Compound Interest Calculator

How compound interest works

Compound interest means you earn interest on your interest. Year one, your money earns a return. Year two, the original money andyear one's interest both earn a return — and so on, every year, each layer feeding the next. Early on the effect looks unremarkable; late in the timeline it turns explosive. Put $10,000 plus $200 a month to work at 7% and after 20 years you have about $144,573 — but only $58,000 of it is money you deposited. The other $86,573 is compound interest, and most of it arrives in the final decade. That back-loaded curve is why the single most powerful input in the calculator above isn't the rate or the amount — it's time.

The cost of waiting makes the point brutally. Invest $200 a month at 7% from age 25 to 65 and you reach about $524,963. Start ten years later at 35 and you reach $243,994 — the decade of delay costs $280,969, even though it only involved $24,000 of missed deposits. Compounding pays whoever starts first.

The compound interest formula, explained

Every compound interest calculation reduces to one formula: A = P(1 + r/n)^nt. P is your starting principal, r the annual rate as a decimal, n the number of times interest compounds each year (365 for daily, 12 for monthly, 1 for yearly), and t the years. So $10,000 at 5% compounded monthly for 10 years is 10,000 × (1 + 0.05/12)¹²⁰ = $16,470.09. Regular deposits simply repeat the formula: each month's deposit becomes its own little P, compounding from the day it lands. The calculator above shows the formula with your own numbers plugged in, live — and its math reproduces the SEC Investor.gov calculator's published example to the cent, so you can trust the output.

Daily vs monthly vs yearly compounding — how much does frequency matter?

Far less than the marketing suggests. On $10,000 at 5% for 10 years, daily compounding yields $16,486.65, monthly $16,470.09 and yearly $16,288.95 — the whole daily-versus-yearly spread is under $200. Frequency gives a real but small boost; the rate and the time do the heavy lifting. Where frequency genuinely matters is in reading bank offers: savings accounts typically compound daily but advertise APY (annual percentage yield), which already includes the compounding. Two accounts with the same APY pay the same, whatever their frequency — so compare APYs, and use the frequency pills above to see the effect for yourself.

The Rule of 72: doubling time in your head

Divide 72 by your rate and you get, almost magically, the years your money takes to double. At 7% that's 72 ÷ 7 ≈ 10.3 years (exact: 10.2). At 10% it's 7.2 (exact: 7.3). The rule stays accurate within months for any realistic rate, which makes it the best sanity check in personal finance: a fund promising to double your money in three years is implicitly claiming a 24% annual return — the kind of implicit claim that exposes a scam in one division. The calculator shows your exact doubling time beside the result.

What rate should you assume?

Match the rate to the vehicle. For a high-yield savings account, use the APY your bank quotes. For fixed deposits and CDs, the contracted rate. For stock index funds, the honest anchor is the S&P 500's long-run average of roughly 10% a year nominal — about 6–7% after inflation — but individual years swing wildly, so conservative planners model 6–8%. Two habits keep the estimate honest: run the ± rate range above and plan around the lower figure, and check the "worth in today's money" tile, because $144,573 twenty years from now buys what about $80,046 buys today at 3% inflation. A big nominal number that ignores inflation is the most common way savers fool themselves.

When you're ready to apply this to a real plan, the same engine powers our SIP calculator for monthly investing in India, the 401(k) calculator for US retirement accounts with an employer match, and the FD calculator for guaranteed quarterly compounding.