Compound Interest Calculator
Estimate future value from an initial amount, recurring contributions, rate, timeline, and compounding frequency.
How to use this compound interest calculator
- Enter an initial deposit
Type the starting amount you already have saved or plan to invest.
- Set a monthly contribution
Add the amount you will deposit each month on a recurring basis.
- Choose an interest rate and term
Enter the expected annual return and the number of years you plan to invest.
- Pick a compounding frequency
Select how often interest compounds — daily, monthly, quarterly, or annually.
- Review the growth projection
The chart and breakdown show total deposits versus interest earned over the full timeline.
How this compound interest calculator works
This compound interest calculator projects how a starting balance grows when returns are reinvested and when recurring contributions continue to build the account. It combines compound growth on the initial deposit with the future value of a monthly contribution stream, using an equivalent monthly growth rate implied by the selected compounding frequency. The result shows how much of your future balance comes from money you deposited versus growth earned through compounding, which is essential for setting realistic savings and investing expectations.
FV = P(1 + r/k)^(kt) + C × [((1 + i_m)^(12t) – 1) / i_m], where i_m = (1 + r/k)^(k/12) − 1 Starting with $15,000, contributing $500 per month at a 7 % annual return over 20 years using the selected compounding frequency: total deposits are $135,000, the projected future value is about $321,044.41, and roughly $186,044.41 comes from growth rather than deposits.
Investing a lump sum of $15,000 with no monthly contributions at 7 % compounded monthly for 20 years: the entire growth comes from compounding alone. Without any additional deposits, the ending balance still exceeds the original amount meaningfully, illustrating how time and reinvested returns do the heavy lifting even when you stop adding new money.
Starting with the same $15,000 but increasing the monthly contribution well above $500 — say doubling it — at 7 % over 20 years shifts the balance dramatically. The extra deposits not only add their face value but also generate their own compounding returns, so the gap between moderate and aggressive contribution levels widens further with each passing year.
- ✓ The estimate assumes a constant average annual return over the full time horizon — actual market returns will vary year to year.
- ✓ Compounding frequency stays fixed for the entire projection; switching between monthly and daily compounding produces slightly different results.
- ✓ Contributions are assumed to continue at a consistent level throughout the period, with no interruptions or increases.
- ✓ Taxes, inflation, and account fees are not deducted from the projection — the output is a nominal gross estimate.
- Longer timelines amplify the effect of compounding dramatically; time in the market often matters as much or more than the rate of return itself.
- Use a conservative return assumption (e.g. 5–6% for equities after inflation) when making planning decisions that depend on the result.
- The growth multiple (future value ÷ total contributions) is a useful sanity check — values above 2× usually indicate a long horizon or aggressive rate assumption.
- Compound interest and future value of annuity formulas — CFA Institute
- SEC Investor.gov compound interest resource
What is compound interest?
Compound interest is the process of earning returns on both your original principal and the interest that has already accumulated. Unlike simple interest, which is calculated only on the initial deposit, compound interest creates a snowball effect: each period's gains become part of the base for the next period's calculation. Over short horizons the difference is modest, but over decades the gap becomes enormous. A practical shortcut for estimating how quickly money doubles is the Rule of 72: divide 72 by the annual rate of return. At a 6 percent return, for instance, the balance roughly doubles every 12 years. At 8 percent, it doubles every 9 years. This mental model highlights why even small differences in return rates matter so much over long periods. The exponential nature of compounding is the single most important concept in personal finance and the primary reason financial advisors emphasize starting to save as early as possible.
Compounding frequency matters
Compounding frequency refers to how often accumulated interest is added back to the principal balance so that it can generate its own returns. Common frequencies include daily, monthly, quarterly, and annually. The more frequently interest compounds, the faster the balance grows, because gains are reinvested sooner. In practice, the difference between monthly and daily compounding is relatively small — usually a fraction of a percent per year — but the gap between annual and monthly compounding is more noticeable, especially at higher rates and over longer time horizons. Savings accounts and certificates of deposit often compound daily, while many investment projections assume monthly or annual compounding. When comparing two products with the same nominal rate, the one with more frequent compounding will deliver a slightly higher effective annual yield. This is why the annual percentage yield (APY) — which accounts for compounding frequency — is a better comparison metric than the stated nominal rate alone.
Compound interest calculator FAQs
What is the difference between compound interest and simple interest?
Simple interest is earned only on the original principal, while compound interest earns returns on both the starting balance and the previously accumulated gains — creating exponential growth over time.
Do monthly contributions matter more than the starting balance?
Over long time horizons, regular contributions typically contribute more to the final balance than the initial deposit because each new contribution also begins compounding.
How should I choose an annual return assumption?
Use a realistic long-run average for the asset class you are modeling. Broad equity markets have often delivered returns in the high single digits over long periods, but actual results vary by country, asset mix, fees, inflation, and starting valuation.
Why does compounding frequency change the result?
More frequent compounding applies returns to the balance more often within each year, slightly increasing the effective yield compared to less frequent compounding at the same nominal rate.
Can I use this for savings instead of investing?
Yes. It works for any balance that grows over time, including high-yield savings accounts, CDs, bonds, and long-term investment portfolios.