Compound Interest Calculator 2025 | Future Value, CAGR & Contribution Growth

APR (Annual Percentage Rate) is the stated yearly interest rate without accounting for compounding within the year. APY (Annual Percentage Yield) adjusts for compounding frequency—a 7% APR compounded monthly yields a 7.23% APY because interest compounds 12 times. CAGR (Compound Annual Growth Rate) is the smoothed annualized return you actually realize over a specific period, accounting for all deposits, withdrawals, and market fluctuations. Example: $10,000 growing to $20,000 in 10 years has a CAGR of ~7.2%, regardless of whether returns were volatile year-to-year. Use APR when comparing loan rates, APY when comparing savings/investment accounts with different compounding frequencies, and CAGR for measuring actual investment performance over time. Always compare APY to APY (not APR to APY) for accurate account comparisons.

Simple interest only earns returns on the principal (initial deposit)—if you invest $10,000 at 5% simple interest for 10 years, you earn $500/year for a total of $5,000 interest ($15,000 final balance). Compound interest earns returns on both principal AND accumulated interest from previous periods—$10,000 at 5% compounded annually for 10 years grows to $16,289 ($6,289 interest), earning $1,289 more than simple interest purely from compounding. The gap widens dramatically over longer periods: at 30 years, compound interest yields $43,219 vs simple interest's $25,000—a $18,219 difference (73% more). This exponential growth is why compound interest is called the 'eighth wonder of the world.' All modern investment accounts (savings, CDs, 401k, IRA, brokerage) use compound interest; simple interest is rare except in certain short-term loans or basic calculations.

Compounding frequency determines how often interest is calculated and added to your balance—more frequent compounding yields higher returns at the same nominal rate. A $10,000 investment at 7% APR: annually compounded = $19,672 after 10 years, quarterly = $19,898 (+$226), monthly = $20,097 (+$425), daily = $20,136 (+$464). The difference seems small annually but compounds significantly over decades. For a $100,000 balance over 30 years at 7%: annual = $761,000, daily = $806,000 (+$45,000). Most savings accounts use daily compounding, investment accounts use monthly, and bonds often use semi-annual. When comparing accounts, always check both the APR AND compounding frequency—a 4.8% APR compounded daily may yield more than a 5.0% APR compounded annually. The calculator's APY (effective annual rate) accounts for compounding frequency, making it easier to compare apples-to-apples.

Beginning-of-period contributions are deposited at the start of each period (month, quarter, year) and earn interest for the entire period. End-of-period contributions are deposited at the end and start earning interest in the next period. Beginning-of-period yields slightly higher returns because each deposit compounds for one extra period. Example: $500/month at 7% for 10 years with beginning-of-period = $87,500 vs end-of-period = $86,700 (+$800 advantage). The difference increases over longer periods and with larger contributions. Real-world: if you deposit on the 1st of each month (paycheck direct deposit), use beginning. If you deposit mid-month or at month-end, use end. Most 401(k) and payroll deductions use beginning-of-period since contributions occur before the period closes. For retirement planning over 30-40 years, beginning-of-period can add $10,000-$30,000 to your final balance—a meaningful difference that requires no extra effort, just earlier timing.

Inflation erodes purchasing power over time—if your investment grows 7% nominally but inflation is 3%, your real return is only ~4%. A $100,000 balance in 20 years may sound impressive, but at 3% inflation it has the purchasing power of only ~$55,400 in today's dollars. This calculator's inflation adjustment shows real (inflation-adjusted) values by discounting nominal balances back to today's dollars. Critical for retirement planning: a $1 million nest egg in 30 years may only have $400,000 in purchasing power at 3% inflation. Historical US inflation averages 2-3%, but varies (1970s: 7-10%, 2010s: 1-2%, 2020s: 5-8% spike). Use conservative estimates (3-3.5%) for planning. Always set goals in real (inflation-adjusted) dollars, not nominal dollars, to avoid undersaving. If you need $50,000/year in retirement income today, plan for $100,000+/year in 25 years to maintain the same lifestyle.

