See how your money grows — with monthly contributions, fund fees, and inflation built in. Compare two scenarios side-by-side, the way the big sites won't.
Long-term US inflation has averaged about 2.5%. Used to convert the future balance into today's money when the real/nominal toggle is on.
795,846 $
Your money doubles every 6.5 years (Rule of 72)
Total contributions
190,000 $
Interest earned
1,479,341 $
All values are shown in today's money
We strip inflation out of your projected balance, so every number you see has the same purchasing power as a dollar today.
How your money grows each year.
| Year | Balance | Contributions to date | Interest earned |
|---|---|---|---|
| 1 | 17,043 $ | 16,000 $ | 1,469 $ |
| 3 | 32,594 $ | 28,000 $ | 7,100 $ |
| 5 | 50,422 $ | 40,000 $ | 17,048 $ |
| 10 | 108,110 $ | 70,000 $ | 68,391 $ |
| 15 | 192,657 $ | 100,000 $ | 179,025 $ |
| 20 | 318,665 $ | 130,000 $ | 392,169 $ |
| 25 | 508,400 $ | 160,000 $ | 782,546 $ |
| 30 | 795,846 $ | 190,000 $ | 1,479,341 $ |
Compound interest is interest earned on both your original money and the interest it has already earned. Each year, the pile that earns interest gets a little bigger, so the dollars stack on top of dollars instead of growing in a straight line.
A simple example: deposit $100 at 5% interest. After year 1, you have $105 (the original $100 plus $5 of interest). In year 2, you earn 5% on $105 — that's $5.25 — so you finish year 2 with $110.25. Year 3 earns interest on the full $110.25, and so on. Over decades, that small bump from "interest on interest" becomes the lion's share of your balance. The compound interest calculator above lets you see this curve for your own numbers, including the parts most sites skip — fund fees, monthly contributions, and inflation.
Get a useful projection in under 30 seconds.
Enter your starting balance
Put in whatever you have invested today. Set this to 0 if you're starting from scratch.
Add your monthly contribution
How much will you invest each month? Even $100 compounds into real money over 20+ years.
Pick years and expected return
7% real (after inflation) or 10% nominal are reasonable defaults for a long-term stock portfolio.
Tune fees and inflation, then optionally compare
Add your fund's expense ratio (0.03–0.75%) and open the advanced panel to set inflation. Click "+ Compare another scenario" to put two strategies head-to-head.
The textbook formula for the future value of a single lump sum is A = P(1 + r/n)^(nt), where P is the principal, r is the annual interest rate, n is how many times per year interest compounds, and t is the number of years. When you also make regular contributions, you add the future-value-of-an-annuity term: PMT × [((1 + r/n)^(nt) − 1) / (r/n)]. For instant compounding (the math limit), the formula collapses to A = Pe^(rt). Our calculator uses monthly compounding under the hood — for the typical investor, the difference between monthly, quarterly, and daily compounding is too small to matter (see the daily-vs-monthly section below).
This calculator shows what fees cost in theory. Plug your actual ETF expense ratios into our portfolio analyzer to see what they're costing you right now.
Open Portfolio AnalyzerA $1,000,000 balance in 30 years sounds great, but a million in 2056 won't buy what a million buys today. "Nominal" means the raw future-dollar number; "real" means after stripping out inflation, so every dollar shown is comparable to a dollar today. Use the formula r_real = (1 + r_nominal) / (1 + r_inflation) − 1 to convert. With 10% nominal returns and 3% inflation, your real return is about 6.8%. Our calculator does this conversion for you when "Show in today's money" is on — you'll see a smaller number, but it's an honest one. Most competitor calculators skip this entirely and quietly inflate their results.
An expense ratio is the annual percentage a fund charges to manage your money. It comes straight out of your return, every year, forever. The headline numbers look small — 0.04% for a Vanguard total-market index fund, 0.75% for a typical actively-managed mutual fund. But because fees compound just like returns do (against you), the gap balloons. A $10,000 starting balance plus $500/month at 7% over 30 years grows to roughly $613,000 with a 0.04% expense ratio — and only about $548,000 with a 0.75% ratio. That's $65,000 the fund company kept instead of you, for a service that on average underperforms a cheap index fund. This is the wedge our calculator exists to surface.
Most compound-interest tools only let you simulate one path at a time. We let you compare two. Click "+ Compare another scenario" and the calculator splits into Scenario A vs Scenario B, with a delta headline showing how much further ahead the winner ends up. Try this on the questions that actually matter: low-fee index fund vs. 0.75% target-date fund. Starting at 25 vs starting at 35. $500/month vs $1,000/month. Three percent vs seven percent real return. The numbers will surprise you — that's the point.
For a diversified stock portfolio, the long-run S&P 500 average return is around 10% nominal — roughly 6–7% real after inflation. Bonds are returning about 4–5% nominal in 2026, and a high-yield savings account is paying around 4%. Pick a rate based on the asset class you're modeling, not on what makes the chart look pretty. If you don't know what to pick, 7% real or 10% nominal is the conventional default for long-term equity investors. For shorter horizons (under 10 years), drop your assumed return — the market can sit flat or negative for a decade at a time.
