Complete the assumptions, then compare.
Growth on top of growth
Compound Interest Calculator
Compare investment growth assumptions side by side and see what fees, taxes, inflation and regular contributions can change over time.
Private by design. Your balances, contributions and scenarios stay in this browser.
Starting assumptions
Build your contribution plan
Positive recurring amounts add money. Use a negative amount to model regular withdrawals.
Growth scenarios
Compare return assumptions and costs
Two example scenarios are ready to compare.
Growth comparison
See where your future balance comes from
Balance over time
Projected compound growth
| Scenario | Annual return | Final balance | Contributed | Gross growth | Fees | Estimated tax | Net growth | Today’s-money value | Goal | Contribution needed |
|---|
Yearly breakdown
Contributions, growth and costs
| Year | Opening balance | Cash flow | Gross growth | Fees | Estimated tax | Closing balance | Today’s money |
|---|
Illustration only: Constant returns are used for clarity. Real investments fluctuate, losses are possible, taxes are simplified, and past S&P 500 performance does not guarantee future results.
The assumptions that matter
A percentage alone does not tell the full story
Time and reinvested growth are powerful, but recurring contributions, costs, tax and inflation can materially change the amount you actually keep.
Compare a range, not one forecast
Try lower and higher return scenarios. Markets do not deliver the same percentage every year, and past averages are not promises.
Make every fee visible
Small annual percentages compound too—but against you. Compare both asset-based and fixed costs across the full time horizon.
Measure purchasing power
A large future balance may buy less than the same number today. The inflation-adjusted result provides that essential context.
Using the calculator well
Stress-test the plan before trusting the headline result
Useful scenarios to compare
- A cautious return below the historical average.
- The same return with realistic fund and account fees.
- A taxable account beside a tax-advantaged account.
- Lower contributions during difficult years or regular withdrawals in retirement.
Limits of a smooth projection
- Real returns arrive unevenly and can be negative.
- Investment returns, inflation, fees and tax rules can all change.
- Tax-deferred does not necessarily mean tax-free.
- Use the result as a planning illustration, not financial advice.
Compound interest questions
Compound Interest Calculator FAQ
What is compound interest?
Compound interest means earning growth on both the original principal and previously accumulated growth. Contributions, time, return, fees, tax and the order in which cash flows occur all affect the result.
Why does the calculator start with a 10% return?
The first scenario uses 10% as a rounded historical illustration of the long-run nominal average annual return commonly associated with the S&P 500, including reinvested dividends. It is not a forecast or guaranteed return, and actual yearly returns can be much higher or lower.
Does this calculator assume smooth market returns?
Yes. It applies the entered annual rate evenly according to the selected compounding frequency. Real investments fluctuate, and the order of gains and losses can matter—especially when withdrawing money.
How are investment fees handled?
The calculator deducts an asset-based percentage fee and an optional fixed annual fee. It reports the cumulative fees paid and allows otherwise identical scenarios to be compared side by side.
How are taxes estimated?
For taxable scenarios, the entered tax rate is applied to positive growth at the end of each year. Tax law is more complex and varies by account, investment, country and holding period, so this is only an illustration. Tax-deferred scenarios postpone tax and do not estimate tax due when money is eventually withdrawn.
What does today’s-money value mean?
Today’s-money value discounts the projected balance by the inflation rate entered. It helps compare a future amount with the purchasing power of money today.
Can I compare more than two growth scenarios?
Yes. Compare up to four scenarios with different returns, compounding frequencies, fees and tax assumptions while keeping the starting amount, contributions and time horizon identical.
Does Bobek save my financial information?
No. Calculations run in your browser. Bobek does not receive or store the balances, contributions, goals or scenario names you enter.