A light-year is the distance light travels in a Julian year.
Light-years to Fathoms Converter — ly to ftm
Convert Light-year (ly) to Fathom (ftm) using the exact conversion factor (1 light-year = 5.17319e+15 fathom). See the formula, worked examples, conversion table, history, and common uses.
Light-years to Fathoms converter
Converter
Length Converter
Convert SI, imperial, US customary, astronomical, scientific, and historical length units.
Context values
Some unit families need one extra value for a physically accurate result.
A fathom is six feet.
Result = input x 9.46073047e+15 / 1.8288.
Length uses the meter as the base unit.
Reserved above the conversion cards and below the converter.
About Converting Light-years to Fathoms
Light-year and Fathom both measure length. Light-year is most often used for measuring distances to stars and galaxies.
Formula
fathoms = light-years × 5.17319e+15
This factor comes from each unit's defined relationship to the category's base unit: 1 light-year equals 9.46073e+15 base units, and 1 fathom equals 1.8288 base units, so dividing one by the other gives the direct light-year-to-fathom factor of 5.17319e+15.
Simple example
1 ly × 5.17319e+15 = 5.17319e+15 ftm
1 light-year = 5.17319e+15 fathoms.
Real-world example
10 ly × 5.17319e+15 = 5.17319e+16 ftm
10 light-years = 5.17319e+16 fathoms.
Conversion table
| Light-year (ly) | Fathom (ftm) |
|---|---|
| 1 ly | 5.17319e+15 ftm |
| 5 ly | 2.586595e+16 ftm |
| 10 ly | 5.17319e+16 ftm |
| 21.0975 ly | 1.091414e+17 ftm |
| 42.195 ly | 2.182828e+17 ftm |
| 100 ly | 5.17319e+17 ftm |
Reverse conversion: Fathom to Light-year
5.17319e+15 ftm × 1.933043e-16 = 0.9999999365 ly
5.17319e+15 fathoms = 0.9999999365 light-years.
light-years = fathoms × 1.933043e-16
For a page dedicated to this direction, see Fathom to Light-year.
Understanding the Light-year
Light-year (ly) measures distance. A light-year is the distance light travels in one Julian year (365.25 days) in vacuum: exactly 9,460,730,472,580,800 meters. It is classified as a Non-SI unit (astronomical).
Where it's used: Used internationally in astronomy and popular science for interstellar distances.
Uses of the Light-year today
- Measuring distances to stars and galaxies
- Popular science communication about the scale of the universe
- Astrophysics research
History of the Light-year
The light-year emerged in 19th-century astronomy as scientists began measuring stellar distances via parallax (Friedrich Bessel's 1838 measurement of 61 Cygni is often cited as the first reliable stellar distance). Expressing these vast distances in light-travel-time became a practical convention, and the unit's value follows directly from the defined speed of light and the length of a Julian year.
Historical origin. Light-year was not the invention of a single named person; it developed through the process described above.
Understanding the Fathom (ftm)
A fathom is six feet.
Light-year — sources
- IAU — Light year definition — Defines the light-year via the Julian year and the speed of light.
Reserved between the cards and the FAQ so the page stays balanced.
Frequently asked questions
How many fathoms are in 1 light-year?
1 light-year equals 5.17319e+15 fathoms, using the exact defined conversion factor rather than an estimate.
How do I convert light-year to fathom?
Multiply the light-year value by 5.17319e+15. The converter above does this instantly to whatever precision you set.
How do I convert fathom back to light-year?
Use the reverse factor: 1 fathom equals 1.933043e-16 light-years. You can also use the swap control in the converter above to flip the direction instantly.
What is a light-year?
Light-year (ly) measures distance. A light-year is the distance light travels in one Julian year (365.25 days) in vacuum: exactly 9,460,730,472,580,800 meters. It is classified as a Non-SI unit (astronomical).
What is a fathom?
Fathom (ftm) is a unit of length.
Is the light-year to fathom conversion exact?
Yes. A light-year is the distance light travels in one Julian year (365.25 days) in vacuum: exactly 9,460,730,472,580,800 meters. That fixed definition is what the converter above uses, so any rounding you see is a display choice, not a limitation of the math.
Where is the light-year still used today?
Used internationally in astronomy and popular science for interstellar distances.