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Megaparsec to Light-Year Converter

Enter a value and pick a direction to convert between megaparsecs and light-years.

Result

2,544,019.7 ly
1 Mpc = 3,261,563.7 ly
0.780000 Mpc

How it is calculated

The conversion uses one fixed factor, 1 Mpc = 3,261,563.7 ly. Going from megaparsecs to light-years, ly = Mpc × 3,261,563.7. Going the other way, with the direction set to light-years to megaparsecs, Mpc = ly ÷ 3,261,563.7. The light-year result is shown with one decimal and the megaparsec result with six. The page also shows the factor used and the reverse conversion as a check. The value cannot be negative. Any direction value other than the light-year to megaparsec option is treated as megaparsec to light-year. The factor is a rounded figure, so very large distances carry the same relative rounding.

Worked example

The Andromeda Galaxy is about 0.78 Mpc away. Multiply by the factor: 0.78 × 3,261,563.7 = 2,544,019.686. The tool rounds this to one decimal and shows 2,544,019.7 ly. The reverse check divides back: 2,544,019.7 ÷ 3,261,563.7 gives 0.780000 Mpc.

Try this example →

Common mistakes

  • Leaving the direction on megaparsec to light-year when you are entering light-years. Typing 2,500,000 would then give a result in the trillions of light-years instead of about 0.77 Mpc.
  • Confusing megaparsecs with parsecs or kiloparsecs. One Mpc is a million parsecs, so a distance given in pc must be divided by 1,000,000 first.
  • Typing the number with a thousands separator, as in 2,500,000. Enter plain digits and a decimal point so the value is read correctly.
  • Treating a light-year distance to a faraway galaxy as the distance today. Light left it that long ago, and in an expanding universe the object is farther away now.
  • Entering a negative distance. The tool does not accept it and shows nothing.

About this calculator

A megaparsec (Mpc) is one million parsecs, and a parsec itself is defined by parallax — the distance at which one astronomical unit subtends an angle of one arcsecond. Because a parsec equals roughly 3.2616 light-years, one megaparsec works out to about 3,261,563.7 light-years. This converter switches between the two units in either direction using that exact factor.

Megaparsecs are the everyday distance unit of extragalactic astronomy and cosmology: the Andromeda Galaxy sits about 0.78 Mpc away, while the observable universe's radius spans thousands of megaparsecs, and quantities like the Hubble constant are conventionally expressed in km/s per Mpc. Astronomers and astrophysics students switch to light-years when communicating distances more intuitively to a general audience, or when comparing a galactic distance to the travel time of the light being observed — since a galaxy's light-years-away figure is literally how far back in time its light was emitted.

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Comments (1)

  • Editorial team

    For nearby galaxies a distance in light-years is a handy picture, but for very distant objects the number is not simply how long the light has travelled. Cosmological distances depend on how they are defined, so converting a catalogue's megaparsecs gives you the same distance in another unit, not a look-back time. Which catalogue or paper do your megaparsec values come from?

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