Why does growth accelerate over time?

Each period earns interest on a slightly larger balance, and those earned dollars then earn interest themselves. Over decades this snowball effect means the majority of a long-horizon balance often comes from interest, not contributions — which is what the contributions-versus-interest bar shows.

What return rate should I use?

Historical average annual returns are often cited around 7-10% for broad stock index funds before inflation, and roughly 2-5% for bonds or savings products. No past average guarantees future results, so it is wise to test several conservative rates.

Does this account for taxes, fees, or inflation?

No. The projection is a pure nominal compounding model. Account fees, fund expense ratios, dividend taxes, and inflation all reduce real results — try lowering the rate to approximate their combined drag.

Investment planning tool

Compound Interest Calculator

Q: What is the compound interest formula?

For a principal P at annual rate r compounded n times per year for t years: A = P(1 + r/n)^(nt). With regular monthly contributions, this calculator simulates each month, adding the contribution and then applying the monthly rate.

Project how savings grow with compound interest and monthly contributions. Watch the growth curve bend upward and see exactly how much of your final balance is interest you earned versus money you put in.

Future balance $0

Total contributed $0

Interest earned $0

Growth multiple 0×

Growth curve

Balance Over Time

The green line is your total balance; the dashed line is what you would have with contributions alone (no compounding). The widening gap is compound interest at work.

Balance with compounding Contributions only

Contributions vs. interest

You contributed: 0% Interest earned: 0%

The longer the horizon, the more interest dominates. Try 10 vs. 30 years with the same inputs to see the difference.

Year	Balance	Contributed

FAQ

Compound Interest Questions

What is the compound interest formula?

Lump-sum compounding: A = P(1 + r/n)^nt, where P is principal, r the annual rate, n the compounding periods per year, and t the years in the market. With recurring contributions, each month is simulated: interest is applied, then the new contribution is added. That is exactly what this calculator does.

Why does the curve steepen so much?

Interest earns interest. In year one, returns apply only to your small starting balance; by year twenty they apply to a balance many times larger. The absolute dollar gains per year therefore keep increasing even at a fixed percentage rate.

What rate of return should I assume?

Long-run nominal averages often cited: roughly 7–10% for broad stock index funds and 2–5% for bonds or high-yield savings. Nothing guarantees future returns — run several scenarios, including a pessimistic one, before making plans.

Are taxes, fees, and inflation included?

No. Results are nominal and pre-tax. To approximate the drag from fund fees and taxes, lower the return rate; to think in today's purchasing power, subtract expected inflation (historically ~2–3%) from the rate.

Does contribution timing matter?

Yes. Contributions made earlier compound for longer. The same monthly amount started ten years earlier can end up worth dramatically more — even if the late starter contributes for most of the same total years.

Is my data stored anywhere?

No. All calculations run locally in your browser. Nothing is uploaded, logged, or shared.

Disclaimer

Investment Projection Disclaimer

Projections are educational estimates, not investment, tax, or financial advice. Actual returns vary and may be negative. Consult a licensed professional before making investment decisions.