How were Juan Carlos I of Spain and Charles III related?

Charles III was the third cousin of Juan Carlos I of Spain.

Their nearest shared ancestors were Prince Alexander of Hesse and by Rhine and Julia, Princess of Battenberg.

That is only the shortest of 5 separate lines of descent between them.

The line between them

1. Juan Carlos I of Spain born 1938
2. Infante Juan, Count of Barcelona 1913–1993
3. Victoria Eugenie of Battenberg 1887–1969
4. Prince Henry of Battenberg 1858–1896
5. Prince Alexander of Hesse and by Rhine 1823–1888
6. Prince Louis of Battenberg 1854–1921
7. Princess Alice of Battenberg 1885–1969
8. Prince Philip, Duke of Edinburgh 1921–2021
9. Charles III born 1948

See the line on the tree