Light Year Conversion

Convert light years into kilometers, miles, and astronomical units.

Supports metric, imperial, and astronomical distance units including mega parsecs.

Updated August 1, 2026

CreatorFrank Zhao

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Introduction / overview

The Light Year Conversion calculator takes a distance in light years and shows it in every scale that actually helps — kilometers, meters, and decimeters on the metric side; miles, kilofeet, yards, feet, and inches on the imperial side; and astronomical units, Earth radii, Sun radii, mega light years, and mega parsecs on the astronomical side.

Type into any box and every other unit updates instantly. You never have to decide which direction the conversion should go — the calculator figures it out from whatever you entered.

Who is this for?

Students working through astronomy homework where a problem mixes light years, kilometers, and astronomical units.
Space enthusiasts reading the news — “this comet is 1.5 billion km away” means a lot more once it is also in light minutes or au.
Teachers and science communicators who want one reliable spot to translate cosmic distances into numbers a classroom can hold onto.

The numbers you get are consistent with the standard definitions astronomers use: a light year is built from the exact speed of light and the Julian year, and the astronomical unit, parsec, and the Earth and Sun radii all follow the values adopted by the IAU. That makes the converter dependable for coursework, fact-checking an article, or just satisfying your own curiosity. If you also need everyday units beyond light years, our Length Converter covers the full length spectrum in one place.

How to use / quick start

1Type a distance into the “Distance in light years” box. It is the only field that always speaks in light years.
2Read the Metric section — kilometers by default. If you want meters or decimeters instead, pick one of those from that field’s unit menu.
3Read the Imperial/US section — miles by default, with kilofeet, yards, feet, and inches available if you need them.
4Read the Astronomical section — astronomical units by default. Switch to Earth radii, Sun radii, mega light years, or mega parsecs when the scale calls for it.
5To convert the other way, type into any of those boxes instead. Type a distance in miles and the light-year box fills in — no need to run the math by hand.

Worked example — what is one light year in everyday units?

1\ \mathrm{ly}

=

9.4607 \times 10^{12}\ \mathrm{km}

=

5.8786 \times 10^{12}\ \mathrm{mi}

\approx

63\,241\ \mathrm{au}

Type $1$ into the light-years box: the kilometers field shows about 9.46 trillion km, the miles field about 5.88 trillion mi, and the astronomical field about 63,241 au. That last number is why astronomers stop using km inside the Solar System — it is simply too many digits.

Worked example — the nearest star to the Sun, in familiar units

4.24 \times 9.4607 \times 10^{12}

=

4.0113 \times 10^{13}\ \mathrm{km}

4.24 \times 5.8786 \times 10^{12}

=

2.4925 \times 10^{13}\ \mathrm{mi}

Proxima Centauri sits about $4.24\ \mathrm{ly}$ away. Type that into the light-years box and you get roughly 40.1 trillion km (about 24.9 trillion mi) — a number that makes it very clear why we prefer light years between the stars.

How to read the results

The value you typed stays as your authored value; every other field is the calculator’s answer and updates live as you edit.
Very large or very small results may be shown in scientific notation (for example $1\ \mathrm{m} \approx 1.057 \times 10^{-16}\ \mathrm{ly}$ ) so the page stays readable.
Long repeating decimals such as light years in miles are rounded for easy reading, while the conversion itself keeps full accuracy on your original number.

Real-world examples

The Andromeda Galaxy in mega light years

Our nearest large galactic neighbor is about $2.5$ million light years away. Switch the astronomical field to mega light years (Mly) and type $2.5$ :

2.5\ \mathrm{Mly}

=

2.5 \times 10^6\ \mathrm{ly}

=

2.3652 \times 10^{19}\ \mathrm{km}

Reading “2.5 Mly” is far friendlier than 2.5 million light years written out as kilometers — this is exactly the job the mega light year unit exists for.

Exploring the Solar System in astronomical units

The Earth–Sun distance is exactly one astronomical unit. Keep the astronomical field onau and type $1$ , or jump to Neptune at about $30\ \mathrm{au}$ :

1\ \mathrm{au}

=

149\,597\,870\,700\ \mathrm{m}

\approx

149.6\ \mathrm{million\ km}

30\ \mathrm{au}

\approx

4.49\ \mathrm{billion\ km}

This is the unit space agencies use when talking about spacecraft distances — much easier to hold in your head than tens of billions of kilometers.

