What exactly is a light-year?

A light-year is the distance light travels through a vacuum in exactly one Julian year (365.25 days). Using the SI definition of the speed of light (299,792,458 m/s exactly), one light-year equals 9,460,730,472,580,800 metres, or about 9.461 trillion kilometres (9.461 × 10^12 km). It is a unit of distance, not time.

How is light travel time calculated?

The formula is simply t = d ÷ c, where d is the distance in metres and c is the speed of light (299,792,458 m/s). For example, the Sun is 1 AU = 149,597,870,700 m away; dividing by c gives 499.0 seconds, or about 8 minutes and 19 seconds — so sunlight we see right now left the Sun over 8 minutes ago.

What is the difference between a light-year and a parsec?

A parsec is defined by parallax geometry: it is the distance at which 1 AU subtends an angle of 1 arcsecond. The exact value is 1 pc = AU × (648,000 ÷ π) ≈ 3.0857 × 10^16 m, which equals roughly 3.2616 light-years. Parsecs are preferred by professional astronomers because they arise directly from trigonometric parallax measurements, while light-years are more intuitive for public science communication.

How far away is Proxima Centauri in light-years?

Proxima Centauri, the closest known star to the Sun, is approximately 4.2441 light-years away. Light from Proxima takes about 4.24 years to reach Earth. If a spacecraft could travel at 0.1c (one-tenth the speed of light), the journey would take around 42 years; even the fastest probes ever launched would take tens of thousands of years.

How big is the observable Universe in light-years?

The comoving radius of the observable Universe is approximately 46.5 billion light-years (4.65 × 10^10 ly). This is larger than 13.8 billion light-years — the age of the Universe in years — because cosmic expansion has carried the most distant objects further away since their light was emitted. The light from the farthest galaxies we can detect was emitted only a few hundred million years after the Big Bang.

Can I convert light-years to kilometres with this tool?

Yes. Enter a value in light-years and the table immediately shows the equivalent in kilometres (and nine other units). One light-year equals exactly 9,460,730,472,580.8 km — just over 9.46 trillion kilometres, or roughly 63,241 AU.

Light-Year Distance Calculator

A light-year distance calculator that converts any cosmic distance between ten units — light-years, kilometres, astronomical units, parsecs, kiloparsecs, megaparsecs, and light-seconds through light-days — and instantly reports how long light takes to cross that distance. Fifteen named cosmic destinations from the Moon to the edge of the observable Universe are built in as one-click presets.

How it works

The calculator is built on three exact physical constants defined by the International Bureau of Weights and Measures (BIPM) and the International Astronomical Union (IAU):

Speed of light: c = 299,792,458 m/s (exact, 2019 SI)
Astronomical Unit: 1 AU = 149,597,870,700 m (IAU 2012, exact)
Julian year: 1 yr = 365.25 × 86,400 s = 31,557,600 s

From these, the other units follow:

Light-year: 1 ly = c × 31,557,600 s = 9.4607304725808 × 10^15 m
Parsec: 1 pc = AU × (648,000 ÷ π) ≈ 3.08568 × 10^16 m = 3.26156 ly

Light travel time is then t = d ÷ c. No approximations are used anywhere in the chain.

Worked example: Proxima Centauri

Proxima Centauri lies 4.2441 light-years away. Converting step by step:

Metres: 4.2441 × 9.4607 × 10^15 = 4.015 × 10^16 m
Kilometres: 4.015 × 10^13 km (40.15 trillion km)
Astronomical Units: 4.015 × 10^13 ÷ 1.496 × 10^11 = 268,440 AU
Parsecs: 4.2441 ÷ 3.26156 = 1.3013 pc
Light travel time: 4.2441 × 365.25 × 86,400 = 1.339 × 10^8 s ≈ 4.244 years

So the light reaching your eyes tonight from Proxima left that star in mid-2021 (if you are reading this in 2026). This is cosmic look-back time: telescopes are time machines.

Formula note

All conversions are a single multiplication or division by the appropriate ratio of metre-exact values. No trigonometry or relativistic corrections are applied — the distances here are proper distances at a given epoch (essentially ruler distances), not light-travel distances corrected for cosmic expansion. For objects within our galaxy the difference is negligible. For the Andromeda Galaxy (~2.5 Mly) and beyond, the comoving vs. light-travel distance distinction starts to matter at the 1–10% level, but for the purposes of education and general reference this calculator is accurate to many significant figures.

Why so many units?

Different communities use different units for practical reasons:

Light-years — science communication, school textbooks, public astronomy
Parsecs / kiloparsecs / megaparsecs — professional astronomy, cosmology papers
Astronomical units — Solar System science, planetary missions
Kilometres — engineering, space-probe navigation
Light-seconds / minutes / hours — mission control signal-delay calculations

This tool covers all of them so you can move fluently between whichever context you are working in.