Compound Interest Calculator
See how a starting amount plus monthly contributions grows over time. Shows future value, total contributions, interest earned, and a year-by-year breakdown — with the formula and assumptions stated. Informational only, not financial advice.
| Year | Balance | Total contributed | Interest earned |
|---|---|---|---|
| 1 | $13,201.42 | $12,400.00 | $801.42 |
| 2 | $16,634.27 | $14,800.00 | $1,834.27 |
| 3 | $20,315.28 | $17,200.00 | $3,115.28 |
| 4 | $24,262.39 | $19,600.00 | $4,662.39 |
| 5 | $28,494.83 | $22,000.00 | $6,494.83 |
| 6 | $33,033.24 | $24,400.00 | $8,633.24 |
| 7 | $37,899.74 | $26,800.00 | $11,099.74 |
| 8 | $43,118.03 | $29,200.00 | $13,918.03 |
| 9 | $48,713.55 | $31,600.00 | $17,113.55 |
| 10 | $54,713.58 | $34,000.00 | $20,713.58 |
| 11 | $61,147.34 | $36,400.00 | $24,747.34 |
| 12 | $68,046.20 | $38,800.00 | $29,246.20 |
| 13 | $75,443.79 | $41,200.00 | $34,243.79 |
| 14 | $83,376.14 | $43,600.00 | $39,776.14 |
| 15 | $91,881.93 | $46,000.00 | $45,881.93 |
| 16 | $101,002.60 | $48,400.00 | $52,602.60 |
| 17 | $110,782.60 | $50,800.00 | $59,982.60 |
| 18 | $121,269.60 | $53,200.00 | $68,069.60 |
| 19 | $132,514.70 | $55,600.00 | $76,914.70 |
| 20 | $144,572.72 | $58,000.00 | $86,572.72 |
How to use this tool
- Enter your starting amount and any regular monthly contribution.
- Set the expected annual interest rate and how often it compounds.
- Choose the number of years — the future value, total contributed, and interest earned update instantly.
- Scan the year-by-year table to see how growth accelerates over time.
How it works
The calculator applies the standard compound interest formula: the balance grows by rate ÷ periods-per-year each period, and monthly contributions are added as they occur. For a lump sum, this is A = P(1 + r/n)^(nt), where P is the principal, r the annual rate, n compounding periods per year, and t years.
Assumptions: the rate stays constant for the whole term, contributions are made at the end of each month, and no taxes or fees are deducted. Real investment returns vary year to year — treat results as a planning estimate, not a prediction.
Sources & references
- SEC Investor.gov — Compound Interest Calculator & formula
- NYU Stern (Damodaran) — Historical returns on stocks, bonds and bills
Content last reviewed: July 2026.
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Frequently asked questions
What is compound interest?
Interest earned on both your original money and on previously earned interest. Each period's growth is added to the balance, so the next period's growth is calculated on a larger amount — which is why long time horizons matter so much.
How much difference does compounding frequency make?
Less than most people expect. $10,000 at 5% for 10 years grows to $16,289 compounded annually versus $16,470 compounded monthly — about 1% difference. Rate and time matter far more than frequency.
What rate should I assume?
That's your call, not ours. Long-run historical US stock returns (see the Damodaran dataset in the sources below) have varied widely by period, and savings accounts pay far less than market averages. Try several rates to see the range of outcomes — past returns don't predict future ones, and this tool isn't financial advice.
Does this account for inflation or taxes?
No. Results are nominal, before taxes and fees. To approximate inflation-adjusted growth, use a rate reduced by expected inflation (for example, 7% nominal minus 3% inflation ≈ 4% real).