Light-Year Distance Calculator

How it is calculated

You enter a distance in light-years and the tool multiplies or divides it by fixed constants: km = ly × 9.4607304725808 × 1012, AU = ly × 63,241.077 and parsecs = ly ÷ 3.2615638. Kilometres are rounded to a whole number, AU to two decimals and parsecs to four.

The conversion goes in one direction only, from light-years to the other three units. To go from parsecs or kilometres back to light-years, use the figures above in reverse. Negative distances give no result, and zero returns zeros. Very large values produce very long kilometre figures, so count the digits carefully when copying them. The constants are exact for the standard Julian light-year, not for the distance light covers in a calendar year.

Common mistakes

• Treating a light-year as a unit of time. It measures distance: the path light covers in one year.
• Confusing light-years with parsecs. One parsec is about 3.26 light-years, so a catalogue distance in pc must be multiplied by 3.2616 before using this field.
• Entering a distance already in light-years from a source that uses thousands or millions of light-years, without scaling it. 2 kly must be typed as 2000.
• Reading the AU figure as a travel estimate. It is only the distance expressed in Earth–Sun units and says nothing about how long a trip would take.
• Dropping digits when copying the long kilometre result, which changes the magnitude by a factor of ten.