Acquire either the radius of the orbit, measured in astronomical units (AU), or the time it takes a planet to complete one full orbit. With one of these measurements, you can calculate the other measurement.
Use this formula: The orbital time in years, squared, equals the orbital radius in AU, cubed.
Solve the formula to calculate whichever measurement you do not have--i.e., orbital time or orbital radius.
Orbital time equals the square root of the cube of the orbital radius.
Orbital radius equals the cube root of the square of the orbital time.
Enter your data into the formulas. As an example, if you had an orbital radius of 2.0 AU or an orbital time of 2.83 years:
Orbital time = square root ( cube ( 2.0 ) )
Orbital time = square root ( 8 )
Orbital time = 2.83 years
Orbital radius = cube root ( square ( 2.83 ) )
Orbital radius = cube root (8.01)
Orbital radius = 2.0 AU