What annual return should I use?

7% is a common real (inflation-adjusted) long-term stock market estimate. 10% is the nominal historical average. Use 5-6% for a conservative bond-heavy portfolio.

Does this account for inflation?

No. The output is in future dollars. If you assumed a 7% real return (already inflation-adjusted), then the number approximates today's purchasing power. If you used 10%, divide by roughly 2-3× over 30 years for a rough real-dollar estimate.

What's the Rule of 72?

A quick mental shortcut: divide 72 by the annual return rate to estimate years to double your money. At 7% return, money doubles in 72/7 = ~10 years. At 10% return, ~7.2 years. At 4% return, ~18 years. Compound interest's power: $10K at 7% becomes $20K in 10 years, $40K in 20 years, $80K in 30 years, $160K in 40 years — the late-stage doubling adds the most absolute dollars.

Should I lump-sum invest or dollar-cost average?

Mathematically: lump-sum wins about 2/3 of the time. The market trends up, so being invested earlier captures more growth. Dollar-cost averaging (splitting a lump-sum into monthly investments over 6-12 months) reduces the risk of buying at a peak. Behaviorally, DCA helps anxious investors stay invested rather than waiting for a 'better' moment that never comes. For new monthly contributions from paycheck, you're effectively DCA'ing already; for windfalls (inheritance, bonus), studies show lump-sum produces ~2.4% higher 10-year returns on average than DCA'ing over 12 months.

What does 'compounding monthly vs daily' actually mean?

Daily compounding gives slightly higher returns than monthly. At 7% APR, monthly compounding yields effective annual rate of 7.23%; daily compounding yields 7.25%. Over 30 years on $100K, that's $735K vs $740K — about 0.7% difference. Most investment accounts compound continuously (every transaction); savings accounts compound daily; bonds typically compound semi-annually. The compounding frequency matters less than the rate and time. The calculator uses monthly compounding by default; switch to annual for back-of-envelope checks against bond yields.

Is 7% a realistic long-term stock market return?

S&P 500 historical: ~10% nominal annual return (1928-2024) before inflation, ~7% real return (after inflation). The 7% real number is what most retirement planners use. Important caveats: returns are not linear (some decades return 0% real, others 12%+); the 'historical average' includes 1929-1933 (-65% nominal cumulative) and 2000-2010 (~0% nominal). Sequence-of-returns risk matters near retirement. Conservative planners use 5-6% real for projection. Aggressive planners use 8-10%. Pick based on your risk tolerance and time horizon.

Is the compound interest calculator accurate for retirement projections?

The math is exact — same future-value formula used by Excel, Vanguard, Fidelity, and every retirement planner. Where the projection can mislead: (1) Constant-rate assumption — real markets fluctuate (some years -30%, others +25%), and a 7% average doesn't guarantee a smooth 7% path. (2) No tax modeling — traditional 401(k) is pre-tax (so $1M shows in calculator but $760K after 24% retirement bracket), Roth IRA stays $1M, taxable brokerage owes capital gains. (3) No inflation toggle — use 7% real to project today's dollars, 10% nominal to project future dollars. (4) No fee modeling — a 1% expense ratio compounds to 25-30% drag over 30 years. For a 'will I have enough' answer: project conservatively (6% real, 80-90% of headline number to absorb shocks), then refine with Monte Carlo for 5-year retirement check.

How do I calculate compound interest by hand?

Single deposit: A = P × (1 + r/n)^(n×t). P = principal, r = annual rate, n = compounding periods per year, t = years. Example: $10,000 at 7% compounded monthly for 10 years: A = 10000 × (1 + 0.07/12)^(12×10) = 10000 × (1.00583)^120 ≈ $20,097. Periodic deposits add a series term: FV = P(1+r)^n + PMT × [((1+r)^n - 1) / r] where PMT is periodic deposit. Quick mental shortcuts: Rule of 72 (72/rate = years to double, so 7% → 10 years); Rule of 114 (114/rate = years to triple). For 'how much do I need to save monthly to hit $X by year Y': PMT = X × r / [(1+r)^n - 1]. Easier in Excel: =FV(rate, periods, -PMT, -PV) returns future value; =PMT(rate, periods, -PV, FV) solves for required deposit.

