Compounding pays interest on interest, which is why the curve steepens the longer you leave the money alone. Enter your own numbers to see the balance, what it earned, and roughly how long it takes to double at that rate.
Compounded once a year, with no additional contributions.
Green is interest on your money; grey is the principal itself.
Compounding pays interest on interest, so the curve steepens later in the period — the same rate over 40 years produces roughly twice the balance of 20 years. A rate you can rely on for decades is the scarce part: savings rates and bond yields move with central bank policy, which is why the interest half of this chart is the half that moves.