Compound Interest Calculator

See how your savings grow over time. Interest earning interest — the most powerful force in personal finance.

$
$0$100000
$
$0$2000
%
0%15%
yrs
1 yrs40 yrs
Adjust for inflation
Tax on interest
$54,713.58Final balance
$34,000.00Total invested
$20,713.58Interest earned
ROI 60.9%
Invested
Interest

How Compound Interest Works

Compound interest is the process of earning interest on both your original principal and the interest you've already accumulated. Unlike simple interest — which only grows based on the starting amount — compound interest accelerates over time because each interest payment becomes part of the base for the next calculation. This creates an exponential growth curve rather than a straight line.

The frequency at which interest compounds makes a meaningful difference over long timeframes. Monthly compounding produces a higher final balance than annual compounding at the same stated rate, because interest is added to the principal twelve times per year instead of once. The difference is small early on but compounds into a significant gap over decades.

Regular contributions amplify the effect further. Adding even a modest monthly deposit means that each new contribution also starts earning compound interest immediately from that point forward. A person who contributes $200 per month for 30 years at 7% will end up with a noticeably larger portfolio than someone who invests a large lump sum and stops — because consistent contributions keep seeding new compounding cycles throughout the entire period.

Time is the most critical variable. Starting five years earlier can produce a larger final balance than doubling the monthly contribution amount. That's why financial advisors consistently emphasize starting early — even small amounts invested in your twenties have more compounding time than large amounts invested in your forties.

Compound Interest Formula

A = P(1 + r/n)^(nt)
  • A — Final amount (what you end up with)
  • P — Principal (your initial investment)
  • r — Annual interest rate expressed as a decimal (e.g. 7% = 0.07)
  • n — Number of times interest compounds per year (12 monthly, 4 quarterly, 1 annually)
  • t — Time in years

For example: $10,000 invested at 7% compounded monthly for 10 years produces A = 10000 × (1 + 0.07/12)^(12×10) ≈ $20,097. Your money roughly doubles in a decade at this rate. With regular monthly contributions of $200, the calculator above accounts for each contribution independently — each deposit starts its own compounding cycle from the moment it is added.

Six currencies, five frequencies, real purchasing power

The calculator now handles six currencies — CAD, USD, EUR, GBP, AUD, CHF — each formatted with its proper locale separator (a period in the US, a comma in Germany, a space in Switzerland). Small detail, but staring at a Swiss savings projection that reads "$1.234.567" breaks the mental model. Five compounding frequencies cover everything from daily to annually. The inflation-adjusted real value display runs alongside the nominal figure so you can see what your $400,000 projection buys in today's dollars — no manual rate subtraction needed. And there's a tax-on-interest toggle that deducts tax annually from the interest earned, which gives a more realistic number for savings held in taxable accounts.

A shareable URL encodes all eight inputs — principal, rate, years, contributions, frequency, currency, inflation rate, and tax toggle — so you can send a spouse or financial advisor the exact scenario without copying numbers into a message. What's not in here: historical rate data, portfolio allocation sliders, or projections tied to any live market feed. This is a deterministic calculator, not a robo-advisor.

Frequently Asked Questions

Does this calculator account for taxes?
No. All figures shown are pre-tax. In practice, investment gains are subject to taxation — capital gains, dividends, and interest income are each taxed differently depending on your jurisdiction and account type. In Canada, contributions to a TFSA grow tax-free, while RRSP withdrawals are taxed as income. In the US, Roth IRA gains are tax-free, while traditional 401(k) withdrawals are taxed. For a realistic projection, consult a financial advisor or use your after-tax expected return rate.
What's the Rule of 72 exactly?
The Rule of 72 is a quick mental shortcut: divide 72 by your annual interest rate to estimate how many years it takes to double your money. At 7%, your investment doubles in approximately 72 ÷ 7 = 10.3 years. At 6%, it takes about 12 years. The rule is an approximation — this calculator gives you the precise figure — but it's useful for fast comparisons without doing the full exponential math.
Does compounding frequency matter much?
Yes, but the difference is smaller than most people expect for practical investment rates. Going from annual to monthly compounding at 7% adds roughly 0.23% to your effective annual yield — meaningful over 30 years but not dramatic over 5. Daily compounding adds only a tiny fraction more than monthly. The stated interest rate and the length of time invested matter far more than the compounding frequency for typical savings rates. Where frequency matters most is at higher rates — think credit card debt.
What interest rate should I use?
Use the expected annual return for whatever account or investment you are modeling. Common benchmarks: high-yield savings accounts currently yield 4–5% (rates change with central bank policy), a broad stock market index fund has historically averaged 7–10% annually before inflation, GICs and CDs range from 3–5% depending on term, and bond funds typically yield 3–5%. For retirement projections, many financial planners use 6–7% as a conservative long-term equity assumption to account for market variability and inflation.
Does this calculator account for inflation?
No — this is a nominal calculation. The figures shown represent actual dollar amounts, not their purchasing power in today's dollars. To estimate real (inflation-adjusted) growth, subtract the expected inflation rate from your interest rate before entering it. For example, if you expect 7% nominal returns and 2.5% inflation, enter 4.5% as your rate to see the inflation-adjusted projection. This gives you a more conservative but realistic picture of what your savings will actually buy in the future.

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