Enter a starting balance, optional periodic contributions, an expected annual return, and a time horizon. The calculator projects nominal and inflation-adjusted (real) future value.
Future value (nominal)
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Total contributions
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Total interest
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Interest as % of total
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Year-by-year balance
| Year | Balance | Contributions YTD | Interest YTD |
|---|---|---|---|
| Fill the form. | |||
Worked example
Start with $10,000, add $6,000 a year (deposited as $500 at the start of each month), earn 7% a year compounded monthly, and let it run for 30 years. The monthly rate is 7% divided by 12, about 0.5833%. Each month the $500 contribution goes in first, then the whole balance earns that month's interest, so every dollar starts compounding immediately. Over 30 years you deposit $10,000 up front plus $180,000 in contributions, $190,000 of your own money in total. Compounding turns that into roughly $694,709. The gap between the ending balance and what you put in, about $504,709, is interest, which is nearly 73% of the final balance. After adjusting for 3% inflation, the $694,709 is worth about $286,211 in today's dollars. The longer the horizon, the more the interest share dominates, which is the core lesson of compounding.
| Item | Amount |
|---|---|
| Starting principal | $10,000 |
| Contributions ($500/mo x 360) | $180,000 |
| Total invested | $190,000 |
| Interest earned | $504,709 |
| Future value (nominal) | $694,709 |
| Real value (after 3% inflation) | $286,211 |
How it is calculated
Compound interest means you earn returns not only on your original principal but also on the interest already credited, so growth accelerates over time. The tool steps period by period using the more frequent of your compounding and contribution schedules. Each period it converts the annual rate to a periodic rate, adds any scheduled contribution at the start of the period as an annuity due, then applies the periodic rate to the running balance. Total contributions are your principal plus every deposit; total interest is the ending balance minus those contributions. Compounding more frequently, monthly versus annually, slightly raises the effective yield because interest starts earning sooner. The real value line discounts the final balance by your inflation rate so you can see its purchasing power rather than just the headline number.