Light-Year Converter

Enter a non-negative distance, choose the source unit, and select the unit you want to convert to.

Use Swap to reverse the direction. When a valid result exists, the unrounded converted value is retained for the reversed calculation.

The input accepts ordinary decimals, grouped values such as 1,000,000, and scientific notation such as 2.5e6.

Each unit has a defined or conventional metres-per-unit factor.

The converter multiplies the source value by the ratio between the source and target factors.

Visible formatting is applied after calculation so swapping does not reuse a visibly rounded result.

A light-year is a unit of length, not time. It is the distance light travels through vacuum during one Julian year.

The speed of light in vacuum is exactly 299,792,458 metres per second. A Julian year contains exactly 31,557,600 seconds.

Multiplying those values gives 9,460,730,472,580,800 metres per light-year.

Light-years are commonly used for stellar distances and public explanations of the scale of space.

The astronomical unit, symbol au, equals exactly 149,597,870,700 metres.

It originated from the Earth–Sun distance scale, but the modern unit is a fixed conventional length rather than a changing orbital measurement.

Astronomical units are especially useful for planetary orbits, spacecraft positions, and distances within planetary systems.

A parsec is the distance associated with one astronomical unit subtending an angle of one arcsecond.

Its exact relation is 1 pc = (648,000/π) au.

One parsec is approximately 3.2616 light-years.

Parsecs and their larger multiples are widely used in stellar, galactic, and extragalactic astronomy.

All three units measure length, but they arise from different ideas.

The light-year connects distance with light travel, the astronomical unit reflects the Solar-System scale, and the parsec comes from parallax geometry.

Changing between them alters only the numerical representation of the same distance.

A metre is the SI base unit of length, and one kilometre equals exactly 1,000 metres.

The international mile equals exactly 1,609.344 metres.

These units provide familiar context, but astronomical distances expressed in them often require very large numbers.

A useful unit keeps the result readable and matches the scale being discussed.

Kilometres and miles work for familiar distances, au suits planetary systems, and light-years or parsecs suit interstellar space.

The table uses the same constants as the live converter.

Non-terminating results are rounded to no more than four decimal places or displayed in scientific notation.

Exact unit definitions do not make an observed astronomical distance exact.

Parallax uncertainty, calibration, model assumptions, source rounding, and the measurement technique still determine practical accuracy.

The interface uses up to four decimal places for readability and scientific notation when a fixed decimal would be impractical.

A light-year describes the distance light covers in one Julian year.

A spacecraft with mass cannot be assumed to travel at the speed of light.

Actual travel time depends on speed, acceleration, propulsion, route, and mission constraints.

The conversion runs entirely in the browser.

Distance values and unit selections are not uploaded, placed in a public URL, or permanently stored.