How this calculator works
The astronomical unit was redefined by the International Astronomical Union in 2012 as exactly 149,597,870,700 m. It is close to the average Earth-Sun distance and is the natural ruler for the solar system. The light year is the distance light travels in one Julian year, and the parsec is the distance at which one astronomical unit subtends one arcsecond. The nominal solar radius is a defined constant of 695,700 km, not a measured photospheric radius.
Formula
metres = value x F(unit). 1 AU = 149,597,870,700 m exactly. 1 ly = 299,792,458 m/s x 365.25 x 86,400 s = 9,460,730,472,580,800 m exactly. 1 pc = 648,000 / pi AU = about 3.0856775814913673e16 m. 1 R-sun = 695,700 km.
Worked example
1 AU is about 149.598 million km, 8.31675 light-minutes and 4.84814e-6 pc. 1 light year is about 63,241.1 AU. 1 parsec is about 206,264.8 AU.
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Frequently asked questions
Is the astronomical unit fixed or measured?
It is fixed by definition at exactly 149,597,870,700 m. The actual Earth-Sun distance varies through the year because Earth's orbit is elliptical, but the unit itself does not change.
Why use parsecs instead of light years?
The parsec follows directly from parallax, the angle shift of a star as Earth moves around the Sun. Astronomers measure parallax in arcseconds, so parsecs make the arithmetic simple.
How big is the solar radius used here?
The IAU nominal solar radius is 695,700 km. The Sun is not a solid body, so its visible edge depends on wavelength and the exact definition, which is why a nominal constant is used.