The compound growth formula
This calculator combines two classic finance formulas: the future value of a lump sum, and the future value of a series of regular deposits (an annuity). Together they answer the question, “if I start with this much, add this much every month, and earn this rate, what will I have?”
Where P is the starting balance, PMT is the contribution each period, i is the periodic return rate (annual rate ÷ 12 for monthly), and n is the number of periods (years × 12). The first term grows your initial lump sum; the second grows the stream of monthly deposits, each of which compounds for a different length of time.
The engine compounds monthly, which mirrors how most people actually invest — a paycheck deposit each month rather than a single yearly lump.
Worked example
Start with $10,000, add $300 a month, earn 7% a year, for 25 years:
Compounding frequency & the rule of 72
Compounding more often — monthly versus annually — adds a little extra because interest is credited sooner and starts earning on itself. A handy mental shortcut is the Rule of 72: divide 72 by your annual return to estimate the years it takes money to double. At 7%, that's about 72 ÷ 7 ≈ 10.3 years per doubling.