Compound Interest Calculator

Compound interest is interest calculated on both your original principal and the interest that has already accumulated. Simple interest, by contrast, is calculated only on the principal, so it grows in a straight line. A $10,000 investment earning 7% simple interest gains exactly $700 every year. The same $10,000 compounding monthly at 7% gains a similar amount in year one, but by year ten each year's gain is larger than the last, because interest is now earning interest on itself. This calculator uses monthly compounding — the more common convention for savings and brokerage accounts — so gains are added to the balance twelve times a year rather than once.

A quick way to estimate how long money takes to double, without touching a calculator, is the rule of 72: divide 72 by the annual rate of return, and the result is roughly the number of years to double. At a 6% annual return, that's 72 ÷ 6 = 12 years, close to the true value of about 11.9 years from the exact compounding formula. At 8%, the rule predicts 9 years, again nearly identical to the actual 9.0 years. The shortcut gets noticeably less accurate at higher rates — at 20%, it predicts 3.6 years while the true doubling time is closer to 3.8 years, and the gap widens further from there. It also silently assumes one constant rate with no withdrawals or added contributions along the way, so treat it as a mental estimate, not a substitute for running the actual numbers.

Time matters more than the rate you earn, because compounding is exponential rather than linear, and a decade's head start can be worth more than most people expect. Take $10,000 invested once, with nothing added afterward, at a steady 7% annual return: left alone for 40 years it grows to roughly $149,700, but the same $10,000 invested for only 30 years — because the investor started a decade later — reaches about $76,120. Waiting ten extra years to begin costs roughly $73,600 in this example, nearly half of what the earlier investor ends up with, even though both contributed the exact same amount of money. Small differences in starting date compound into large differences in outcome, which is why the biggest lever most people have isn't finding a higher return, it's starting sooner.

Regular monthly contributions change the shape of the curve. Adding even a modest amount every month — the same principle behind automatic 401(k) or IRA contributions — means new money keeps entering the account at today's dollar amount while older contributions keep compounding. Over a working career, most of the final balance in a retirement account often comes from contributions plus compounding on those contributions, not just the initial deposit. Long-run U.S. stock market returns, such as the S&P 500's historical average, are sometimes used as a rough reference point for the rate field, but past performance never guarantees future results.

How often interest compounds also matters, though usually less than people assume. Take $10,000 growing at a nominal 6% annual rate for 20 years: compounded once a year it reaches about $32,071; compounded monthly, the same nominal rate produces about $33,106 — roughly $1,000 more, a difference of about 3%. Switching from monthly to daily compounding adds only about another $95, well under a tenth of a percent. Moving from annual to monthly compounding has a real but modest effect, while moving from monthly to daily makes almost no practical difference — the rate itself matters far more than how finely it's sliced.

A steady annual rate is a simplification real markets don't follow, and volatility itself quietly erodes returns in a way a simple average return can hide. Imagine a portfolio that gains 50% in one year and then loses 50% the next: the arithmetic average of those two returns is 0%, which sounds like breaking even, but the actual result is a 25% loss overall — $10,000 grows to $15,000 and then falls back to $7,500. The rate that truly describes what happened is the geometric mean, about −13.4% per year, not the 0% arithmetic average. This gap between the two kinds of average is sometimes called volatility drag: the more a return swings up and down around its average, the more it eats into the compounding you actually experience, even when the simple average return still looks perfectly respectable.

This calculator has real limits worth understanding. It assumes the same rate every single year, which, as the point above shows, doesn't reflect how real returns behave. It also ignores taxes, account fees, and expense ratios, and even a small ongoing cost compounds too: a 1-percentage-point annual fee turns a 7% gross return into a 6% net one, and over 30 years that alone shrinks $10,000 from about $76,120 to about $57,435 — a difference of nearly $18,700, almost a quarter of the final balance, from costs alone. Most importantly, the results shown are nominal growth, not real purchasing power: if your investment grows 7% a year while inflation runs at 3%, your money's actual buying power grows closer to 4% a year. Treat every number here as an educational simulation of how compounding works under a chosen set of assumptions, not a forecast, a recommendation, or a promise of what any real investment will do.