Comparing sizes with Earth radii and Sun radii

The Sun’s radius is about $109$ times Earth’s. Switch the astronomical field to Earth radii (R⊕) and type $109.2$ :

109.2\ \mathrm{R_\oplus}

=

695\,700\ \mathrm{km}

=

1\ \mathrm{R_\odot}

Because the calculator stores every distance in meters, the same number shows up in the metric section as 695,700 km — a handy way to see both the relative and the absolute size at a glance.

Cosmology distances in megaparsecs

Astronomers quote distances to distant galaxy clusters in megaparsecs. Switch the astronomical field to mega parsecs (Mpcs) and type $100$ :

1\ \mathrm{Mpc}

\approx

3.26 \times 10^6\ \mathrm{ly}

100\ \mathrm{Mpc}

\approx

3.26 \times 10^8\ \mathrm{ly}

One hundred megaparsecs is roughly 326 million light years. Converting this way helps you translate a cosmology paper’s units into the light-year picture you already understand.

Common scenarios / when to use

Astronomy coursework

Homework problems love mixing units. Enter the value once and read it in the unit the question asks for, then sanity-check the magnitude against the reference values in this guide.

Teaching & science writing

When an article says “a galaxy 100 million light years away,” readers need a km or mile figure to grasp the scale. This converter gives you a consistent, accurate translation to include.

Stargazing & astrophotography

Target catalogs often list distances in light years or parsecs. Convert them to au or km when you want to compare targets, and use the light-year value to think about how far back in time you are looking.

Following space news

Mission updates quote distances in au or km; discovery announcements use light years. A quick conversion keeps the two stories consistent so you can compare a comet’s distance with a star’s.

When it’s less useful

This tool is built for astronomy. For everyday measuring — a room, a fence, a hiking trail — the units here are overkill, and a light year is far too large to be useful. Use it for cosmic scales and reach for an everyday length tool when you are back on the ground.

Tips & best practices

Type into the unit you know

If your source is in kilometers, type into the kilometers box rather than converting by hand first. Everything else follows, and there is one fewer place to make a mistake.

Match the unit to the scale

Use au inside the Solar System, light years to nearby stars, and mega light years or megaparsecs across galaxies. The right unit keeps results readable instead of a wall of digits.

Memorize one sanity-check number

Remember that $1\ \mathrm{ly} \approx 9.46 \times 10^{12}\ \mathrm{km}$ . If a result is wildly different from that scale, re-check what you typed.

Know the mile you are using

The imperial field uses the statute mile ( $1609.344\ \mathrm{m}$ ), the everyday road mile. Aviation and marine charts use the longer nautical mile instead — not part of this tool, so don’t mix them up in your own notes.

Getting the most accurate result

Enter as many digits as you have. The conversion is exact for the defined units, so the result is only as precise as your input.
Remember the Earth and Sun radii are nominal reference values, not measurements taken on any particular day — they are the standard numbers astronomy uses.
Double-check the unit you selected in each field. Choosing miles instead of meters changes the result by more than a thousand times.

Calculation method / formula explanation

Every unit is defined by how many meters it equals. The converter turns your number into meters first, then into the target unit — so any two units can talk to each other without a separate formula for each pair.

The general conversion

L_t = L_s \times \frac{f_s}{f_t}

where $L_s$ is the value in the source unit, $L_t$ is the value in the target unit, $f_s$ is meters per source unit, and $f_t$ is meters per target unit.

The light year itself

1\ \mathrm{ly}

=

c \times 365.25 \times 86\,400

=

9\,460\,730\,472\,580\,800\ \mathrm{m}

A light year is the distance light travels in one Julian year: the exact speed of light $c = 299\,792\,458\ \mathrm{m/s}$ multiplied by the number of seconds in $365.25$ days.

The astronomical unit and the parsec

1\ \mathrm{au}

=

149\,597\,870\,700\ \mathrm{m}

1\ \mathrm{pc}

=

\frac{648\,000}{\pi}\ \mathrm{au}

1\ \mathrm{Mpc}

=

10^6\ \mathrm{pc}

The astronomical unit is the exact Earth–Sun distance used by convention. A parsec is defined by geometry — the distance at which one au subtends one arcsecond — and a megaparsec is simply a million parsecs, the scale astronomers use for galaxy surveys.

