Compound Interest Calculator
Work out the future value of your savings or investment with regular contributions and any compounding frequency — then open Compare scenarios to model different starting amounts, rates, durations and contribution stop dates side by side.
✓ Your inputs are saved automatically in this browser — close the tab and your model will still be here when you return.
■ Total balance ■ Amount invested
| Year | Contributions | Total contributions | Interest | Total interest | End balance |
|---|
Add one or more scenarios. Leave a field blank to keep the same value as the main inputs above — change only the variables you want to test (different starting capital, interest rate, duration, or the year you stop contributing). Then press Run comparison.
Balance growth — base vs scenarios
Scenario summary
Year-by-year comparison
End-of-year balance for each scenario. Scroll sideways to see all scenarios.
What is compound interest?
Compound interest is interest earned on both your original principal and on the interest that has already been added to your balance. Unlike simple interest — which only ever pays on the original amount — compound interest creates a snowball effect: each period's interest joins the pot and goes on to earn interest of its own. Over long horizons this is the single most powerful force in personal investing.
The compound interest formula
The future value of a lump sum is:
A = P × (1 + r/n)n·t
where P is the principal, r the annual interest rate (as a decimal), n the number of compounding periods per year, and t the number of years. When you also make regular contributions, each deposit is added to the balance and then compounds for the remaining periods — this calculator simulates every period for you, including contributions that increase each year or stop after a chosen year.
Worked example: $10,000 for 20 years at 5%
| Year | Interest earned | End balance |
|---|---|---|
| 1 | $500.00 | $10,500.00 |
| 2 | $525.00 | $11,025.00 |
| 3 | $551.25 | $11,576.25 |
| … | … | … |
| 20 | — | $26,533 |
With no further deposits, $10,000 grows to about $26,533 — roughly $16,533 of interest, a 165% return. Add just $100 a month and the balance after 20 years jumps to around $67,000. Try it above and use the comparison tool to see how much difference an extra 1–2% rate, a few more years, or continuing to contribute makes.
How to make compound interest work for you
- Start early. Time is the biggest lever — money invested in your twenties compounds for decades.
- Contribute regularly. Consistent deposits feed the snowball; even small amounts add up.
- Don't stop too soon. Use the "stop contributing after year" field to see the cost of pausing.
- Favour higher compounding frequency. Daily or monthly compounding beats yearly for the same nominal rate.
Frequently asked questions
How is compound interest calculated?
Using A = P(1 + r/n)nt for a lump sum. Regular contributions are added each period and then also compound, which this tool simulates period by period.
Does compounding frequency matter?
Yes — the more often interest compounds, the higher the effective annual rate. The calculator shows the effective annual rate so you can compare products fairly.
What happens if I stop contributing early?
Your existing balance keeps compounding, but growth slows. Set a stop year and run a comparison to quantify the difference.
What is the effective annual rate?
It's the real return once compounding is included. A 6% nominal rate compounded monthly gives an effective rate of about 6.17%.