Compound growth, not stock picking or market timing, is the main driver of long-term investment outcomes. This calculator projects a portfolio forward from a starting balance, an expected annual return, and monthly contributions, then separates what you put in from what the market added. For a plain compounding calculation with no ongoing deposits, see the compound interest calculator.
The investment growth equation
Future value combines compound growth on the starting balance with the future value of a stream of monthly deposits, the second term is the standard annuity formula. Starting with $10,000, adding $500 every month, and earning 8% annually for 25 years produces about $548,900: roughly $160,000 from the deposits themselves and about $388,900 from compound growth on both the principal and the contributions.
Rule of 72
At 8% your money doubles every 9 years. $10,000 becomes $20,000 in 9 years, $40,000 in 18, $80,000 in 27, $160,000 in 36. Each doubling adds more than all previous gains combined. To measure the actual annualized return a portfolio delivered after the fact, rather than assuming one, use the growth rate calculator.
Compounding frequency
The 8% figure in these calculations is a nominal annual rate compounded monthly, which is not the same as compounding it once a year. Holding $10,000 for 25 years with no further deposits, monthly compounding at 8% nominal grows it to about $73,400, versus about $68,485 if the same 8% only compounds annually, a difference of roughly $4,900, or about 7%, from compounding frequency alone.
How the balance breaks down over time
Using the same $10,000 start and $500 monthly contribution at 8%, the gap between money contributed and money earned widens dramatically with time.
| Years | Final value | Contributed | Compound gain |
|---|---|---|---|
| 10 | $113,700 | $70,000 | $43,700 |
| 25 | $548,900 | $160,000 | $388,900 |
| 40 | $1,988,000 | $250,000 | $1,738,000 |
At 10 years, contributions still outweigh gains. By 25 years, gains are more than double the amount contributed. By 40 years, gains outweigh contributions nearly 7 to 1, illustrating why the last decade of a long investing horizon usually adds more dollars than all the earlier decades combined.
Why starting early matters
A single $500 contribution invested at age 25 and left untouched until age 65 (40 years at 8%) grows to about $500 × 1.08^40 ≈ $10,870. The same $500 invested 10 years later, at age 35, has only 30 years to grow, reaching about $500 × 1.08^30 ≈ $5,030, less than half as much despite being an identical contribution at an identical rate. The extra decade of compounding, not a higher return, explains the difference.
Reinvestment yield
Reinvesting dividends and interest is the engine of long-term wealth. Pulling those distributions as cash converts compound growth into linear growth and dramatically reduces the long-run value. A stock returning 7% a year with dividends reinvested compounds to about 1.07^20 ≈ 3.87 times its starting value over 20 years; if the 3-percentage-point dividend portion is instead taken as cash each year and only the 4% price appreciation compounds, the same stock reaches only about 1.04^20 ≈ 2.19 times its starting value, nearly 77% less growth from that one choice alone.
Adjusting for inflation
Every result above is nominal, meaning it ignores the falling purchasing power of money over time. To see the value in today's dollars, divide the nominal total by (1 + inflation)^t. Applying 3% average inflation to the 25-year, $548,900 nominal result gives $548,900 / 1.03^25 ≈ $262,200 in real terms, roughly half the nominal figure. The gap grows with both the inflation rate and the number of years, so a long-term projection is more informative when it reports the real, inflation-adjusted number alongside the more impressive-looking nominal one. A shorter, 10-year horizon feels the same effect less severely: the $113,700 nominal result from that table shrinks to about $84,600 in real terms at 3% inflation, a smaller but still meaningful haircut.
FAQ
How is investment growth calculated?
Future value with monthly contributions is A = P × (1 + r/12)^(12t) + C × [((1 + r/12)^(12t) − 1) / (r/12)], where P is the starting balance, C the monthly contribution, r the annual return and t the years. For example, $10,000 growing at 8% for 25 years with $500 monthly contributions reaches roughly $549,000: about $160,000 from contributions and about $389,000 from compound growth.
How is CAGR used in investment projections?
CAGR, compound annual growth rate, is the steady annual rate that turns a starting balance into an ending balance, calculated as (ending value / starting value)^(1/years) − 1. It is the single-rate figure this calculator's own annual return input represents: a portfolio that grows from $10,000 to $25,000 over 10 years has a CAGR of (2.5)^(1/10) − 1, about 9.6% per year, even though no single year likely returned exactly that.
What is the Rule of 72?
The Rule of 72 estimates doubling time by dividing 72 by the annual percent return. An 8% annual return doubles an investment in about 9 years, a 12% return in about 6 years, and a 6% return in about 12 years. It stays accurate within a few percent for rates roughly between 4% and 15%.
Should I trust 10% annual return assumptions?
Historically, 10% reflects nominal long-run US stock market averages before inflation; after subtracting typical 2-3% inflation, the real return figure is closer to 6-7%. Projections using 10% show a larger nominal number, while projections using 6-7% show what that money is likely worth in today's purchasing power. Any single decade can return far less or more than either figure.
Do contributions matter more than rate?
In the first 10-15 years, contributions matter more than the return rate because compound growth has not yet built up enough gains to outweigh new deposits. After 20-plus years, the rate typically dominates. Mathematically, a higher contribution rate has more effect on the early-year balance, while a higher return rate has more effect on the late-year balance, because compounding needs a larger existing base before its effect outweighs new contributions.
Are taxes included?
No, this calculator reports pre-tax growth only. In a taxable brokerage account, subtract capital gains and dividend taxes from the final figure to estimate what you actually keep. In tax-advantaged accounts such as a 401(k), traditional IRA, or ISA, the pre-tax number shown here is much closer to the amount available at withdrawal.