Estimate long-term growth with compound interest and monthly contributions.
Enter a starting amount, an expected annual return, the number of years you plan to invest, and an optional monthly contribution. The calculator compounds the return every month over that period, adding your monthly contribution before applying that month's growth, and reports the estimated final value alongside how much of it came from contributions versus growth.
Even a modest monthly contribution compounds alongside the starting amount, so two savers with the same starting balance and return rate can end up with very different totals depending on whether they kept adding money. Try the calculator with the monthly contribution set to 0 to see how much of your result is pure growth versus your own deposits.
Long before spreadsheets, investors used a simple trick to estimate doubling time without a calculator: divide 72 by the annual return percentage. At this page's default 5% return, 72 ÷ 5 ≈ 14.4 years to double. At a more aggressive 8% return, it's 72 ÷ 8 = 9 years. The Rule of 72 is an approximation — it gets less accurate at very high or very low rates — but it's accurate enough to sanity-check the calculator's output or compare two return rates in your head.
Monthly. The annual return you enter is divided by 12 and applied each month, along with that month's contribution, for the number of months in your chosen term.
No — it's a projection based on the constant annual return you entered. Real investment returns vary year to year, so treat this as an estimate for planning, not a promise.
Yes. Enter your savings account's interest rate as the annual return and leave contributions at whatever you plan to deposit monthly.
Yes — the Rule of 72. Divide 72 by your annual return percentage to get an approximate number of years. At the calculator's default 5% return, that's about 14.4 years; at 8%, about 9 years. It's a rough mental shortcut, not a substitute for running the full calculation.