What's the best account type for compound growth?

Tax-advantaged ALWAYS beats taxable for long-horizon compounding. Hierarchy: (1) HSA (triple tax-free in US — best account in the tax code if eligible). (2) Roth IRA / Roth 401(k) — pay tax now, growth is tax-free forever. (3) Traditional 401(k) / Traditional IRA — deduct now, taxed on withdrawal (but lower bracket usually). (4) 529 college savings — tax-free if used for education. (5) Taxable brokerage — owes annual dividend tax + capital gains on sale. The 30-year compound difference: $500/month for 30 years at 7% = $620K in tax-advantaged vs ~$520K in taxable (after capital gains and annual dividend tax). For retirement: max Roth IRA + max HSA + 401(k) to match. For shorter goals (<10 years): high-yield savings (no tax-advantaged option for short-term). For taxable brokerage: hold tax-efficient index funds (low turnover, qualified dividends).

Money & Finance · Free tool

Compound Interest Calculator

Calculate how your money grows with compound interest. Add starting balance and monthly contributions to watch the curve — free online tool in seconds.

Updated June 2026

Starting balance ($)Monthly deposit ($)Annual return (%)Years

Final balance

$134,270

You contributed

$61,000

Interest earned

$73,270

Year-by-year growthContributionsInterest

21 rows — one per year plus the starting balance

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What it does

A compound interest calculator that shows what a starting balance plus regular monthly contributions becomes over time at a given return rate. The number at 30 years is often shocking — this is the magic of compounding.

$200/month at 7% for 30 years = $245,000. At 40 years = $525,000. Time is the dominant variable, not amount. Starting early beats contributing more. Pair with our investing guide.

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Example input & output

Input

Starting balance: $5,000
Monthly contribution: $500
Annual return: 7%
Years: 30

Output

Ending balance: $650,000
Total contributed: $185,000
Interest earned: $465,000

Interest earned exceeds contributions by 2.5×. This is why time matters more than amount.

How to use it

Enter starting balance and monthly contribution.
Set expected annual return (7% is a common long-term stock market assumption).
Set number of years to invest.
See the ending balance and total contributed vs. interest earned.

How it works

Key takeaways

$500/month at 7% for 30 years grows to $650K. Interest alone exceeds total contributions by 2.5× — that’s the entire compounding argument in one number.
Use real (inflation-adjusted) returns of ~7%, not nominal 10%. The 3% gap is inflation, and projecting nominal will overstate your retirement purchasing power by 2-3× over 30 years.
Rule of 72: divide 72 by your annual rate to get years-to-double. At 7%, money doubles every ~10 years; at 4%, every 18 years.
Starting at age 25 vs 35 typically produces 2× the final balance — not because of contributions, but because the last decade of compounding adds the most absolute dollars.

Uses the future-value formula for a series with compound interest: FV = P(1+r)^n + PMT × [((1+r)^n - 1) / r] where P is starting balance, PMT is periodic contribution, r is periodic rate, and n is number of periods. We compound monthly to match how most retirement accounts work.

Advanced: real returns, sequence risk, and tax treatment

The biggest hidden distortion: nominal vs real returns. The S&P 500’s historical 10% return is nominal — before inflation. Real return (after inflation) is ~7% historically and is what matters for purchasing power. Default to 7% for long-horizon projections; the calculator’s output then approximates today’s dollars. Use the retirement calculator for a more complete model that includes inflation, Social Security, and withdrawal-rate analysis.

Sequence-of-returns risk hits near retirement: the same average return distributed differently produces wildly different ending balances if you’re drawing down during the bad years. A 30-year-old can absorb a 30% drawdown; a 65-year-old in year 1 of retirement can’t. This calculator assumes a constant rate — for retirement modeling specifically, run multiple scenarios at ±30% of your assumed return. Tax treatment is the other quiet variable: pre-tax (traditional 401k / IRA) and after-tax (Roth) contributions reach different ending balances after tax even with identical inputs. See the Roth IRA calculator and 401k calculator for tax-aware projections.

