Light-Year to AU Converter

How it is calculated

Both directions use one fixed factor, 1 ly = 63,241.077 AU. Light-years to AU: AU = ly × 63241.077, shown with two decimals. AU to light-years: ly = AU / 63241.077, shown with six decimals. The tool also prints the rate line (for the reverse direction, 1 AU = 0.00001581 ly) and a check line that restates the conversion with the original number.

Choose the direction with the selector; the value is read in whichever unit that direction starts from. Negative numbers and non-numeric text return no result, and zero converts to zero. Because the AU-to-light-year answer is rounded to six decimals, very small distances lose precision: 1 AU shows as 0.000016 ly, so for small values use the rate line (0.00001581 per AU) or work in AU.

Common mistakes

• Leaving the selector on the wrong direction. Entering 4.25 while it is set to AU to light-years gives a tiny 0.000067 ly, not 268,774 AU.
• Expecting more than six decimals when converting small AU values. A single AU shows as 0.000016 ly, which is rounded; the exact factor is 1 AU = 0.00001581 ly.
• Typing a negative distance or including units or text. Only non-negative plain numbers are accepted.
• Confusing light-years with parsecs or with kilometres. This tool only converts between light-years and AU; one parsec is about 3.26 light-years and needs a different factor.
• Rounding the AU result and then converting it back by hand. The small rounding error in the printed figure is multiplied, so use the unrounded number you entered.