Compound Interest Calculator

Compound interest means you earn returns not just on what you put in, but on the returns you've already earned. Early on, the "Contributed vs. Interest by Year" chart above is dominated by blue — your own contributions. Given enough years, the green interest bars start to outweigh them, because each year's gains are now themselves earning more.

That's why time in the market tends to matter more than timing the market: a modest sum invested a decade earlier often ends up ahead of a much larger sum invested later, purely because it had more years to compound. The gap between "started at 25" and "started at 35" is rarely about how much was contributed — it's almost entirely about how many extra years of compounding those early contributions got to enjoy.

Simple interest is calculated only on the original principal — a flat percentage of the starting amount, paid out the same way every period. Compound interest, by contrast, is calculated on the principal plus any interest already earned, so the base it's calculated on grows every period. Over short periods the difference is small, but stretched across decades it becomes substantial — a big part of why long-term investing accounts are almost always described in terms of compound, not simple, returns.

The frequency of compounding matters too. Interest that compounds monthly grows slightly faster than interest that compounds annually at the same stated rate, because each month's gains start earning their own return a little sooner. This calculator compounds monthly, which matches how most savings and investment accounts actually work.

A quick mental shortcut for compound growth is the Rule of 72: divide 72 by your annual rate of return to estimate how many years it takes for your money to double. At a 7% return, that's roughly 72 ÷ 7 ≈ 10.3 years to double. It's not exact, but it's a handy way to sanity-check the kind of long-term growth this calculator projects without running the full math.

The "Final Balance by Interest Rate" chart shows how sensitive your outcome is to the return you assume. A couple of percentage points doesn't sound like much, but stretched over 20–30 years it can mean a meaningfully different ending balance. Since future returns aren't guaranteed, it's usually safer to test a range of rates — a conservative one and an optimistic one — rather than anchoring on a single number.

Historically, broad stock market indices have returned somewhere in the high single digits annually over long stretches, before inflation, though any individual decade can look very different from the long-run average. Bonds and savings accounts tend to offer lower but more stable returns. The right assumption depends heavily on what you're actually invested in, and it's worth revisiting periodically rather than treating it as fixed.

This same math underlies retirement accounts, education savings plans, and any account where money sits and grows over years. It also works in reverse — credit card balances and other high-APR debts compound against you the same way, which is part of why carrying a balance on a card can be so much more expensive than it first appears. If you're weighing paying off debt against investing, the debt payoff calculator uses the same underlying math from the other direction.

This calculator assumes a constant rate of return and doesn't account for taxes, fees, or the ups and downs a real investment would actually experience along the way. Real portfolios rarely grow in a straight line — some years will be well above the assumed rate, others below or even negative — but over long enough horizons, the average tends to smooth toward a long-run rate. Treat the projection as a reasonable estimate for planning purposes, not a guarantee of the balance you'll actually see.

• Principal — the initial amount of money invested or deposited, before any growth.
• Compounding period — how often interest is calculated and added to the balance (monthly, in this calculator).
• Annual Percentage Yield (APY) — the real, effective annual return once compounding frequency is accounted for; slightly higher than the stated annual rate if compounding more often than yearly.
• Contribution — money added to the account on a recurring schedule, on top of the original principal.
• Future value — what the account is projected to be worth at the end of the time horizon.

The compound growth formula with regular contributions is:

Where P is the starting principal, r is the monthly rate, t is the number of months, and PMT is the monthly contribution.

Example: $10,000 starting balance, $200/month contribution, 7% annual return, 20 years.

• P = $10,000
• r = 7% ÷ 12 = 0.5833% = 0.005833
• t = 20 × 12 = 240 months
• PMT = $200

Total contributed over 20 years was $10,000 + ($200 × 240) = $58,000. The remaining roughly $86,600 came purely from compounding — more than the total amount contributed, which is the core reason long time horizons matter so much for compound growth.

• Compound interest is sometimes called "interest on interest" — each period's earnings become part of the base for the next period.
• The Rule of 72 estimates doubling time: 72 ÷ annual rate ≈ years to double.
• More frequent compounding (daily vs. monthly vs. annually) produces slightly higher effective returns at the same stated rate.
• Compound interest works identically in reverse against debt — it's why credit card balances can grow quickly if only minimum payments are made.
• Albert Einstein is widely (though apocryphally) quoted as calling compound interest "the eighth wonder of the world" — a popular but unverified attribution.

On $10,000 at 7% annual rate for 10 years, comparing compounding frequency:

More frequent compounding produces a higher final balance at the same stated annual rate, though the difference between monthly and daily compounding is usually small in practice.

• Assuming a constant rate every year — real markets fluctuate; a smooth average return rarely happens in a straight line.
• Ignoring fees — even a 1% annual fee can meaningfully reduce long-term compound growth.
• Forgetting taxes on gains — outside tax-advantaged accounts, growth may be reduced by capital gains tax when realized.
• Underestimating how much starting early matters — delaying contributions is one of the costliest mistakes in long-term planning.