How this compares to alternatives

vs Excel FV(): same math; spreadsheet is faster for custom modeling, web tool is faster for one-off scenario comparison. vs Vanguard / Fidelity / Schwab calculators: those usually assume their own fund mix and bake in expense ratios. Our generic version forces you to specify the return rate explicitly, which is more honest. vs Personal Capital / Empower retirement planner: those handle Monte Carlo simulation (running 1000+ market-history scenarios). For binary “will I have enough?” questions in retirement planning, Monte Carlo is more accurate. For “what does $X/month become?” this calculator is fine. vs the Rule of 72 mental shortcut: 72 / rate = years to double. Useful for back-of-envelope; this calculator is for actual numbers.

Common mistakes when using this tool

Using nominal returns and forgetting inflation. 10% nominal looks great until you realize $1M at age 65 in 2055 is roughly $400-500K in 2025 purchasing power. Always model in real terms (~7%).
Optimistic return assumptions. 8-10% works for 30-year horizons in stocks; 5-6% is more realistic for balanced portfolios; 3-4% for bonds. Don’t project a 90/10 stock portfolio’s historical return on a 60/40 portfolio you actually have.
Ignoring contribution limits. Roth IRA capped at $7K/year (2024, $8K if 50+). 401k capped at $23K (2024, $30.5K if 50+). The calculator accepts any number; reality has annual ceilings. See the 401k contribution limit glossary.
Underestimating fee drag. A 1% expense ratio looks small; over 30 years it eats 25-30% of total return. Use index funds (0.03-0.10% expense ratios) and re-run the calculator at (your-rate − 0.05%) to see what you actually get; high-fee target-date funds may need (rate − 1%).
Stopping early when life happens. The calculator assumes uninterrupted contributions. Real life: layoffs, kids, home purchases all create gaps. Conservatively model 80-90% of the headline number to account for these gaps.

Learn more about compound growth

The Rule of 72 — the mental shortcut for doubling time, when it’s accurate, and the “rule of 114” for tripling.
Nominal vs real returns — why 10% on paper is 7% in groceries, and how to project in today’s dollars.
Compound interest glossary — the short definition and the simple-vs-compound distinction.
APY glossary — how compounding frequency converts a nominal rate into an effective annual yield.

When to use this tool

Long-horizon planning (10+ years) where compounding dominates.
Comparing consistent-contribution strategies over time.
Illustrating the cost of waiting to invest.

When not to use it

Short-term savings under 5 years — use the savings goal calculator.
Debt payoff — the math runs the opposite direction (use the debt payoff calculator).
Tax-advantaged accounts where contribution limits and tax treatment matter (use a proper retirement planner).

Common use cases

Projecting a retirement account balance at age 65.
Showing a teenager what $100/month becomes over 40 years.
Comparing the impact of starting 5 years earlier vs saving 50% more.
Modeling a 529 college savings account growth.

