See how your money can grow over time with compound interest and regular monthly contributions. Enter your starting amount, how much you plan to add each month, an annual return, and how long you plan to invest — the calculator updates instantly and shows your projected balance year by year.
Current as of November 2026.
Future value
$144,573
Total contributions
$58,000
Total interest earned
$86,573
Growth multiple
2.49×
Monthly contributions are treated as end-of-month deposits. Results are estimates based on a constant annual rate — actual investment returns vary year to year and are never guaranteed.
Growth chart
Year-by-year breakdown
Year
Balance
Contributed so far
Interest earned so far
Worked example: Start with $10,000, add $200 per month, earn 7% per year compounded monthly, for 20 years.
Future value of the initial $10,000: $10,000 × (1 + 0.07/12)240 = $40,387.39
Future value of the monthly contributions: $200 × [((1 + 0.07/12)240 − 1) / (0.07/12)] = $104,185.33
Total future value: $144,572.72 | Total contributed: $58,000 | Total interest earned: $86,572.72
Year 1 balance: $13,201.42 | Year 5: $28,494.83 | Year 10: $54,713.58 | Year 20: $144,572.72
How It Works
Compound interest means you earn returns not only on your original deposit, but also on all the interest you have already earned. Over long periods, that snowball effect does most of the work — which is why starting early and contributing regularly matters more than chasing a slightly higher rate.
Standard formula: A = P(1 + r/n)nt + PMT × [((1 + r/n)nt − 1) / (r/n)]
P is your initial investment, r is the annual rate as a decimal (7% = 0.07), n is how many times interest compounds per year, t is the number of years, and PMT is your monthly contribution (assumed to be deposited at the end of each month). The first part grows your lump sum; the second part grows all of your regular contributions. When compounding is not monthly, contributions are valued at the equivalent monthly rate so the maths stays consistent.
What return should you assume? The S&P 500 has averaged roughly 10% per year nominally since the 1920s, or about 7% after inflation — a commonly cited long-term planning assumption. Many planners use 7% as a conservative nominal figure (or lower, after fees and taxes). Treat any assumed rate as a planning scenario, not a prediction: individual years can be far higher or far lower, and losses are possible.
Try it: raise the monthly contribution by $100 and watch how much the 20-year balance moves — regular deposits usually matter more than the starting amount. Then try a lower rate, like 5%, to see how sensitive long-term results are to the return you assume.
Frequently Asked Questions
What is compound interest?Compound interest is growth earned on your money and on the growth it has already produced. Unlike simple interest, which only pays on the original amount, compound interest reinvests each period's earnings so your balance grows faster over time.
What annual return should I use in this calculator?As a planning assumption, many people use 7% — roughly the long-run inflation-adjusted average return of the US stock market (about 10% nominal before inflation). Conservative planners often use 5–6%. The rate you enter is only an assumption, not a guarantee, and actual returns vary widely from year to year.
How do monthly contributions change the result?Regular contributions add up fast. In the worked example above, $200 a month for 20 years is $48,000 of deposits, but those deposits grow to $104,185 — more than double what was paid in. Over long horizons, contributions and their growth usually make up most of the final balance.
Does compounding frequency matter much?Yes, but modestly. Monthly compounding beats annual compounding slightly at the same rate, and daily compounding adds only a little more. The rate itself and the number of years matter far more than whether interest compounds monthly or daily.
Are the results guaranteed?No. This calculator assumes a constant annual rate, no fees, no taxes, and no inflation adjustment. Real investments fluctuate, can lose value, and are affected by fees, taxes, and inflation. Use the results for planning and comparison, not as a promise of what you will receive.
Does this calculator include taxes, fees, or inflation?No. The figures are before tax, before fees, and in future (nominal) dollars. To allow for inflation, try a lower rate — for example, enter 4% instead of 7% to roughly show growth in today's purchasing power if inflation averages about 3%.
Disclaimer: This calculator provides estimates for educational and planning purposes only and is not financial, investment, or tax advice. Returns are assumed to be constant and are not guaranteed; investments can lose value. Past performance does not predict future results. Figures do not include taxes, fees, or inflation. Consult a licensed financial adviser before making investment decisions.