Light year converter: from kilometres to parsecs and back
This light year converter changes any astronomical distance into every unit you are likely to meet — light years, parsecs, kiloparsecs and megaparsecs, astronomical units, kilometres, miles, metres, and light-seconds, light-minutes, light-hours and light-days. Type a number, pick its unit, and the full table updates as you type, with a copy button on every row.
It is also a light travel time calculator. Every distance shows how long light — or a radio signal — takes to cover it, and a table gives today’s light time to the Moon, the Sun and every planet, one way and round trip. A speed picker shows how long a trip would take at walking pace, Voyager 1’s speed or a fraction of light speed. The constants are the IAU and SI definitions, and the arithmetic runs in your browser.
How to use the Light Year & Parsec Converter
- Enter a Distance and choose its Unit, or click one of the example buttons (Moon, Sun, Proxima Centauri, Sirius, Andromeda galaxy).
- Read the headline conversion in the panel — kilometres for most inputs, light years for a kilometre input — and the full table of units underneath.
- Click the copy icon on any row to copy that value with full precision.
- Pick a Speed from the list or choose “Custom…” and type a speed in km/h to see the travel time, how long light takes, and the speed as a fraction of light speed.
- Scroll to Light travel time today for the Moon, Sun and planets on any date; press Convert on a row to load that distance.
Light Year & Parsec Converter features
Twelve units
From metres to megaparsecs, including the light-time units used for the solar system (a light-minute, a light-hour) and the parsec family used for stars and galaxies.
Exact constants
The light year is defined as light speed × one Julian year (9 460 730 472 580.8 km), the parsec as 648 000/π AU, and the AU as 149 597 870 700 m (IAU 2012).
Travel-time calculator
Real speeds from walking to the fastest spacecraft ever flown, plus light-speed fractions and a custom field.
Light time today
Earth–Moon, Earth–Sun and Earth–planet distances for any date from 1900 to 2100, with one-way and round-trip light time.
Distance examples
Fifteen reference objects with their distance, light travel time and the trip time at your chosen speed.
What a light year actually is
A light year is a distance, not a time: how far light travels through a vacuum in one Julian year of 365.25 days. At 299 792.458 km/s that is about 9.46 trillion kilometres or 5.88 trillion miles — roughly 63 241 astronomical units, where one AU is the mean Earth–Sun distance. Inside the solar system light-minutes and light-hours are handier.
The parsec comes from a different idea. A star one parsec away shows a parallax of one arcsecond as the Earth moves from one side of its orbit to the other, which puts it at 648 000/π ≈ 206 265 AU, or 3.26 light years. Professional papers use parsecs, kiloparsecs (the Milky Way is about 30 kpc across) and megaparsecs (the Virgo cluster is about 16 Mpc away); popular writing uses light years.
Conversion factors
Light years to kilometres: multiply by 9.4607 × 10¹² (divide by 1.609344 for miles). Parsecs to light years: × 3.26156. For everyday lengths use the length converter.
| Unit | In kilometres | In light years |
|---|---|---|
| 1 light-second | 299 792.458 | 3.169 × 10⁻⁸ |
| 1 light-minute | 17 987 547.48 | 1.901 × 10⁻⁶ |
| 1 astronomical unit (AU) | 149 597 870.7 | 1.581 × 10⁻⁵ |
| 1 light-day | 2.590 × 10¹⁰ | 0.002738 |
| 1 light year (ly) | 9.4607 × 10¹² | 1 |
| 1 parsec (pc) | 3.0857 × 10¹³ | 3.2616 |
| 1 kiloparsec (kpc) | 3.0857 × 10¹⁶ | 3261.6 |
| 1 megaparsec (Mpc) | 3.0857 × 10¹⁹ | 3 261 564 |
Light travel time to the Moon, Sun and planets
Light time is t = d ÷ c, with c = 299 792.458 km/s exactly. The Moon is 1.2 to 1.35 light-seconds away, the Sun 8 min 10 s to 8 min 27 s through the year, and Mars about 3 to 22 light-minutes, which is why rovers cannot be driven in real time. A radio command and its reply take twice the one-way time. The table uses each body’s distance from Earth on your date; for closest approaches use the planet distance calculator.
How long would the trip take?
Travel time is distance divided by speed. Voyager 1, the fastest object leaving the solar system, moves at about 17 km/s (61 200 km/h), so the 4.25 light years to Proxima Centauri take roughly 74 000 years. The Parker Solar Probe briefly reached 192 km/s near the Sun in 2024, only 0.064% of light speed. Even at 10% of light speed the nearest star is a 42-year journey. The times ignore acceleration and relativity, which matter above about 10% of c; for those use the space travel time calculator. The speed converter turns km/s and mph into km/h.
Light Year & Parsec Converter FAQ
How many kilometres are in a light year?
Exactly 9 460 730 472 580.8 km, because the light year is defined as the speed of light (299 792.458 km/s) multiplied by a Julian year of 31 557 600 seconds. In miles that is about 5.879 trillion.
Is a parsec bigger than a light year?
Yes. One parsec is 3.26 light years. It is defined by parallax — the distance at which 1 AU subtends one arcsecond — which is why astronomers measure in parsecs.
How far away is the nearest star in light years?
Proxima Centauri is about 4.25 light years (1.30 parsecs), roughly 40 trillion km, so we see it as it was four years ago.
How long would it take to travel one light year?
At the speed of light, one year by definition. At Voyager 1’s 61 200 km/h about 17 600 years; in a car at 100 km/h about 10.8 million years. Pick a speed in the Travel time section to see other cases.
How far is Mars from Earth in light minutes?
It changes daily, from about 3 light-minutes at a close opposition to about 22 at conjunction. On 1 January 2026 it was about 20 light-minutes; the table shows any date.
What is a light-minute?
The distance light travels in one minute — 17 987 547.48 km. The Sun is 8.3 light-minutes away, which is also how old the sunlight you see is.
Why are some stellar distances marked “approx.”?
Distances to distant stars such as Betelgeuse are uncertain by tens of percent because their parallaxes are tiny. The examples use commonly quoted values; the conversions themselves are exact.