Light-Year Converter & Cosmic Distance Explorer
How Far Is a Light-Year? Convert Light-Years to Kilometers and Miles
A light-year measures distance: how far light travels through a vacuum in one Julian year.
How Far Is 1 Light-Year?
9,460,730,472,580.8 kmApproximately 5.879 trillion miles
Convert Any Distance ↓Light-Year to Kilometers and Miles Converter
Enter any distance and change either unit. The result updates as you type.
Use a decimal point or scientific notation, such as 2.5 or 1e-6.
1 ly → km
Quick conversions
Conversions use the exact vacuum speed of light and a Julian year.
Explore the Scale of a Light-Year
Move from the Moon’s neighborhood to stellar distances. Each equal step on this logarithmic distance scale represents multiplication, not an equal number of kilometers.
One light-year · defined length
Earth is an origin for Earth-centered distances, not a zero position on a logarithmic axis. References mix explicitly labeled object distances and structural sizes: 1 AU is nominal, Neptune’s orbit is Sun-centered, and the Milky Way value is a disk diameter. These colored points compare lengths; they are not a sky map or objects lying on one line from Earth.
How Far Away Are the Nearest Stars?
The Sun is our nearest star. Beyond it, Proxima Centauri is roughly 4.25 light-years away. Select a star to update the converter, cosmic ruler and travel times together.
Logarithmic comparison: a marker’s position compares the selected reference lengths between the Sun and Sirius. This is not a map or a list of every nearby star.
Distances here are rounded educational references from NASA and Observatoire de Paris, with source dates in the reference notes and source list below. They are not a new astrometric catalog or live positions. The Sun uses a fixed 1 AU baseline; all other listed stellar distances are approximate distances from our solar neighborhood.
How Long Does Light Take to Travel?
Light travel time for the current converter distance. Edit its value or units above, or select a reference here.
Time = distance ÷ c, for a fixed vacuum path. Endpoint motion, gravity and the expansion of the universe are omitted. This is not a cosmological lookback-time calculator.
How Long Would It Take a Spacecraft to Travel One Light-Year?
Start with 1 ly, or use the current converter length. Choose a hypothetical constant cruising speed.
Earth/rest-frame time = distance ÷ constant speed, with the endpoints at rest in that frame. This is a hypothetical kinematic estimate, not a feasible mission plan. No acceleration, braking, gravity or fuel model is included. No spacecraft-clock or relativistic onboard time is calculated. Speeds must be at least 0.000000001 km/s and below c.
One Light-Year Compared With Familiar Distances
At the default 1 ly, this ratio shows how many reference lengths fit into one light-year. Changing the converter updates the numerator.
The mean Earth–Moon center distance is 384,400 km; the Sun reference is exactly 1 AU. Earth’s idealized orbit diameter is 2 AU and Neptune’s nominal orbital radius is 30.06 AU. The Solar System has no single sharp edge, so an orbital radius should not be read as its total size.
How Many Kilometers Are in a Light-Year?
One light-year is exactly 9,460,730,472,580.8 kilometers. To calculate it, multiply the exact vacuum speed of light by the number of SI seconds in a Julian year.
365.25 × 86,400 = 31,557,600 seconds
299,792,458 m/s × 31,557,600 s = 9,460,730,472,580,800 m
Divide by 1,000 → 9,460,730,472,580.8 km
A Julian year is a fixed 365.25 days, each exactly 86,400 SI seconds. It does not switch between 365 and 366 days like a calendar year. Light-years therefore stay the same length in leap years. For light-years to km, multiply by the exact kilometer value; for km to light-years, divide by it.
How Many Miles Are in a Light-Year?
One light-year is approximately 5,878,625,373,183.608 international miles, or 5.879 trillion miles. An international mile is exactly 1,609.344 meters, so divide the meter distance by that value. The definition is exact, while this displayed decimal quotient is rounded.
“Trillion” on this English-language page means 10¹²; “billion” means 10⁹.
Is a Light-Year a Unit of Time or Distance?
A light-year is a distance. The year tells us how long light travels, just as one hour at a given speed defines a travel distance. Multiplying meters per second by seconds leaves meters, a unit of length.
For a nearby star about four light-years away, the light arriving now has traveled for roughly four years in the usual fixed-distance approximation. We see the star’s past, not its present state. That connection between distance and the age of incoming light makes the unit useful for astronomy.
Light-Year vs AU vs Parsec: What’s the Difference?
The astronomical unit is exactly 149,597,870.7 km, a convenient scale for planetary orbits. A light-year describes the vacuum distance covered in one Julian year. A parsec comes from stellar-parallax geometry: astronomers compare a nearby star’s apparent position from different points in Earth’s orbit.
The formal IAU parsec used here is exactly (648,000 / π) AU, following the 2015 definition. It is approximately 3.261563777167434 ly. We use that angular convention, rather than substituting AU/tan(1 arcsecond). One light-year is about 63,241.07708426628 AU.
Enter a value in light-years, AU or parsecs above to update this three-unit comparison using the same converter.