Exact definitions behind the units

Unit	Exact value in meters
kilometer (km)	1,000
mile (mi)	1,609.344
Earth radius (R⊕)	6,371,000
Sun radius (R☉)	695,700,000
astronomical unit (au)	149,597,870,700
light year (ly)	9,460,730,472,580,800
mega light year (Mly)	9.4607 × 10²¹
megaparsec (Mpc)	≈ 3.0857 × 10²²

Variable guide

$L_s$ — numeric distance in the source unit
$L_t$ — numeric distance in the target unit
$f_s$ — meters per source unit
$f_t$ — meters per target unit
$c$ — speed of light, exactly $299\,792\,458\ \mathrm{m/s}$

Related concepts / background info

A light year is a distance, not a time

It is easy to trip over the name. A light year measures how far light travels in a year, so it is a length — about $9.46 \times 10^{15}\ \mathrm{m}$ . What makes it useful in astronomy is that it doubles as a time machine: light from an object one light year away left it one year ago, so the distance also tells you how far back in time you are looking.

Astronomical units for the Solar System

Inside the Solar System the au is the natural yardstick — one au is the Earth–Sun distance, and the farthest planets sit tens of au out. Beyond the Solar System those numbers climb so fast that astronomers switch to light years, and then to parsecs and megaparsecs for galaxies.

Parsecs are born from parallax

A parsec is defined so that one au subtends one arcsecond of angle — the classic parallax setup for measuring nearby stars. One parsec is about $3.26\ \mathrm{ly}$ , and megaparsecs are the unit of choice for galaxy surveys and cosmology. The converter includes them so you can translate between the two families of units astronomers actually use.

Earth radii and Sun radii as reference sizes

Radii are handy reference sizes: the Sun is about 109 Earth radii across, and stellar sizes are often quoted in solar radii. Using them as distance units lets you compare the size of stars and planetary systems on a scale people can picture.

Go further: once you’re comfortable with light years, use the same fields to compare a whole Solar System worth of distances in au, or jump up to Mly and Mpc when a news story crosses from our neighborhood to the wider cosmos.

Frequently asked questions

Is a light year a measure of time or distance?

It is a distance — the distance light travels in one year. The name is misleading, but the number is fixed: $1\ \mathrm{ly} \approx 9.46 \times 10^{15}\ \mathrm{m}$ .

How far is one light year in miles or kilometers?

One light year is about $9.46 \times 10^{12}\ \mathrm{km}$ or $5.88 \times 10^{12}\ \mathrm{mi}$ — roughly 9.46 trillion km, or 5.88 trillion miles. Just type $1$ into the light-years box to see it for yourself.

What’s the difference between a light year and an astronomical unit?

An au is the Earth–Sun distance ( $149.6\ \mathrm{million\ km}$ ), great for the Solar System. A light year is about $63\,241\ \mathrm{au}$ , so it fits distances between stars. Use au for planets and comets, light years for stars and galaxies.

What are parsecs and megaparsecs, and when do I use them?

A parsec is the parallax-based unit astronomers prefer for stars — $1\ \mathrm{pc} \approx 3.26\ \mathrm{ly}$ . A megaparsec is a million parsecs, used for galaxy clusters and cosmology. Switch the astronomical field toMpcs whenever you see these in a paper or news item.

Can I convert kilometers or miles into light years?

Yes. Type a distance into the kilometers or miles box and the light-years field fills in the other way. For example, $1\ \mathrm{km} \approx 1.057 \times 10^{-13}\ \mathrm{ly}$ — tiny, which is exactly why light years exist.

Why do some results use scientific notation?

Cosmic distances span too many orders of magnitude to write out fully. Scientific notation like $2.3652 \times 10^{19}\ \mathrm{km}$ keeps the page readable for enormous and tiny numbers alike, while the conversion itself keeps full accuracy.

What are Earth radii and Sun radii used for?

They are reference sizes: the Sun’s radius is about $109\ \mathrm{R_\oplus}$ , and astronomers quote stellar sizes in solar radii. Using them as distance units makes “how big” comparisons easy — for example, the Sun as $695\,700\ \mathrm{km}$ .

Limitations / disclaimers

This is a unit conversion tool — the math is exact for the defined units it uses. A few practical boundaries are worth knowing:

•The light year, astronomical unit, and parsec use the standard IAU definitions, while the Earth and Sun radii are nominal reference values. These are the right numbers for astronomy, but they are conventions you should know you’re using.
•Results are only as precise as the numbers you enter. The converter keeps your full value, but the visible display rounds long decimals and uses scientific notation for readability.
•This tool is intended for education, astronomy, and general reference. It is not a surveying or navigation instrument, and distances here are not legal measurements.

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