Compound Interest Calculator
This calculator estimates how an initial investment can grow when interest compounds over time, with optional recurring contributions. Use it to explore the effect of contribution size, contribution timing, compounding frequency, and time horizon. Results are hypothetical illustrations — not guaranteed investment outcomes.
Your results will appear here
Enter your assumptions and calculate to see the estimated result and supporting details.
How this calculator works
Enter an initial investment, optional recurring contribution, contribution frequency, annual rate of return, investment period in whole years, compounding frequency, and whether contributions are made at the beginning or end of each contribution period.
The primary result is ending balance. Secondary metrics show total principal contributed, growth earned, and the share of the ending balance attributable to growth. An annual projection table and chart summarize year-by-year progress.
Formula / methodology
FV_initial = P × (1 + r/n)^(n×t) FV_contribution = C × (1 + r/n)^[n(t − tc)] FV = FV_initial + Σ FV_contribution
- P is initial principal, r is the annual nominal rate as a decimal, n is compounding periods per year, and t is years.
- Each contribution C made at time tc grows for the remaining time t − tc. Contribution frequency may differ from compounding frequency.
- When those frequencies differ, exponents may be fractional (n × years remaining). This closed-form model is intentional for a consumer calculator and is not a discrete compounding-date ledger.
- End-of-period timing starts the first contribution at 1 / contributionFrequency years. Beginning-of-period timing starts at time 0.
Worked example
Invest $10,000 at 7% with monthly compounding for 10 years and no recurring contributions. Ending balance is about $20,096.61.
Add $500 monthly contributions at end of each month with a 6% annual rate for 10 years (monthly compounding). Ending balance is about $100,133.64. Beginning-of-period contributions raise that illustration to about $100,543.34.
Important assumptions
- The annual rate is a constant nominal assumption you supply — not a forecast.
- Investment periods are whole years (1–100) in Phase 2.
- Taxes, fees, inflation, and sequence-of-returns risk are not modeled.
- Contribution and compounding frequencies are independent.
- Mixed frequencies use fractional compounding periods in the closed-form growth factor.
Common questions
For a given nominal rate, more frequent compounding usually increases ending value slightly. The difference is often modest compared with contribution size and time invested.
Contributions invested earlier have more time to compound. With a positive assumed return, beginning-of-period results are higher than end-of-period results for the same inputs.
Yes, for illustrative declines greater than −100% are rejected. Negative rates reduce ending balance relative to total contributions.
No. Investment returns vary and can be negative. This tool illustrates math under your assumptions only.