Frequently asked questions

What annual return should I use?: 7% is a common real (inflation-adjusted) long-term stock market estimate. 10% is the nominal historical average. Use 5-6% for a conservative bond-heavy portfolio.
Does this account for inflation?: No. The output is in future dollars. If you assumed a 7% real return (already inflation-adjusted), then the number approximates today's purchasing power. If you used 10%, divide by roughly 2-3× over 30 years for a rough real-dollar estimate.
What's the Rule of 72?: A quick mental shortcut: divide 72 by the annual return rate to estimate years to double your money. At 7% return, money doubles in 72/7 = ~10 years. At 10% return, ~7.2 years. At 4% return, ~18 years. Compound interest's power: $10K at 7% becomes $20K in 10 years, $40K in 20 years, $80K in 30 years, $160K in 40 years — the late-stage doubling adds the most absolute dollars.
Should I lump-sum invest or dollar-cost average?: Mathematically: lump-sum wins about 2/3 of the time. The market trends up, so being invested earlier captures more growth. Dollar-cost averaging (splitting a lump-sum into monthly investments over 6-12 months) reduces the risk of buying at a peak. Behaviorally, DCA helps anxious investors stay invested rather than waiting for a 'better' moment that never comes. For new monthly contributions from paycheck, you're effectively DCA'ing already; for windfalls (inheritance, bonus), studies show lump-sum produces ~2.4% higher 10-year returns on average than DCA'ing over 12 months.
What does 'compounding monthly vs daily' actually mean?: Daily compounding gives slightly higher returns than monthly. At 7% APR, monthly compounding yields effective annual rate of 7.23%; daily compounding yields 7.25%. Over 30 years on $100K, that's $735K vs $740K — about 0.7% difference. Most investment accounts compound continuously (every transaction); savings accounts compound daily; bonds typically compound semi-annually. The compounding frequency matters less than the rate and time. The calculator uses monthly compounding by default; switch to annual for back-of-envelope checks against bond yields.
Is 7% a realistic long-term stock market return?: S&P 500 historical: ~10% nominal annual return (1928-2024) before inflation, ~7% real return (after inflation). The 7% real number is what most retirement planners use. Important caveats: returns are not linear (some decades return 0% real, others 12%+); the 'historical average' includes 1929-1933 (-65% nominal cumulative) and 2000-2010 (~0% nominal). Sequence-of-returns risk matters near retirement. Conservative planners use 5-6% real for projection. Aggressive planners use 8-10%. Pick based on your risk tolerance and time horizon.
Is the compound interest calculator accurate for retirement projections?: The math is exact — same future-value formula used by Excel, Vanguard, Fidelity, and every retirement planner. Where the projection can mislead: (1) Constant-rate assumption — real markets fluctuate (some years -30%, others +25%), and a 7% average doesn't guarantee a smooth 7% path. (2) No tax modeling — traditional 401(k) is pre-tax (so $1M shows in calculator but $760K after 24% retirement bracket), Roth IRA stays $1M, taxable brokerage owes capital gains. (3) No inflation toggle — use 7% real to project today's dollars, 10% nominal to project future dollars. (4) No fee modeling — a 1% expense ratio compounds to 25-30% drag over 30 years. For a 'will I have enough' answer: project conservatively (6% real, 80-90% of headline number to absorb shocks), then refine with Monte Carlo for 5-year retirement check.
How do I calculate compound interest by hand?: Single deposit: A = P × (1 + r/n)^(n×t). P = principal, r = annual rate, n = compounding periods per year, t = years. Example: $10,000 at 7% compounded monthly for 10 years: A = 10000 × (1 + 0.07/12)^(12×10) = 10000 × (1.00583)^120 ≈ $20,097. Periodic deposits add a series term: FV = P(1+r)^n + PMT × [((1+r)^n - 1) / r] where PMT is periodic deposit. Quick mental shortcuts: Rule of 72 (72/rate = years to double, so 7% → 10 years); Rule of 114 (114/rate = years to triple). For 'how much do I need to save monthly to hit $X by year Y': PMT = X × r / [(1+r)^n - 1]. Easier in Excel: =FV(rate, periods, -PMT, -PV) returns future value; =PMT(rate, periods, -PV, FV) solves for required deposit.
What's the best account type for compound growth?: Tax-advantaged ALWAYS beats taxable for long-horizon compounding. Hierarchy: (1) HSA (triple tax-free in US — best account in the tax code if eligible). (2) Roth IRA / Roth 401(k) — pay tax now, growth is tax-free forever. (3) Traditional 401(k) / Traditional IRA — deduct now, taxed on withdrawal (but lower bracket usually). (4) 529 college savings — tax-free if used for education. (5) Taxable brokerage — owes annual dividend tax + capital gains on sale. The 30-year compound difference: $500/month for 30 years at 7% = $620K in tax-advantaged vs ~$520K in taxable (after capital gains and annual dividend tax). For retirement: max Roth IRA + max HSA + 401(k) to match. For shorter goals (<10 years): high-yield savings (no tax-advantaged option for short-term). For taxable brokerage: hold tax-efficient index funds (low turnover, qualified dividends).

See how this compares

Head-to-headRoth IRA vs Traditional IRARoth IRA vs Traditional IRA: how each is taxed, who each is for, and which gives you more retirement spending power. Free compound interest calculator included.

Show the math + sources

Formula

Future value A = P(1 + r/n)^(n × t). With monthly contributions C: A = P(1+r/n)^(n×t) + C × [((1+r/n)^(n×t) − 1) / (r/n)].

What this assumes

Constant interest rate, no fees, no taxes. Contributions added at end of each compounding period; no withdrawals.

Sources

SEC Investor.gov — Compound Interest Calculator

Methodology last verified: 2026-04-30

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