Year

1 mm = 1.057 × 10^-19 ly

Mile to Light Year

1 mi = 1.701 × 10^-13 ly

Yard to Light Year

1 yd = 9.665 × 10^-17 ly

Foot to Light Year

1 ft = 3.222 × 10^-17 ly

Inch to Light Year

1 in = 2.685 × 10^-18 ly

Nautical Mile to Light Year

1 nmi = 1.958 × 10^-13 ly

Micrometer to Light Year

1 µm = 1.057 × 10^-22 ly

Nanometer to Light Year

1 nm = 1.057 × 10^-25 ly

Furlong to Light Year

1 fur = 2.126 × 10^-14 ly

Fathom to Light Year

1 ftm = 1.933 × 10^-16 ly

Angstrom to Light Year

1 Å = 1.057 × 10^-26 ly

FAQ About the Light Year

3 questions

What does the Light Year (ly) measure?

The light year measures length. The light-year is the standard astronomical distance unit, equal to 9.461 trillion kilometers traveled in one year.

When is the Light Year used?

The light year is used in: Interstellar and galactic distances in astronomy; Popular-science descriptions of the observable universe; Exoplanet distance reporting in the media. Proxima Centauri, the nearest star beyond the Sun, is 4.24 ly away. The Milky Way is about 100,000 ly across.

How accurate are conversions involving the Light Year?

All conversions on Units Converter use NIST SP 811 and BIPM reference values, accurate to 8 significant figures.