Use realistic, conservative rates based on asset class and risk: Savings accounts/CDs: 1-5% (check current HYSA rates—4-5% in 2025). Money market funds: 2-4%. Bonds (investment grade): 3-5%. Balanced portfolio (60% stocks/40% bonds): 5-7%. Diversified stock portfolio (S&P 500 index): 7-9% (historical average ~10% nominal, ~7% real after inflation). Aggressive growth stocks: 10-12% (higher risk). Conservative rule: use 6-7% for retirement planning with stock-heavy portfolios, 4-5% for balanced, 2-3% for conservative bond-heavy portfolios. NEVER assume 12-15% consistently—that's unrealistic and leads to undersaving. Individual years vary wildly (-40% to +30%), but CAGR smooths out volatility. It's better to exceed conservative 7% projections than fall short of aggressive 12% assumptions and run out of money in retirement. Adjust annually based on actual performance and rebalance as needed.

Yes—compound interest works both ways. For debt: enter your current balance as principal, the APR as interest rate (use actual credit card/loan rate like 18-24%), $0 contributions, and your payoff timeline. The result shows how much interest you'll pay if you don't pay down the balance. Then model extra payments as negative contributions to see savings. Example: $10,000 credit card at 20% APR grows to $61,917 in 10 years with no payments (compound interest working against you). Paying $200/month drops the balance to $0 in 7.8 years and saves $13,000+ in interest. For loans with compound interest (student loans during deferment, capitalized interest): model how unpaid interest adds to your principal and compounds. This visualizes the true cost of debt and motivates aggressive payoff. Always pay off high-interest debt (>7-8% APR) before investing in accounts earning lower returns—paying off an 18% credit card is equivalent to an 18% guaranteed return.

The Rule of 72 is a quick mental shortcut to estimate how long it takes your money to double: divide 72 by your annual interest rate. At 8%, money doubles in 72 ÷ 8 = 9 years. At 6%, it takes 72 ÷ 6 = 12 years. At 10%, it doubles in 7.2 years. This rule is remarkably accurate for rates between 6-10% (the range most investors care about). Example: $10,000 at 8% actually doubles in 9.01 years—Rule of 72 predicts 9.0 years (99.9% accurate). The rule becomes less accurate at very low (<3%) or very high (>15%) rates, but still useful for rough estimates. Practical use: quickly assess different scenarios without calculators. If you're 30 and want to retire at 65 (35 years), your money doubles ~4 times at 8% (every 9 years), turning $50,000 into $800,000. For precise calculations, use this calculator; for quick mental math, use the Rule of 72.

Small differences are normal due to: (1) Rounding—banks round daily interest to 2 decimals; over time this creates tiny discrepancies. (2) Timing—if you made contributions on different dates than modeled (mid-month vs beginning/end), actual compounding periods differ. (3) Fees—account fees, expense ratios, transaction costs reduce net returns but aren't modeled here. (4) Variable rates—if your actual rate changed (promotional APY expired, market fluctuations), your real returns differ from fixed-rate projections. (5) Partial periods—starting mid-month or mid-year means actual periods don't align with the calculator's full periods. (6) Dividend reinvestment—stocks/funds may reinvest dividends on specific dates, not continuously. To match your statement: use the exact APY from your account disclosure, match contribution dates precisely, account for fees separately, and use the calculator's year-by-year table to compare period-by-period. For brokerage accounts with market volatility, this calculator models fixed returns—real market returns fluctuate annually, so CAGR won't match year-to-year.

Taxes significantly reduce net compound growth depending on account type. Tax-deferred (401k, Traditional IRA): grow tax-free until withdrawal, then taxed as ordinary income (10-37% brackets). Tax-free (Roth IRA, Roth 401k): contributions are after-tax, but growth and withdrawals are 100% tax-free forever—max compounding benefit. Taxable (brokerage): pay 15-20% long-term capital gains tax on profits, plus annual taxes on dividends/interest. HSA: triple tax advantage (deductible contributions, tax-free growth, tax-free medical withdrawals). Example: $100,000 growing to $1M over 30 years. Roth IRA: keep full $1M. Traditional IRA: $700,000-$800,000 after taxes. Taxable account: $750,000-$850,000 after capital gains taxes (depends on holding period, tax bracket). This calculator doesn't model taxes—reduce your final balance by 15-30% for taxable accounts to estimate after-tax value. Prioritize tax-advantaged accounts (401k, IRA, HSA) to maximize compound growth.