Light-Years to Kilometers and Miles Conversion Table
| Light-years | Kilometers | Miles |
|---|---|---|
| 0.1 | 946,073,047,258.08 | 587,862,537,318.3608 |
| 0.5 | 4,730,365,236,290.4 | 2,939,312,686,591.804 |
| 1 | 9,460,730,472,580.8 | 5,878,625,373,183.608 |
| 2 | 18,921,460,945,161.6 | 11,757,250,746,367.22 |
| 5 | 47,303,652,362,904 | 29,393,126,865,918.04 |
| 10 | 94,607,304,725,808 | 58,786,253,731,836.08 |
| 50 | 473,036,523,629,040 | 293,931,268,659,180.4 |
| 100 | 946,073,047,258,080 | 587,862,537,318,360.8 |
| 1,000 | 9,460,730,472,580,800 | 5,878,625,373,183,608 |
How Far Does Light Travel in a Second, Minute, Hour and Year?
| Time | Kilometers |
|---|---|
| One second | 299,792.458 |
| One minute | 17,987,547.48 |
| One hour | 1,079,252,848.8 |
| One day | 25,902,068,371.2 |
| One Julian year | 9,460,730,472,580.8 |
Frequently Asked Questions
How many kilometers are in one light-year?
Exactly 9,460,730,472,580.8 kilometers, using the IAU Julian-year convention and the exact vacuum speed of light.
How many miles are in one light-year?
Approximately 5,878,625,373,183.608 international miles, or 5.879 trillion miles. The mile is exactly 1,609.344 meters; the displayed quotient is rounded.
Is a light-year a unit of distance or time?
Distance. The year specifies how long light travels; the resulting light-year measures the length of that journey through a vacuum.
How do you convert light-years to kilometers?
Multiply the number of light-years by 9,460,730,472,580.8. For example, 2 light-years equal 18,921,460,945,161.6 km.
How do you convert kilometers to light-years?
Divide the distance in kilometers by 9,460,730,472,580.8. You can also choose Kilometers as the input unit and Light-years as the output unit in the converter.
How many astronomical units are in one light-year?
About 63,241.07708426628 AU. One astronomical unit is exactly 149,597,870.7 km.
How many light-years are in one parsec?
About 3.261563777167434 light-years. The formal parsec used here is exactly 648,000/π astronomical units.
How far is the nearest star in light-years?
The Sun is the nearest star: its nominal 1 AU reference distance is about 0.0000158125 light-years. Beyond the Sun, Proxima Centauri is roughly 4.25 light-years away. Stellar distances are measurements, so those reference values are rounded.
How long would it take to travel one light-year?
It depends on speed. At a hypothetical constant 1% of light speed, the Earth/rest-frame journey takes 100 Julian years; at 10%, it takes 10. These estimates omit acceleration and braking and do not give time elapsed on a spacecraft’s clock.
How long does light take to travel one light-year?
Exactly one Julian year in a vacuum: 365.25 days, or 31,557,600 SI seconds, in the fixed-distance model used to define the unit.
How far is 100 light-years in kilometers?
946,073,047,258,080 kilometers, about 946.1 trillion km.
How far is 1,000 light-years?
9,460,730,472,580,800 km, or approximately 5,878,625,373,183,608 miles. Light takes 1,000 Julian years to cross that fixed length in a vacuum.
Why do astronomers use light-years?
They make stellar distances easier to read: a nearby star a few light-years away would require tens of trillions of kilometers. They also connect distance with the travel time of the light we receive.
Is anything faster than light?
In special relativity, an object with mass cannot reach or exceed the vacuum speed of light, and information cannot be sent locally faster than c. The expansion of space and some apparent motions require different interpretations; they do not let a spacecraft cruise locally faster than light.
Definitions, Reference Distances and Sources
The converter uses 60 significant digits internally and rounds only displayed results. Inputs support up to 40 significant digits; swapping units places up to 40 significant digits into the new input. The displayed output can extend beyond the supported input range; entering or swapping such a value shows a range message. Parsecs involve π and are evaluated numerically to the internal precision.
Exact unit definitions and rounded astronomical measurements serve different purposes. Showing more converted digits cannot make a source distance more accurate. Reference distances are fixed explanatory values, not current ephemerides; all source links were checked in November 2026.
- IAU: measuring the universe — light-year, Julian year and the exact AU.
- NIST: defining constants — the exact vacuum speed of light.
- NIST Guide to the SI — international-mile conversion.
- IAU Working Group: 2015 Resolution B2 — parsec = 648,000/π AU.
- NASA/JPL lunar-distance definition — Moon: 384400 km. Mean Earth–Moon distance, not tonight’s changing separation. Reference checked November 2026.
- NASA/Hubble Neptune orbital reference — Neptune’s orbit · radius: 30.06 AU. Rounded semimajor axis of Neptune’s orbit around the Sun. This is not the current Earth–Neptune distance. NASA reference published 2013.
- NASA/JPL nearest stellar neighborhood — Proxima Centauri: 4.25 ly. Nearest star beyond the Sun. Rounded educational reference, not a live parallax or exact measurement. NASA reference published 2025.
- Observatoire de Paris / LESIA — Alpha Centauri A/B: 4.37 ly. The nearby binary is farther away than Proxima. Conversion precision does not increase this rounded distance’s measurement precision. Reference published 2016.
- NASA/Chandra Barnard’s Star reference — Barnard’s Star: 6 ly. A deliberately rounded six-light-year reference. Proper motion and later astrometry can change more precise catalog distances. NASA reference published 2020.
- NASA/Hubble Sirius reference — Sirius: 8.6 ly. Rounded distance to the Sirius binary system. This is a reference distance, not a real-time position. NASA reference published 2005.
- NASA/GSFC Milky Way stellar-disk scale — Milky Way · disk diameter: 100000 ly. A conventional roughly 100,000-light-year stellar-disk diameter. We are inside the Milky Way; this is not a destination distance or the size of its dark-matter halo. Approximate reference checked November 2026.
Published November 3, 2026 · Fixed unit definitions and rounded astronomical references · No location permission or external data request is needed for these calculations.
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