Compound Interest Calculator

Compound interest is interest that earns interest. Instead of paying out only on the money you originally set aside, each round of interest is added back to your balance — and the next round is then calculated on that larger total. People often call this effect “interest on interest,” and it is the reason a modest sum left to grow can snowball into a much bigger one over time.

Simple interest, by contrast, is always worked out on the original amount alone. Over a year or two the gap is small, but across decades compounding pulls steadily ahead — a difference that grows wider the longer your money stays invested.

Three levers move the outcome more than anything else. The first is time: the earlier you start, the more compounding cycles your money passes through, and because the latest cycles act on the largest balances, the final years do the heaviest lifting. The second is the rate of return — even one extra percentage point, compounded for decades, changes the result dramatically. The third is the compounding frequency: daily or monthly compounding edges out annual compounding for the same headline rate, because interest is put back to work sooner.

The fourth lever, which the calculator above lets you add, is regular contributions. Steadily paying money in gives compounding more principal to act on and usually matters more than any single tweak to the rate.

The standard formula is A = P(1 + r/n)^nt, where A is the final amount, P is the principal, r is the annual rate as a decimal, n is the number of times interest compounds per year, and t is the number of years.

As a worked example, $1,000.00 at 5% compounded monthly for one year gives 1000 × (1 + 0.05/12)¹² ≈ $1,051.16 — about 16 cents more than the $1,051.00 you would get from simple interest, thanks to compounding. That tiny first-year edge is what widens into a large gap over decades.

The table below is produced by the same engine that powers the calculator above, using monthly compounding. Notice how the yearly interest figure keeps rising even though the rate never changes — that is compounding at work.

To see frequency in isolation, here is $10,000.00 growing at 5% for ten years, compounded at different intervals. The balance climbs as compounding gets more frequent, but notice how the gains taper: the jump from annual to monthly is far larger than the jump from monthly to daily.

This is why banks advertise an APY rather than a bare rate — it captures the frequency so you can compare accounts on equal terms. To convert any nominal rate and frequency into its effective yield, use the APY calculator.

The difference is the base each calculation uses. Simple interest is always figured on the original principal, so the interest added each year never changes. Compound interest is figured on the principal plus all the interest accumulated so far, so the amount added grows every period.

On $10,000.00 at 5% for 20 years, simple interest pays a flat $500.00 a year for a final balance of $20,000.00. Compounded monthly over the same period, the balance instead approaches $27,126.00 — the extra $7,126.00 is entirely “interest on interest.” For a flat-rate calculation, use the Simple Interest Calculator; to compound every single day, use the Daily Compound Interest Calculator.

When you want a fast, mental estimate rather than an exact figure, divide 72 by the annual rate to approximate how many years it takes to double your money. At 6% that is 72 ÷ 6 = 12 years; at 9% it is 8 years. The rule is an approximation — it is most accurate for rates between roughly 4% and 12% — but it is a handy sanity check before you reach for the calculator.

Adding money on a regular schedule changes the picture dramatically. Each deposit begins compounding from the moment it lands, so a steady monthly contribution over many years can end up contributing more growth than the original lump sum. Switch the contributions control above to Deposits to see the effect on your own numbers, and try an annual increase to mimic a rising income. If your plan is a fixed monthly investment into a mutual fund, the SIP calculator models that pattern directly.

On the saving side, compounding drives high-yield savings accounts, certificates of deposit, bonds, dividend-reinvesting funds and broad index funds. Each carries its own balance of risk, return and access to your money, and the headline rate is only part of the story — the compounding frequency and your own contribution discipline matter just as much.

The same force runs in reverse on debt. Credit-card balances, payday loans and unpaid interest compound against you, which is why a small balance left unpaid can balloon. Understanding compounding helps on both sides of the ledger. This tool is for illustration only — for decisions about your own situation, speak with a qualified financial advisor.

Compound interest rewards patience more than timing. The figures here assume a constant rate and the exact contributions you enter, which real-world investments rarely guarantee. Treat the results as a guide for understanding the mechanics — not as a forecast or as financial advice.