Convert between light years, kilometers, miles, astronomical units, and parsecs, and find the light-travel time for any distance.
Inputs
The distance to convert, in the "From" unit below.
Result
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Enter a value above to compute.
How long light takes to cross that distance
0years
0days
0hr
0min
0sec
Enter a value above to compute.
Conversion Factors (1 unit in meters)
Conversion factor for each supported distance unit, expressed in meters
Unit
Symbol
Meters
Real-World Distances
Example astronomical distances and how long light takes to cross each one
Distance
Approx. value
Light-travel time
Earth to the Moon
384,400 km
≈ 1.28 light-seconds
Earth to the Sun (1 AU)
149.6 million km
≈ 8.3 light-minutes
Nearest star (Proxima Centauri)
4.24 ly
≈ 4.24 years
Milky Way galaxy diameter
≈ 100,000 ly
≈ 100,000 years
Andromeda Galaxy
≈ 2.5 million ly
≈ 2.5 million years
3 min read3 steps7 terms3 examples6 FAQsresult = value × factor(from) / factor(to)
A light-year sounds like a unit of time, but it's actually a unit of distance — one of the largest ones humans use.
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Walk-through
How to Use This Calculator
3 steps▸
1
Enter a distance and pick your units
Type a distance value, then choose the unit it's measured in ('From') and the unit you want to convert it to ('To') — light-year, kilometer, mile, astronomical unit, parsec, light-second, or light-minute. The result updates instantly.
2
Read the converted result
The Convert tab shows the converted distance plus a plain-language interpretation. Use the swap button to flip the From and To units without retyping the value.
3
Check the light-travel time
Switch to the Light-Travel Time tab to see how long light takes to cross that same distance, broken into years, days, hours, minutes, and seconds — useful for putting astronomical scales in human terms.
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Reference
Formula & Methodology
2 formulas▸
Unit conversion via a common meters base
result = value × factor(from) / factor(to)
Every supported unit has a fixed conversion factor into meters (1 ly = 9.461×10¹⁵ m, 1 AU = 1.496×10¹¹ m, 1 parsec = 3.0857×10¹⁶ m ≈ 3.2616 ly, 1 light-second = 299,792,458 m). To convert, the calculator multiplies the input value by the 'from' unit's meters factor, then divides by the 'to' unit's meters factor — the same approach used across all the site's unit converters.
Light-travel time
time = distance (m) / c
c is the speed of light in a vacuum, 299,792,458 m/s (an exact SI value). Dividing any distance in meters by c gives the time light takes to cross it. A light-year is, by definition, the distance light travels in one Julian year — so converting 1 ly always yields a light-travel time of almost exactly one year.
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Glossary
Key Terms Explained
7 terms▸
Light Year ↗The distance light travels in a vacuum in one Julian year (365.25 days): about 9.461 trillion kilometers (5.879 trillion miles). Despite the name, it measures distance, not time — it's the standard yardstick for interstellar distances.
Light-Travel Time ↗How long light takes to cross a given distance, calculated as distance ÷ the speed of light (c). Because light has a finite speed, looking at a distant star means seeing light that left it long ago — its light-travel time in the past.
Astronomical Unit (AU) ↗The average distance from Earth to the Sun, about 149.6 million kilometers (93 million miles). AU is the standard unit for distances within a solar system, where a light-year would be inconveniently large.
Parsec ↗A distance unit used in professional astronomy, defined via stellar parallax: about 3.2616 light-years, or roughly 3.0857×10¹⁶ meters. Parsecs (and kiloparsecs, megaparsecs) are the default units in most astrophysics literature.
Speed of Light (c) ↗The speed at which light travels in a vacuum: exactly 299,792,458 meters per second. It's a fixed constant of nature and the basis for both the light-year and every light-travel-time calculation.
Light-Second ↗The distance light travels in one second: 299,792,458 meters. Light-seconds and light-minutes are convenient for solar-system-scale distances — the Moon is about 1.28 light-seconds away, and the Sun is about 8.3 light-minutes away.
Distance ↗In this calculator, distance is always expressed as a positive length in one of seven units (light-year, kilometer, mile, astronomical unit, parsec, light-second, light-minute), all converted through a common meters base for consistency.
