A compound interest calculator estimates how an initial balance may grow when interest is added over time and optional contributions are made each month. Enter the starting amount, monthly contribution, annual interest rate, time horizon, and compounding frequency; you can also choose an inflation rate to view an estimate of the future balance in today's purchasing power.
The result is a projection based on a fixed rate and regular end-of-month contributions. It does not predict investment performance or account for changing rates, taxes, fees, withdrawals, or missed deposits.
Figures in this article were reproduced from the calculator itself, so the worked examples match what the tool returns for the same inputs.
How to Use the Compound Interest Calculator
- Enter the starting amount: Type the Initial Investment, the lump sum you are starting with, and a Monthly Contribution if you plan to keep adding money each month.
- Set the rate and horizon: Enter the Annual Interest Rate (% p.a.) and the Time Horizon in years.
- Choose the compounding: Pick a Compounding Frequency of Monthly, Daily, Quarterly, or Annually; more frequent compounding grows the money faster.
- Optionally adjust for inflation: Tick the Adjust for Inflation box and enter a rate to see what the future balance is worth in today's money.
- Calculate and read the results: Click Calculate to see the Future Value, total contributions, earned interest, and a year-by-year table of balances.
Leave the Monthly Contribution at 0 for a lump-sum-only projection, and treat the fixed-rate result as a mathematical projection rather than a forecast of returns.
How Compound Interest Works
With simple interest, interest is calculated only on the original principal. With compound interest, previously credited interest remains in the balance and can earn interest in later periods. As the balance grows, the amount of interest may also grow, assuming the rate and compounding terms stay the same.
| $10,000 at a fixed 8% for 30 years | Balance after 30 years | Interest earned |
|---|---|---|
| Simple interest, no reinvestment | $34,000.00 | $24,000.00 |
| Monthly compounding, no further contributions | $109,357.30 | $99,357.30 |
These are mathematical illustrations, not a forecast or a guaranteed return. The compound row is the calculator's lump-sum result for the same rate and horizon.
The Mathematical Foundation of Compounding
For one initial balance with no contributions, the standard formula is A = P(1 + r/n)nt, where P is the initial amount, r is the nominal annual rate as a decimal, n is the number of compounding periods per year, and t is the time in years.
This calculator also accepts monthly contributions, so it converts the selected compounding assumption into an effective monthly rate. Let i = (1 + r/n)n/12 - 1 and let M be the number of months in the selected horizon. For end-of-month contributions C, the projection is:
Future value = P(1 + i)M + C × [((1 + i)M - 1) / i]
When the rate is 0%, the result is the initial amount plus monthly contributions multiplied by the number of months. The calculator rounds the selected time horizon to a whole number of months for the projection.
Compounding Frequency Example
The table compares the calculator's results for a $10,000 starting balance, $500 added at the end of each month, an 8% nominal annual rate, and a 20-year horizon. Total contributions are $130,000.
| Selected frequency | Periods per year | Projected future value | Modeled interest | Difference from annual example |
|---|---|---|---|---|
| Annually | 1 | $331,109.11 | $201,109.11 | n/a |
| Quarterly | 4 | $341,341.52 | $211,341.52 | +$10,232.41 |
| Monthly | 12 | $343,778.24 | $213,778.24 | +$12,669.13 |
| Daily | 365 | $344,979.49 | $214,979.49 | +$13,870.38 |
This comparison holds the same nominal annual rate constant, so the effective annual rate differs by frequency. It illustrates the calculator's assumptions; it is not an apples-to-apples comparison of real accounts. When comparing account disclosures, use comparable effective annual yields such as APYs.
What Affects the Projection?
The rate is usually the biggest lever. Holding everything else constant — $10,000 starting balance, $500 added at the end of each month, 20-year horizon, monthly compounding — the calculator returns three very different projections for three rates:
| Assumed annual rate | Projected future value | Modeled interest |
|---|---|---|
| 5% | $232,643.24 | $102,643.24 |
| 8% | $343,778.24 | $213,778.24 |
| 11% | $522,169.17 | $392,169.17 |
Contributions are identical ($130,000) in all three rows; the modeled interest ranges from $102,643.24 to $392,169.17. More time works the same way: it gives existing balances and earlier contributions more periods to grow, but the calculator assumes the same rate and monthly deposit throughout the selected horizon. Actual balances can differ if rates change, contributions vary, or money is withdrawn.
Monthly contributions are modeled at the end of each month. Deposits made earlier could earn more in an account that credits interest during that period, but this calculator does not provide a beginning-of-month contribution option.
Rule of 72: A Rough Doubling Estimate
For a quick estimate, divide 72 by a fixed annual rate expressed as a percentage. At 8%, for example, 72 ÷ 8 suggests about nine years to double. This shortcut is approximate; it does not include contributions, taxes, fees, inflation, or changing returns.
Inflation Adjustment and Important Limits
The optional inflation adjustment divides the projected future balance by the cumulative effect of the annual inflation rate you enter. For example, a projected $343,778.24 after 20 years becomes about $190,341.67 in today's dollars with a constant 3% annual inflation assumption. This is a scenario, not a forecast of future inflation or purchasing power.
The calculator does not subtract taxes or fees, model withdrawals, or account for variable returns. Its compounding-frequency comparison uses the same nominal annual rate for each option, which creates different effective annual yields. For real savings or investment products, compare the rate basis and disclosed APY or equivalent yield. The currency selector changes how amounts are displayed; it does not convert values between currencies.
For a target-based monthly savings estimate, try the Savings Goal Calculator. To compare a stated rate with its effective annual yield, use the APY Calculator. For a return estimate based on costs and revenue, see the Investment ROI Calculator.
The same formula applies to any balance that compounds — savings, investments, or retirement accounts — while loans compound in reverse, which is why the same math underpins amortization in the loan calculators here.
Last verified: October 2026. Calculator figures are outputs for the inputs entered; they are not live quotes, current rates, or an offer.