Withdrawing funds early permanently reduces your compounding potential—you lose not just the withdrawn amount, but all future growth that money would have generated. Example: withdrawing $10,000 from your retirement account at age 35 doesn't just cost you $10,000—it costs you the 30 years of compounding until age 65. At 8%, that $10,000 would grow to $100,600 by retirement. You've actually lost $100,600 in future wealth, plus immediate taxes and penalties (10% penalty + 10-37% income tax on Traditional IRA/401k = 20-47% total hit). A $10,000 withdrawal may only net you $5,300-$8,000 after penalties and taxes. To model: reduce your initial principal or contributions by the withdrawal amount, or restart the calculator from the withdrawal date with the new lower balance. Avoid early withdrawals by maintaining a separate emergency fund (3-6 months expenses in HYSA). Only withdraw from retirement accounts for true emergencies, and understand the massive long-term cost of breaking the compounding chain.

Lump sum investing (investing all money immediately) historically outperforms dollar-cost averaging (spreading investments over time) about 66% of the time because markets trend upward, and time in the market beats timing the market. However, DCA reduces risk from market timing—if you invest a lump sum right before a crash, you lock in losses; DCA buys more shares when prices drop, averaging your cost. Use this calculator to compare: (1) Lump sum: enter full amount as initial principal, $0 contributions. (2) DCA: enter $0 initial, divide lump sum by number of periods as monthly contribution. Example: $50,000 invested immediately at 8% for 20 years = $233,050. $50,000 spread over 5 years ($833/month) at 8% for 20 years = $195,940 (lump sum wins by $37,110). But if a 30% market crash occurs in year 1, DCA may outperform by buying shares at lower prices. Best strategy: lump sum for long time horizons (20+ years) where volatility smooths out; DCA for shorter horizons (5-10 years) or if you're risk-averse and would panic-sell during a crash.

More frequent contributions generally yield slightly higher returns because money enters the market sooner and compounds longer. Weekly contributions compound more than monthly, which compound more than annual. However, the difference is small—$6,000 annual contribution at 7% for 10 years: weekly = $87,650, monthly = $87,300, quarterly = $86,950, annual = $86,500. Weekly beats annual by only $1,150 (1.3%). The bigger impact is consistency and automation—whatever frequency matches your paycheck (weekly, biweekly, monthly) ensures you never miss contributions. Transaction fees can negate benefits: if your broker charges per-transaction fees, monthly beats weekly to minimize costs. Best practice: align with your paycheck (biweekly for most W-2 employees, monthly for salaried), automate the transfer so it's effortless, and ensure contributions happen every period without fail. Missing even 2-3 contributions per year costs more than the difference between weekly and monthly frequency.

Reverse-engineer your goals: (1) Enter your target amount as 'future value' mentally, then adjust principal and contributions until results match. (2) For retirement: calculate monthly expenses in retirement, multiply by 12 × 25-30 years (safe withdrawal period), adjust for inflation over your working years. Example: need $50,000/year for 25 years = $1.25M in today's dollars. In 30 years at 3% inflation, that's $3M nominal. Work backward: at 8% return for 30 years, you need to contribute ~$2,100/month starting from $0. (3) For college (18 years): 4 years of college costs $120,000 today = ~$180,000 in 18 years at 3% inflation. At 6% return, contribute $400/month from birth. (4) For house down payment (5 years): $50,000 down payment, no inflation adjustment. At 4% HYSA return, contribute $750/month + $5,000 initial deposit. Use the calculator iteratively: start with your goal, estimate returns and timeline, adjust contributions until you hit your target. Review annually and adjust as needed.