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final balance
Compound interest pays interest on interest: each period the balance grows by the periodic rate and the new interest joins the principal for the next round. Add monthly contributions and the effect stacks — in the example defaults, a 5,000 start plus 200 a month at 6% becomes roughly double the money contributed after fifteen years, with the interest share accelerating over time. The frequency selector shows why more frequent compounding nudges the effective annual rate above the quoted one.
The rule of 72 (years to double ≈ 72 ÷ rate) makes the timescale visceral: 6% doubles money every twelve years, so a 24-year horizon means two doublings on every unit invested early. That's why the halfway-point row typically shows well under half the final balance — the second half of the timeline does most of the work, and why starting early beats starting big. Against savings, the same curve is your mortgage's total-interest figure.