How to use Compound Interest Calculator
This compound interest calculator shows how a lump sum and regular contributions grow over time as interest earns interest. Set your starting balance, monthly or annual deposits, an expected rate and the compounding frequency to see the future value, total contributions and total interest earned — plus a growth chart that makes the power of compounding visible. It is the single most useful tool for understanding long-term saving and investing.
- Enter your starting principal (or 0 if starting from scratch).
- Add a regular contribution and choose how often you deposit.
- Set the expected annual interest/return rate.
- Choose the compounding frequency (daily, monthly, annually).
- Set the number of years and review the future value and growth chart.
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How compound interest works
Compound interest is calculated with A = P(1 + r/n)^(nt), where P is the principal, r is the annual rate, n is the number of times interest compounds per year and t is the number of years. The key difference from simple interest is that each period’s interest is added to the balance, so the next period earns interest on a larger amount. Over long horizons this snowball effect dominates — most of a 30-year balance is interest, not contributions.
Why frequency and time matter
More frequent compounding (daily vs annually) increases the effective yield slightly, but time is the far more powerful lever. Starting ten years earlier typically beats contributing more later, because the earliest dollars compound the longest. This is why financial planners stress beginning to invest as early as possible, even with small amounts.
| Years | Future value | Interest earned |
|---|---|---|
| 10 years | $19,672 | $9,672 |
| 20 years | $38,697 | $28,697 |
| 30 years | $76,123 | $66,123 |
Worked examples
Lump sum
Inputs: $10,000 · 7% · 30 yr · monthly
Result: ~$81,165 future value
With $300/mo added
Inputs: $10,000 + $300/mo · 7% · 30 yr
Result: ~$444,000 future value
Glossary
- Principal
- The initial amount you start with before any interest.
- Compounding frequency
- How often earned interest is added to the balance (daily, monthly, yearly).
- APY
- Annual percentage yield — the real rate of return once compounding is included.
- Future value
- What your balance is projected to be worth at the end of the period.
Related reading
Frequently Asked Questions
Why use Compound Interest Calculator?
- See how interest-on-interest compounds across monthly, quarterly or yearly frequencies
- Growth chart splits your own contributions from interest earned at every year mark
- Model a lump-sum start plus recurring deposits together in one projection
- Compare daily, monthly and annual compounding frequencies on the same starting balance
Common use cases
- Compare a one-time lump-sum investment against the same amount spread as monthly deposits
- Show a teenager how starting at 18 versus 28 dramatically changes the final balance
- Estimate how much a savings account at a new APY would grow over 10 years
- Find the annual rate needed to reach a target amount by a specific deadline
- Test a retirement projection by toggling the compounding frequency from annual to monthly
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