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Scenarios
Real-World Examples
3 worked examples▸
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Quick Unit Check
One light-year in kilometers
Value 1From Light-yearTo Kilometer
1 light-year converts to about 9.461 trillion km (9.461 × 10¹² km). This is the number most often quoted for 'how big is a light-year' — useful context whenever kilometers feel too small a unit for interstellar distances.
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Stargazer
Distance to Proxima Centauri
Value 4.24From Light-yearTo Parsec
Proxima Centauri, the nearest star to the Sun, sits 4.24 light-years away — about 1.3 parsecs. On the Light-Travel Time tab, that same 4.24 ly shows a travel time of 4.24 years: the starlight you see left Proxima roughly four years before you observed it.
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Solar System Scale
Light-minute from the Sun
Value 1From Astronomical unitTo Light-minute
1 AU (the Earth–Sun distance) equals about 8.317 light-minutes. That's the light-travel time you'd see on the Light-Travel Time tab — roughly 8 minutes and 19 seconds for sunlight to reach Earth, the reason solar flares are already minutes old by the time we observe them.
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Reference
Cite This Calculator
APA & MLA▸
Use either format to cite this calculator in a paper, report, or resource list.
A light-year sounds like a unit of time, but it's actually a unit of distance — one of the largest ones humans use. This calculator converts between light-years and the other units astronomers rely on (kilometers, miles, astronomical units, parsecs, light-seconds, and light-minutes), and shows how long light itself takes to cross any distance you enter.
Why Astronomers Measure Distance in Light-Years
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Kilometers and miles work fine on Earth, but they become unwieldy at interstellar scales — the nearest star beyond the Sun is roughly 40 trillion kilometers away. A light-year (the distance light travels in one Julian year, about 9.461×10¹⁵ meters) gives astronomers a far more manageable number: that same star is 4.24 light-years away. Within a single solar system, the astronomical unit (the Earth–Sun distance) is the more practical scale, while professional astrophysics papers usually default to the parsec, defined from stellar parallax rather than travel time.
What Light-Travel Time Actually Tells You
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Because light has a finite speed — exactly 299,792,458 meters per second — it takes real time to cross any distance. Divide a distance in meters by that speed and you get the light-travel time: how long ago the light you're seeing right now actually left its source. Sunlight is about 8.3 minutes old by the time it reaches Earth; light from Proxima Centauri is about 4.24 years old. At galactic scales the effect becomes dramatic — light from the Andromeda Galaxy left roughly 2.5 million years ago, meaning we see it as it looked in the distant past, not as it looks today.
How the Conversion Works
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Every unit this calculator supports has a fixed, well-defined length in meters. To convert between any two of them, the calculator multiplies your input by the 'from' unit's meters-per-unit factor, then divides by the 'to' unit's factor — the same meters-base approach used throughout the site's unit converters. Because a light-year is defined directly from the speed of light and a Julian year, converting 1 light-year on the Light-Travel Time tab always returns almost exactly one year, which is a handy sanity check that the math is behaving.
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Questions
Frequently Asked Questions
6 questions▸
How many kilometers are in a light year?+
About 9.461 trillion kilometers (9.461 × 10¹² km), or roughly 5.879 trillion miles. This calculator computes the exact figure — 9,460,730,472,580.8 km — from the definition of a light-year as the distance light travels in one Julian year.
What is a parsec, and how does it compare to a light-year?+
A parsec is a distance unit defined from stellar parallax, equal to about 3.2616 light-years (roughly 3.0857 × 10¹⁶ meters). It's the default distance unit in most professional astronomy literature, where light-years are more common in popular science writing.
What is an astronomical unit (AU)?+
An astronomical unit is the average distance from Earth to the Sun: about 149.6 million kilometers (93 million miles). It's the standard unit for describing distances within a solar system, where a light-year would be far too large to be practical.
How is light-travel time calculated?+
Light-travel time equals distance divided by the speed of light, c = 299,792,458 meters per second. Enter any distance and unit, and the Light-Travel Time tab converts it to meters and divides by c to show the years, days, hours, minutes, and seconds light takes to cross it.
How far away is the nearest star?+
Proxima Centauri, the closest star to the Sun besides the Sun itself, is about 4.24 light-years away (roughly 1.3 parsecs, or about 40.1 trillion kilometers). Its light takes about 4.24 years to reach Earth.
What units does this calculator support?+
Light-year, kilometer, mile, astronomical unit (AU), parsec, light-second, and light-minute — all distance units, converted through a common meters base so any pair can be compared directly.
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