Investment planning tool
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.
Growth curve
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.
The longer the horizon, the more interest dominates. Try 10 vs. 30 years with the same inputs to see the difference.
| Year | Balance | Contributed |
|---|
The eighth wonder, explained
Compound interest is often called the most powerful force in personal finance, but it only reveals its strength over time. The sections below contrast it with simple interest, show which inputs matter most, and explain why the same engine works for you when saving and against you when borrowing.
Simple interest is paid only on the original amount, so it grows in a straight line. Compound interest is paid on the original amount plus all the interest already earned, so the base keeps getting larger and growth curves upward instead of staying flat. Over a single year the difference is modest, but across decades the gap becomes dramatic. Our calculator plots the growth curve so you can see visually how the same rate behaves very differently once earnings start earning their own earnings.
Three inputs decide the outcome: how much you start with, how much you add along the way, and how long the money stays invested. The rate matters too, but time is the quiet hero, because compounding needs years to show its full effect. A saver who begins early with modest contributions often ends ahead of a later saver who puts in far more, simply because the earlier money had longer to compound. Small, regular additions do more heavy lifting than people expect when the horizon is long.
Compounding frequency, whether yearly, monthly, or daily, changes the final total because interest is credited more often and then earns interest sooner. The more frequently compounding occurs, the slightly larger the outcome, all else equal. The effect is real but smaller than the effect of a better rate or a longer timeline, so it is worth understanding without obsessing over it. Our tool lets you compare frequencies directly to see how much, if anything, the difference actually amounts to.
The same mathematics that grows savings also grows debt. Credit card balances and high interest loans compound against you, which is why a balance that is only partially paid keeps climbing. Understanding the engine cuts both directions: it argues for starting investments early and for clearing expensive debt quickly. Nothing here is financial advice, and returns are never guaranteed, but the arithmetic of compounding is one of the most reliable forces in personal finance, and seeing it spelled out makes the trade-offs concrete.
A concrete example
The levers above are easier to feel with a real scenario. The two notes below show how time and inflation reshape the same monthly contribution.
Imagine two people each investing $200 a month at a 7 percent annual return. The first starts at age 25 and stops at 35, putting in $24,000 total. The second starts at 35 and continues to 65, putting in $72,000. At 65 the first saver has about $241,000 and the second about $244,000. The early starter contributed a third as much yet finishes nearly even, purely because ten extra years of compounding did the heavier lifting.
Headline growth is not the whole story, because prices rise too. If inflation runs at 3 percent, a nominal return of 7 percent is really about 4 percent in purchasing power. The calculator shows nominal growth; subtracting a reasonable inflation assumption from your rate gives a more honest sense of how much your future spending power actually grows. Compounding still wins, but the real target is spending power, not just a bigger number.
The difference between 5 and 7 percent feels tiny on paper, yet over thirty years it can nearly double the final balance. That sensitivity is why the rate field deserves more of your attention than any single contribution amount. Run the same plan at a few rates, including a pessimistic one, before you commit to a long-term expectation.
FAQ
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.
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.
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.
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.
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.
No. All calculations run locally in your browser. Nothing is uploaded, logged, or shared.
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.