| Year | Balance | Total Invested | Interest (cumul.) | Annual Growth | Return % |
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Enter your starting balance (principal), annual interest rate, and time horizon. Choose a compounding frequency — daily, monthly, or yearly. Optionally add regular contributions (monthly deposits) or periodic withdrawals to model real-world savings or investment scenarios. The chart and table update instantly as you type.
Compound interest is interest calculated on both your initial principal and the accumulated interest from all previous periods. Unlike simple interest — which is calculated only on the original principal — compound interest grows exponentially. The longer your money compounds, the more powerful the effect. This is why starting to invest early, even with small amounts, has such an outsized long-term impact.
The standard formula is: A = P(1 + r/n)^(nt), where A is the final amount, P is the principal, r is the annual interest rate (as a decimal), n is the number of compounding periods per year, and t is the time in years. When you add regular contributions, each contribution also compounds for its remaining time — the calculator handles all of this automatically, including mid-period deposits and withdrawals.
The more frequently interest is compounded, the faster your balance grows — but the differences are smaller than many people expect. On $10,000 at 7% over 20 years: yearly compounding yields approximately $38,697; monthly compounding yields approximately $40,318; daily compounding yields approximately $40,388. The compounding frequency matters most at high interest rates and over very long time horizons. For most savings accounts and index fund estimates, monthly is a reasonable default.
The Rule of 72 is a quick mental shortcut for estimating how long it takes to double your money: divide 72 by your annual rate. At 6%, your money doubles in roughly 12 years. At 9%, roughly 8 years. At 12%, roughly 6 years. It's an approximation, not exact, but it's useful for quickly comparing options or stress-testing an investment thesis without a calculator.
The answer depends heavily on your rate, time, and contributions. A single $10,000 investment at 7% compounded annually grows to approximately $19,672 after 10 years, $38,697 after 20 years, and $76,123 after 30 years. If you add $500/month to that starting balance at 7% over 30 years, your ending balance exceeds $650,000. Time is the most powerful variable — a 25-year-old who invests $200/month at 7% ends up with more than a 35-year-old investing $400/month at the same rate, simply because of compounding time.
Compound interest works the same way on debt — but against you. Credit card debt at 24% APR compounded daily means a $5,000 balance left unpaid for one year grows to over $6,200. Student loans, personal loans, and buy-now-pay-later products all use compound interest. This is why paying off high-interest debt as quickly as possible is one of the highest-return financial moves available — you earn a guaranteed "return" equal to the interest rate you're eliminating.
Start as early as possible — time is the variable with the most leverage. Make consistent contributions, even small ones; regularity beats timing. Reinvest all dividends and interest so they compound too. Minimize fees: a 1% annual fund fee seems small but reduces your ending balance by 20–25% over 30 years. Keep investments in tax-advantaged accounts (RRSP, TFSA, 401k, IRA) where possible so compound growth isn't eroded by annual tax on gains.
Tools that pair well with compound interest planning: