Savings
Compound Interest Calculator
See how your savings grow when interest earns interest. Enter a starting balance, regular contributions, and an expected annual rate to project the ending balance, total contributions, and interest earned over time.
Results
Ending balance: $106,639.02, Total contributions: $70,000.00, Interest earned: $36,639.02, Interest share of balance: 34.4%
- Ending balance
- $106,639.02
- Total contributions
- $70,000.00
- Interest earned
- $36,639.02
- Interest share of balance
- 34.4%
- Portion of the final balance that came from growth
Year-by-year growth
| Year | Contributions | Interest | Balance |
|---|---|---|---|
| 1 | $16,000.00 | $919.19 | $16,919.19 |
| 2 | $22,000.00 | $2,338.58 | $24,338.58 |
| 3 | $28,000.00 | $4,294.31 | $32,294.31 |
| 4 | $34,000.00 | $6,825.16 | $40,825.16 |
| 5 | $40,000.00 | $9,972.70 | $49,972.70 |
| 6 | $46,000.00 | $13,781.53 | $59,781.53 |
| 7 | $52,000.00 | $18,299.43 | $70,299.43 |
| 8 | $58,000.00 | $23,577.68 | $81,577.68 |
| 9 | $64,000.00 | $29,671.22 | $93,671.22 |
| 10 | $70,000.00 | $36,639.02 | $106,639.02 |
- Projections are hypothetical and do not guarantee investment returns.
- Contributions are added at the end of each period; taxes and fees are not modeled.
How to use this calculator
- Enter your initial investment — the lump sum you are starting with today. Use 0 if you are starting from scratch.
- Enter your regular contribution and choose how often you plan to contribute: weekly, monthly, quarterly, or annually.
- Enter the annual interest rate you expect to earn. For reference, the S&P 500 has historically averaged around 10% per year before inflation, while savings accounts earn far less.
- Choose how many years you plan to keep the money invested, and how often interest compounds.
- Review the results: ending balance, total contributions, interest earned, and a year-by-year growth chart.
How the calculation works
monthly growth factor = (1 + r/m)^(m/12) balance₍ₖ₎ = balance₍ₖ₋₁₎ × growth + monthly contribution
where r is the annual interest rate as a decimal and m is the number of compounding periods per year (365 for daily, 12 for monthly, 1 for annual). Contributions are converted to a monthly equivalent and added at the end of each month.
Worked example
Input
$10,000 initial investment, $500 contributed every month, 7% annual rate compounded monthly, for 10 years.
Result
Total contributions: $70,000. Ending balance: $106,639 — including $36,639 of interest earned.
Assumptions and important notes
- The rate you enter is a constant annual rate held for the entire period. Real investment returns vary year to year.
- Contributions are added at the end of each period (ordinary annuity).
- Taxes, fees, and inflation are not modeled.
- Compounding uses the nominal annual rate divided by the compounding frequency — the standard convention for savings accounts and CDs.
Frequently asked questions
What is compound interest?
Compound interest is interest earned on both your original balance and on interest you have already earned. Over long periods, compounding causes balances to grow faster and faster — which is why starting early matters more than investing large amounts.
What interest rate should I use?
It depends on where the money is invested. High-yield savings accounts might earn 3–5%, CDs offer fixed rates, and broad stock market index funds have historically averaged near 10% per year — but with real risk of down years. Choose a rate that matches your actual investment.
Does the compounding frequency matter?
Yes, but modestly. At the same annual rate, daily compounding produces slightly more interest than monthly, which produces slightly more than annual. The bigger levers are time, the rate itself, and the size of your contributions.
Is this projection a guarantee?
No. This calculator applies a fixed rate to illustrate how compounding works. Real investments fluctuate, and past performance does not guarantee future results. Treat the result as an educational estimate, not financial advice.
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