Watch how an initial amount plus steady contributions grow over time.
A compound interest calculator shows what your savings can grow into over time as you earn returns on both your money and your prior returns. Enter a starting amount, a monthly contribution, a time horizon, and a rate to see the future value and how much of it is pure growth.
Compounding starts slow and then accelerates, because you earn returns on your past returns. Over a few years the effect is modest; over decades it dominates. That's why starting early, even with small amounts, usually beats starting later with larger ones: a dollar invested in your 20s has far more compounding cycles ahead of it than a dollar invested in your 40s.
There's no guaranteed number. Historically a broadly diversified stock portfolio has returned roughly 7% a year after inflation over long periods, but any single year can be far higher or lower, and future returns aren't promised. Use a conservative rate for planning, and remember that fees and taxes reduce what you keep. The point isn't a precise prediction, it's to show how powerfully consistency and time work together.
A quick way to grasp compounding is the Rule of 72: divide 72 by your annual return to estimate the years it takes money to double. At 8% that is about nine years; at 6%, twelve. The rule shows why the rate you earn matters so much over time, money doubling every nine years versus every twelve leads to dramatically different ending balances across a working life. It also makes inflation vivid: at 3%, prices double in about 24 years, halving what your money buys.
The biggest driver of compound growth is not picking the perfect moment to invest but the number of years your money stays invested. Because each year's gains earn gains of their own, the curve bends upward most in the later years, which is why an early start is so powerful and why trying to time the market usually backfires. Staying invested through ups and downs, and continuing to contribute during downturns when prices are lower, captures the compounding that market-timers routinely miss.
The same math that rewards a higher return punishes fees, because a 1% annual fee is a 1% lower return every single year, compounded. Over an investing lifetime, a seemingly small expense ratio can consume a large share of your final balance, sometimes a quarter or more. This is why low-cost index funds have become the default choice for long-term investors: minimizing costs is one of the few reliable ways to keep more of the growth compounding for you rather than for a fund company.
How often interest compounds, annually, monthly, or daily, affects the result, though less than the rate and the time horizon. More frequent compounding means interest starts earning interest sooner, so daily compounding edges out monthly, which edges out annual, for the same nominal rate. The effective annual yield (APY) captures this, which is why savings accounts are quoted in APY. For long-term investing, the difference between compounding frequencies is small next to the effect of decades of growth, but it is real and worth understanding.
Compounding only works if the earnings stay invested. Reinvesting dividends from stocks and interest from bonds or savings automatically buys more shares or adds to the balance, so those earnings begin compounding too. Historically, reinvested dividends have accounted for a large portion of total stock-market returns. Spending the income instead breaks the compounding chain and slows growth. Setting investments to reinvest automatically, and leaving them alone, is one of the simplest ways to let compounding do its work.
Most people build wealth not with a single lump sum but by investing steadily over time, an approach called dollar-cost averaging. By contributing a fixed amount on a regular schedule, you automatically buy more shares when prices are low and fewer when they are high, which smooths out your average cost and removes the temptation to time the market. The harder part is behavioral: staying invested through downturns, when headlines are frightening and balances drop, is what lets compounding work. Investors who sell in a panic lock in losses and miss the recovery that historically follows. Automating contributions and avoiding frequent check-ins both help. The math of compounding rewards patience above almost everything else, so the most valuable habit is simply to keep contributing and leave the account alone for the years or decades until you need it.
Interest earned on both your original money and the interest it has already earned. Over long periods it becomes the dominant driver of growth.
Future value = PV × (1+i)^n + PMT × [(1+i)^n − 1] / i, where i is the periodic rate and n the number of periods. The full example is on our methodology page.
Because compounding accelerates over time, the earliest dollars have the most years to grow, so a few extra years early can outweigh larger contributions later.
More frequent compounding helps, but time and rate matter far more. Daily beats monthly beats annual for the same nominal rate, yet the gap is small compared with the effect of leaving money invested for decades.
For a diversified stock-and-bond portfolio, many planners use a long-run nominal return in the mid-single digits to around 7%, before inflation. Using a conservative figure avoids overestimating your future balance.
See the exact formula and a worked example on our methodology page.