Compound Interest Calculator - Future Value and Long-Term Growth

Compound interest means earning returns on both your original principal and on all previously accumulated returns. A $10,000 investment at 6% earns $600 in year 1, then $636 in year 2 (6% on $10,600), and so on. Over 30 years the balance reaches about $57,400, even though only $10,000 was deposited. The longer the period, the larger the share of the final value that comes from compounding rather than from the original deposit.

The effective monthly rate is the annual return converted to a monthly equivalent using m = (1 + r)^(1/12) − 1. At 6% annual, the effective monthly rate is about 0.487%, not simply 6% divided by 12 (0.5%). The difference matters at high rates or long horizons. Using the correct formula ensures that 12 monthly steps produce exactly the same result as one annual step at the same nominal rate.

The rule of 72 is a quick mental shortcut: divide 72 by the annual return percentage to estimate the number of years needed to double your money. At 6% the answer is 12 years; at 8% it is 9 years; at 4% it is 18 years. The rule is an approximation but works well for rates between 3% and 12%. It helps quickly compare how long different return assumptions take to produce the same result.

$10,000 at 6% for 30 years grows to about $57,400. The same $10,000 at 6% for 40 years grows to about $103,000. The extra 10 years roughly double the final balance without any additional deposit. Early contributions and principal are worth more because their returns have more years to compound. A common illustration: $1 invested at age 25 is worth about twice as much at retirement as $1 invested at age 35.

Principal (or starting amount) is the lump sum you deposit at the beginning. Contributions are additional deposits made regularly over the investment period. Both earn compound returns, but the starting amount has the longest compounding runway. In the worked example above, $10,000 in principal plus $36,000 in contributions ($100 per month for 30 years) produce a total balance of roughly $161,000, with about $115,000 coming from growth on top of both.

Monthly compounding applies the return 12 times per year instead of once. At 6% nominal, monthly compounding produces an effective annual rate of about 6.17% ((1.005)^12 − 1). Over 30 years on $10,000, that adds roughly $2,800 compared with annual compounding, a modest but consistent gain. This calculator uses monthly compounding because it matches how most funds and savings accounts actually credit returns.

Broad global equity index funds have returned roughly 7 to 10 percent annually before inflation over the long term. A common conservative planning assumption is 5 to 7 percent, which leaves room for fees, taxes, and below-average periods. To model real (inflation-adjusted) growth, subtract expected inflation (typically 2 to 3 percent) from the nominal return. Always test several scenarios rather than committing to a single number, since a 2 percentage point difference in assumed return changes the 30-year outcome dramatically.

A 0.2% TER on a $10,000 investment at 6% over 30 years leaves roughly $55,600 (effective return 5.8%). Raising the TER to 0.5% leaves roughly $49,800. That 0.3 percentage point difference costs about $5,800 over 30 years. On a larger portfolio or with monthly contributions, the gap widens further because contributions also compound at the lower effective rate for the full remaining period.

This option raises your recurring contribution by a fixed amount at the start of each year. Starting at $100 per month with a $10 annual increase means year 2 uses $110, year 3 uses $120, and so on. It models a salary that rises over a career. Even a small annual raise makes a meaningful difference at long horizons because the higher contributions also compound over the remaining years.

All seven frequencies (Daily=365, Weekly=52, Biweekly=26, Semimonthly=24, Monthly=12, Quarterly=4, Annually=1) are normalized to a monthly timeline internally. Higher frequency means contributions enter the portfolio sooner and benefit from slightly more compounding cycles. The difference between weekly and monthly is small over 30 years. Where frequency matters more is with transaction costs: at $2 per trade, weekly contributions cost $104 per year in fees versus $24 for monthly.

Nominal return is the raw annual growth percentage before correcting for inflation. Real return is what remains after inflation has eroded purchasing power. At 6% nominal return and 3% inflation, the real return is roughly 3%. This calculator outputs nominal values. To model real purchasing power, subtract your inflation assumption from the return before entering it, or divide the final balance by (1 + inflation rate)^years.

This calculator models transaction costs and TER but not income or capital gains taxes. In a taxable account, taxes may be owed on dividends or realized gains each year, which reduces the compounding base. In a tax-advantaged account (ISA, 401k, pension), growth is sheltered and compounds untaxed. A practical workaround for taxable accounts is to reduce the assumed annual return by 0.5 to 1 percent to get a rough after-tax estimate.

Nominal means values are shown in future currency units without adjusting for the loss of purchasing power from inflation. A balance of $161,000 in 30 years buys less than $161,000 buys today if inflation averages 3 percent per year. To convert to today's purchasing power, divide by (1.03)^30, which gives roughly $66,000. Entering a return already reduced by your inflation estimate (e.g. 3% instead of 6%) is a simpler way to get approximate real values.