Compounding more often makes your balance grow faster, but for a long-term investor the difference between daily, monthly, and yearly compounding is barely noticeable. The table below shows $10,000 at 5% over 10, 20, and 30 years across compounding frequencies. The gap between annual and daily compounding is around 1% over 30 years — meaningful at scale, but dwarfed by the fee and contribution decisions you make. Our calculator uses monthly compounding because it matches how most real-world accounts actually pay interest.
| Years | Yearly | Monthly | Daily |
|---|---|---|---|
| 10 | $16,289 | $16,470 | $16,487 |
| 20 | $26,533 | $27,126 | $27,181 |
| 30 | $43,219 | $44,677 | $44,812 |
Divide 72 by your annual rate of return to estimate how many years it takes for your money to double. At 6% it's 12 years. At 8% it's 9 years. At 10% it's about 7.2 years. The Rule of 72 is a back-of-envelope approximation (the exact formula uses ln(2) ≈ 0.693), but it's close enough to be useful in conversation. Subtract your fund's expense ratio from your assumed return before dividing — that's the only honest way to use it.
Compound interest in its purest form. Each cell shows what $10,000 becomes after that many years at that annual rate, with monthly compounding and no fees.
| Years | 4% per year | 6% per year | 8% per year | 10% per year |
|---|---|---|---|---|
| 1 | $10,407 | $10,617 | $10,830 | $11,047 |
| 5 | $12,210 | $13,489 | $14,898 | $16,453 |
| 10 | $14,908 | $18,194 | $22,196 | $27,070 |
| 20 | $22,226 | $33,102 | $49,268 | $73,281 |
| 30 | $33,135 | $60,226 | $109,357 | $198,374 |
| Term | Meaning |
|---|---|
| Principal | The original amount of money you invest before any interest is earned. |
| Annual return | The percentage your investment grows in one year, before fees and inflation. |
| Compounding frequency | How often interest is added to the balance — daily, monthly, quarterly, or yearly. |
| Expense ratio | The annual percentage fee a fund charges to manage your money. Comes straight out of your return. |
| Real return | Your annual return after subtracting inflation. The honest measure of purchasing-power growth. |
| Nominal return | Your annual return before subtracting inflation. The number marketing material likes to quote. |
| Future value | What your money will be worth at the end of the investment period, including compounded growth. |
| APY vs APR | APY (annual percentage yield) includes compounding. APR (annual percentage rate) does not. APY > APR for the same nominal rate. |
| Rule of 72 | A mental shortcut: divide 72 by your annual rate of return to estimate doubling time in years. |
Compound interest is interest earned on both your original money and on the interest that money has already earned. Over long horizons, the "interest on interest" effect becomes the dominant driver of your balance.
Each compounding period (monthly, daily, etc.) your balance gets a small percentage added to it. Next period, that bigger balance gets the same percentage added. Repeat for years and the growth curve bends sharply upward.
A = P(1 + r/n)^(nt), where P is the principal, r is the annual rate, n is compounding periods per year, and t is the number of years. With recurring contributions you add the annuity term PMT × [((1 + r/n)^(nt) − 1) / (r/n)].
Simple interest only earns on the original principal. Compound interest earns on the principal plus all previously earned interest. After 30 years at 7%, $10,000 grows to $31,000 with simple interest, but about $76,000 with monthly compounding.
Yes, but the difference is small. At 5% over 30 years, daily compounding produces about 1% more than yearly compounding. The gap is real but minor compared to fees and contribution rate.
Use the Rule of 72: divide 72 by your annual rate of return. At 6% it takes about 12 years. At 8%, about 9 years. Subtract your fund's expense ratio from your return before dividing.
For a diversified stock portfolio, the long-run S&P 500 average is roughly 10% nominal / 6–7% real after inflation. Use lower numbers for shorter horizons or more conservative allocations.
Fees compound against you exactly the way returns compound for you. A 0.75% expense ratio instead of 0.04% can cost a long-term investor tens of thousands of dollars over 30 years. The fee drag callout in this calculator quantifies it for your inputs.
Yes. A million dollars in 30 years isn't worth a million dollars today. Use real (inflation-adjusted) returns to see your future balance in today's purchasing power. Our "Show in today's money" toggle does this automatically.
APY (annual percentage yield) includes the effect of compounding. APR (annual percentage rate) does not. For the same nominal rate, APY is always greater than or equal to APR.
Yes — this calculator handles both at once. Enter your starting balance and your monthly contribution and the projection combines them. Set either to 0 if it doesn't apply.
Closely related. Coast FIRE asks: how much do I need invested today so that compound interest alone (no more contributions) carries me to retirement? Open our Coast FIRE Calculator to find that specific number.
Compound interest math is exact, but real-world returns are not. Markets vary year to year, sequence-of-returns risk matters for retirees, and tax treatment depends on the account type. Use this as a planning tool, not a forecast.
Plug your real balances and contributions into My Financial Freedom Tracker and see compound interest at work every month.
Get started freeCalculate your path to financial independence with Coast FIRE, Barista FIRE, and custom withdrawal strategies.
Try Free FIRE CalculatorFind the amount you need today so compound interest carries you to FIRE — no more contributions needed.
Find your Coast FIRE numberFind the portfolio size that lets you downshift to part-time work, with healthcare bridge costs built in.
Try Free Barista FIRE CalculatorAnalyze ETF holdings, sector exposure, run Monte Carlo simulations, and stress test your portfolio against historical crises.
Try Free Portfolio AnalyzerFind your net worth percentile vs your age group and country.
Find your percentileCompare two mortgages side-by-side and see how extra payments shave years off your loan.
Open the calculatorShould you pay off the mortgage early or invest the spare cash? Compare three strategies side-by-side in today's money.
Compare strategies free