Calculators
Compound interest calculator
Enter a starting balance, what you add each month, a rate and a horizon. The chart splits the ending balance into what you put in and what compounding added — which is usually the more surprising half.
Your assumptions
Balance after 25 years
$462,290
Growth overtakes your contributions in year 17.
You contributed
$160,000
Growth added
$302,290
Growth share
65.4%
of the ending balance
Each month the balance grows by (1 + r ÷ n)n ÷ 12 − 1 and the contribution is added afterwards, where r is the annual return and n the compounding periods per year. Growth first, contribution second — an end-of-month deposit earns nothing that month.
Contributions vs growth, by year
- Contributed
- Growth
| Year | Contributed | Growth | Balance |
|---|---|---|---|
| 1 | $16,000 | $919.19 | $16,919 |
| 2 | $22,000 | $2,339 | $24,339 |
| 3 | $28,000 | $4,294 | $32,294 |
| 4 | $34,000 | $6,825 | $40,825 |
| 5 | $40,000 | $9,973 | $49,973 |
| 6 | $46,000 | $13,782 | $59,782 |
| 7 | $52,000 | $18,299 | $70,299 |
| 8 | $58,000 | $23,578 | $81,578 |
| 9 | $64,000 | $29,671 | $93,671 |
| 10 | $70,000 | $36,639 | $106,639 |
| 11 | $76,000 | $44,544 | $120,544 |
| 12 | $82,000 | $53,455 | $135,455 |
| 13 | $88,000 | $63,443 | $151,443 |
| 14 | $94,000 | $74,587 | $168,587 |
| 15 | $100,000 | $86,971 | $186,971 |
| 16 | $106,000 | $100,683 | $206,683 |
| 17 | $112,000 | $115,820 | $227,820 |
| 18 | $118,000 | $132,486 | $250,486 |
| 19 | $124,000 | $150,790 | $274,790 |
| 20 | $130,000 | $170,851 | $300,851 |
| 21 | $136,000 | $192,796 | $328,796 |
| 22 | $142,000 | $216,760 | $358,760 |
| 23 | $148,000 | $242,892 | $390,892 |
| 24 | $154,000 | $271,345 | $425,345 |
| 25 | $160,000 | $302,290 | $462,290 |
What the chart is actually showing
The two colours are your money and the market's. In the early years the bar is almost entirely the first colour: nothing has had time to compound, and the balance is close to the sum of the deposits. Somewhere in the middle the second colour overtakes the first, and from that point on the portfolio is growing faster than you are funding it. The calculator names the year that happens, because it is the only milestone in the whole projection that means anything.
That crossover year is far more sensitive to the horizon than to the rate. Adding two percentage points to the return moves it a few years earlier; adding ten years to the horizon moves it into existence at all. This is why the standard advice is about starting early rather than about picking better, and it is visible directly in the bars.
One caution about the rate. A long-run equity return is an average of years that looked nothing like the average — the S&P 500's own record contains a year down 47% and a year up 45%. A smooth curve at 7% is a reasonable way to size a plan and a poor way to predict any particular decade.
Questions people ask about this
- What compounding frequency should I use?
- For a savings account or a bond, use the frequency the product actually states — monthly and daily are both common, and the difference over long horizons is small but real. For a stock portfolio there is no compounding frequency at all; annual is the honest choice, because a market return is a change in price, not interest credited on a schedule.
- Is the contribution added before or after the period's growth?
- After. Each period the existing balance grows first, then the contribution is added. That is the conservative convention: contributing at the end of the month rather than the start, which matches how most people are actually paid.
- Should I use a nominal or a real rate of return?
- If you want to know what the balance will buy rather than what the statement will say, subtract inflation from the rate before entering it. A 7% nominal return with 3% inflation is closer to 4% in purchasing power, and over thirty years the two answers differ by more than the starting balance.
Sources and method
- Data
- None. This tool sends nothing anywhere — every figure is computed in your browser from the values you type, and no input is stored, logged or transmitted.
- How it was calculated
- Simulated month by month in your browser. Each month the balance grows by (1 + r ÷ n)^(n ÷ 12) − 1, then the contribution is added; n is the compounding periods per year. Contributions step up once a year by the increase you set. No fees, taxes or inflation are applied.
- How often it changes
- Never. The formula is fixed; the answer changes only when you change an input.
- Citing this page
Free to quote — please link rather than copy the table.
Plutux. "Compound interest calculator." https://plutux.ai/resources/tools/compound-interest-calculator
Historical figures for information only — not investment advice, and not a forecast.
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Plutux is not an investment adviser. Market data and AI-generated analysis are for information and education only, not investment advice. Disclaimer