Investment Growth Calculator
Compounding turns steady contributions into serious growth over time. Enter what you're starting with, what you add each month, and the return you expect — see the projected future value, and how much of it is growth you never had to save.
Projected future value
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Enter your details and calculate.
Future value compounds your starting amount and every monthly contribution at the return you enter. This is a projection based on your assumptions, not a guarantee — real returns vary year to year.
Why compounding accelerates growth
Simple growth adds the same amount every period. Compound growth adds a percentage of whatever the balance already is — including everything it has already earned. Early on, the difference is small. After a decade or two, it isn't: in the example below, more than half the final balance is money that was never actually contributed, just earned.
That's also why this calculator treats your starting amount and your monthly contributions separately. A dollar invested today has decades to compound; a dollar invested in the calculator's final month barely has any time at all. When money goes in matters almost as much as how much goes in.
The math behind the projection
The calculator treats your investment as two streams that both compound monthly: the amount you start with, and the amount you add every month afterward.
where r = annual return ÷ 12, n = years × 12, P = starting amount, C = monthly contribution
The first term compounds your starting balance for the full period. The second term adds up every monthly contribution, each compounding from the month it's added until the end — so a contribution made in year one has far longer to grow than one made in year nineteen.
A worked example, step by step
Say you start with $10,000 already invested, add $500 every month, and assume a 7% average annual return, compounded monthly, for 20 years — the same numbers loaded into the calculator above by default.
- Step 1 — monthly rate. Convert the 7% annual return to a monthly rate: 7% ÷ 12 ≈ 0.583% per month.
- Step 2 — growth factor. Over 240 months, that monthly rate compounds to (1.00583)²⁴⁰ ≈ 4.039 — your money is set to grow roughly fourfold from compounding alone.
- Step 3 — growth on your starting amount. $10,000 × 4.039 ≈ $40,387.
- Step 4 — growth on your contributions. $500 a month, compounding the same way, grows to about $260,463 over 20 years.
- Step 5 — total future value. Add the two: $40,387 + $260,463 ≈ $300,851.
- Step 6 — how much is actually yours vs. growth. You'll have contributed $10,000 + ($500 × 240 months) = $130,000 out of pocket. The remaining $170,851 — well over half the final balance — is investment growth you never had to save.
Change the assumed rate, the years, or the monthly amount, and the calculator above recalculates all of this instantly.
Frequently asked questions
What average annual return should I use for stocks?
There's no single right number. The S&P 500's long-run historical average is often cited around 10% before inflation, or roughly 6%–7% after inflation. Many planners use a more conservative 5%–7% to avoid overstating future growth — try a few rates here to see how sensitive your result is.
Does this account for taxes or investment fees?
No. This models gross growth only. Taxes on gains, and any account fees or fund expense ratios, will reduce your real-world result, so consider using a lower assumed rate to approximate their drag, or model the account type separately.
Does the return I enter include dividends?
It should. Use a total-return assumption — one that includes reinvested dividends — since that's how most long-run stock market averages are quoted. Entering a price-only return will understate real-world growth.
Does this account for market volatility or bad years?
No. It applies one smoothed, constant annual return every year, while real markets rise and fall unevenly. Treat the result as a long-run trendline, not a prediction of any specific year, and consider running the numbers at a lower rate for a more conservative case.
Is investing a lump sum better than investing monthly?
Historically, investing a lump sum immediately has outperformed spreading it out (dollar-cost averaging) more often than not, since markets trend upward over time. But dollar-cost averaging reduces the risk of investing everything right before a downturn, which is why many people prefer the steadier, monthly approach this calculator models.
How much does starting early actually matter?
A lot — because of compounding, money invested early has more time to earn returns on its own returns. Two savers contributing the same amount can end up with very different balances if one starts even five or ten years earlier. Try shortening the years in this calculator to see the effect for yourself.
Should I pay off my mortgage early or invest instead?
It depends on your mortgage rate versus your expected investment return, and your comfort with risk. Paying down a mortgage is a guaranteed, risk-free return equal to your interest rate; investing is a variable, market-dependent return that's historically been higher over long periods but isn't guaranteed. Compare this calculator against the Extra Payment Calculator to weigh both paths side by side.