Light Year Converter

What the Light Year Converter does

Light Year Converter: 1 ly = 9,460,730,472,580.8 km, so multiply a light year value by 9,460,730,472,580.8 to get kilometre, or divide to go back. Example: 1 ly = 9,460,730,472,580.8 km.

With Value, From and To entered, the Light Year Converter works out Result, How it was calculated and Reverse conversion. Change any input and the output follows immediately.

Converting once from the original figure beats chaining approximations. That is usually what brings engineers checking a datasheet here.

The work is done by JavaScript already loaded in your browser, so your input never crosses the network.

Field definitions, the method, and a worked example from the default values are all set out further down the page.

What do the Light Year Converter fields mean?

The Light Year Converter uses 3 inputs. None of the boxes start empty. The page loads with a worked set of values you can edit or replace.

How does the Light Year Converter work?

Both units are converted through length's base unit (m), so any of the 19 units in the dropdowns can be paired in either direction without loss of accuracy.

Exact definitions are preferred throughout: the metre, the inch and the pound all have them. Only where a unit depends on a measured constant is an accepted published value substituted.

Worked example

These are the values the Light Year Converter loads by default, and the result it produces from them. Open the tool above and you will find precisely these figures already in place.

The light year

A light year is a unit of distance, not time. It is how far light travels in one Julian year (365.25 days), which works out to exactly 9,460,730,472,580.8 kilometres using the defined speed of light (299,792,458 m/s).

Worth knowing

• Because the speed of light is an exact defined constant (it fixes the length of the metre, not the other way around, since 1983), and the Julian year is also an exact defined length, the light year itself is an exact figure, not a measured approximation, despite representing an astronomically large number.
• The nearest star system beyond the Sun, Proxima Centauri, is about 4.25 light years away, meaning the light reaching us from it left over four years ago, a genuinely common point of confusion since it sounds like a statement about time rather than distance.
• Astronomers more often use the parsec (about 3.26 light years) for professional work, because it relates directly to a star’s observed parallax angle rather than requiring a separate distance calculation.

Definitions: BIPM SI Brochure · NIST reference on units

How to use it

1. Type Value.
2. Pick a value for From: the options are Nanometre (nm), Micrometre (µm), Angstrom (Å), Millimetre (mm), Centimetre (cm) or Metre (m).
3. Pick a value for To: the options are Nanometre (nm), Micrometre (µm), Angstrom (Å), Millimetre (mm), Centimetre (cm) or Metre (m).
4. The answer follows your cursor, in effect. Change a field and read the new figure.
5. Look at Result, How it was calculated and Reverse conversion in the output panel.
6. Use Copy Link if you want the same setup again later, or Print for a paper copy.

What this tool does not do

• Conversion factors use SI definitions; some industries use their own rounded factors.
• Repeated conversions compound rounding, so convert once from the original value where possible.
• The tool answers exactly the question it is given; it cannot know the context around your numbers.
• Garbage in, garbage out: confirm each figure means what you think it does before trusting the result.

Frequently asked questions

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