How this calculator works
Divide 72 by the annual return and you get, near enough, the number of years for money to double. At 7% that is about 10.3 years — the exact answer is 10.2. The shortcut is accurate enough for mental maths between roughly 5% and 12%, and drifts at the extremes, which is exactly what the two numbers above show.
This calculator has no currency because the answer is the same whatever you hold: doubling is doubling.