| Year | Contributions | Balance | Growth |
|---|---|---|---|
| 1 | $4,600 | $4,790 | $190 |
| 5 | $19,000 | $22,895 | $3,895 |
| 10 | $37,000 | $53,935 | $16,935 |
| 15 | $55,000 | $97,938 | $42,938 |
| 20 | $73,000 | $160,317 | $87,317 |
| 25 | $91,000 | $248,747 | $157,747 |
| 30 | $109,000 | $374,108 | $265,108 |
Monthly compounding, contributions made at month-end. Results are illustrative and don't account for taxes, fees, or inflation.
How compound interest actually works
Compound interest is interest earning interest. Instead of your gains sitting idle, they get reinvested — and then they earn returns too. That's the entire engine behind long-term wealth building.
Year one, your gains are small. Year five, they're noticeable. By year 20, the interest is doing more work than your contributions. By year 30, the graph starts going almost vertical.
The formula
FV = P (1 + r/n)^(nt) + PMT × [((1 + r/n)^(nt) − 1) / (r/n)]
- FV = future value
- P = initial principal
- PMT = periodic contribution (per compounding period)
- r = annual return (decimal)
- n = compounding periods per year
- t = years
How to calculate compound interest by hand
The calculator does this instantly, but running it once yourself makes the result impossible to misread. This worked run uses a $5,000 start, $300 a month, 7% a year, monthly compounding, over 30 years.
- Write down your starting balance. The principal (P) is whatever is invested on day one — $5,000, for example. If you are starting from zero, P is 0 and only the contributions compound.
- Convert the annual return to a periodic rate. Divide the annual rate by the number of compounding periods per year. 7% compounded monthly is 0.07 ÷ 12 = 0.005833 per month.
- Count the number of periods. Multiply years by periods per year. 30 years of monthly compounding is 30 × 12 = 360 periods.
- Compound the starting balance. P × (1 + r/n)^(nt). $5,000 × 1.005833^360 = about $40,650.
- Compound the contributions. PMT × [((1 + r/n)^(nt) − 1) ÷ (r/n)]. $300 a month gives 300 × [(8.1297 − 1) ÷ 0.005833] = about $366,700.
- Add the two parts and subtract what you paid in. $40,650 + $366,700 = roughly $407,350 future value. Contributions were $5,000 + ($300 × 360) = $113,000, so about $294,350 is compound growth.
Sanity check
Contributions of $113,000 turning into roughly $407,000 means compounding did more than twice the work you did. For the shortcut version, the Rule of 72 estimates doubling time: 72 ÷ 7 ≈ 10.3 years. Related definitions: compound interest, APY, and CAGR, which measures the same growth backwards — see how to calculate CAGR.
Three real-life examples
- The 25-year-old with $300/month at 7%. By age 65, they have roughly $720,000. Total contributions: $144,000. The rest is compounding.
- The 35-year-old with $500/month at 7%. By age 65, they have about $567,000. Ten years later start, and it costs them more per month to end up with less.
- The one-time $10,000 gift at age 20, untouched at 8%. At age 65, it's grown to about $319,000 — with zero additional contributions.
Practical tips for making compounding work for you
- Automate it. Set a monthly transfer on payday. Don't rely on willpower.
- Use tax-advantaged accounts first. Compounding tax-free (Roth IRA, ISA, TFSA) beats compounding taxed every year.
- Reinvest dividends. Turn on DRIP so payments buy more shares automatically.
- Ignore short-term volatility. The engine needs decades to spin up. Checking daily makes it feel like nothing's happening.
- Increase contributions with every raise. Even bumping your monthly amount by 5% per year makes a huge difference in the final number.
Frequently asked questions
How does compound interest work?
Compound interest is interest earned on both your original money and on the interest already earned. In investing, it's the mechanism by which small, consistent contributions become large sums over decades — because each year's growth becomes the base for the next year's growth.
What return should I assume?
For long-term stock market investing, 7% (after inflation) or 9–10% (before inflation) are common historical assumptions for a diversified index fund. Bonds have historically returned 2–4% after inflation. Be conservative — real returns vary year to year.
How often does interest compound?
For investments, compounding happens as frequently as your gains are reinvested — daily for interest-bearing accounts, and effectively continuously for reinvested dividends and growth in a stock fund. This calculator uses monthly compounding, which closely matches most real accounts.
Does this calculator account for taxes or inflation?
No — it shows raw nominal returns. To estimate real (inflation-adjusted) returns, subtract 2–3% from your assumed annual return. For taxes, use a tax-advantaged account (Roth IRA, 401(k), ISA, TFSA) whenever possible so the results here more closely reflect what you'll actually keep.
How long does it take to double my money?
The Rule of 72 gives a quick estimate: divide 72 by your annual return. At 7%, your money doubles in about 10.3 years. At 10%, in about 7.2 years.
Put this into practice
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