Future value answers one question: if a lump sum sits untouched and grows at a fixed annual rate, what does it become after a set number of years? The output is not a forecast of any specific investment; it is the mechanical result of compounding, useful for comparing options or setting expectations before fees, taxes and real-world volatility get involved.
Reading the multiple, not just the dollar figure
The most portable number this tool produces is the multiple: future value divided by the starting sum. At 7% for 20 years, money multiplies by about 3.87 times. At 9% for 30 years, it multiplies by roughly 13.27 times. Because the multiple does not depend on the size of the lump sum, it is the figure worth remembering and reapplying to whatever amount you are actually working with.
A useful benchmark: doubling takes about ten years at 7%, about eight years at 9%, and about eighteen years at 4%. If a projection implies your money doubles faster than that without a matching jump in risk, treat the assumed rate as optimistic rather than the arithmetic as wrong.
Growth concentrates late. In the 20-year, 7% example below, the final year alone adds more than the first six years combined, because each year's gain is calculated on an ever-larger balance.
Three lump sums, three outcomes
20,000 growing at 7% for 20 years reaches about 77,394, a gain of roughly 57,394 and a multiple of 3.87 times the starting amount.
A smaller sum with a longer runway can overtake it: 5,000 growing at 9% for 30 years reaches about 66,338, a gain of roughly 61,338 and a multiple of 13.27 times, even though the starting sum was a quarter of the size. Time did more work than the initial deposit did.
Rate sensitivity over a long horizon is where the compounding effect is easiest to underestimate: 50,000 growing for 25 years reaches about 133,292 at 4% but about 271,372 at 7%. That three-point gap in the assumed rate is worth roughly 138,080, more than double the entire starting sum.
Why the assumed rate carries almost all the risk
Because the formula raises one plus the rate to the power of the number of years, small changes in the rate produce large changes in the result once the horizon stretches past a decade or two. A projection that quietly swaps a 5% assumption for an 8% one over 30 years is not describing a slightly rosier outcome; it is describing a result roughly two and a half times larger.
This is also why a single average rate flatters a genuinely lumpy market. Real returns arrive as a sequence of up and down years, and the order matters if any money is ever added or withdrawn along the way. A pure future-value calculation with no contributions or withdrawals is immune to sequence risk, but the moment you start adding annual deposits, the constant-rate assumption stops matching how markets actually move.
Where the constant-rate formula breaks down
The formula compounds once per year at a single rate with no additions, no withdrawals, and no fees. Real accounts rarely fit that shape: brokerage and fund fees typically shave 0.1 to 1 percentage point a year off the stated return, and that drag compounds exactly like the return itself does.
Inflation is not built into the output. A future value of 77,394 in twenty years' time buys noticeably less than 77,394 buys today; running the same amount through an inflation-impact calculation alongside this one shows the purchasing-power gap rather than just the nominal growth.
Taxes are also outside the model. Interest and dividends taxed annually, or a capital-gains bill due on withdrawal, both reduce the effective compounding rate below whatever headline return you entered here.
Tax-wrapper and currency timing to check separately
In the United States, growth inside a traditional or Roth IRA compounds without annual tax drag, while the same lump sum in a taxable brokerage account loses a slice of return to yearly dividend and interest taxation, so two accounts holding identical investments will not reach the same future value.
In the United Kingdom, an ISA shelters growth from income and capital gains tax entirely, while money held outside a wrapper is subject to both, and the annual ISA subscription limit caps how much of a large lump sum can be sheltered in a single tax year.
Whatever the currency, the rate you assume should already reflect the environment it was earned in; a return quoted after inflation is a materially different number from one quoted before it, and mixing the two understates or overstates the result.
Frequently asked questions
- What is the future value formula?
- Future value equals the starting amount multiplied by one plus the annual rate, raised to the power of the number of years. A 20,000 lump sum at 7% for 20 years works out to 20,000 times 1.07 to the power of 20, or about 77,394.
- How is future value different from compound interest on savings?
- They use the same formula, but future value is the general version applied to any growth rate, including expected investment returns, while compound interest usually refers specifically to a bank account or CD paying a stated interest rate. Neither assumes extra deposits are being made.
- Does this calculation account for inflation?
- No. It compounds a nominal rate, so the result is a future dollar figure, not future purchasing power. Pair it with an inflation-impact calculation using an assumed inflation rate to see what the result is worth in today's terms.
- Why does a small rate difference matter so much over a long horizon?
- Because the rate is raised to the power of the number of years, the gap compounds rather than adding up. Over 25 years, the difference between growing 50,000 at 4% versus 7% is about 138,080, roughly 2.7 times larger than the starting sum, even though the rate gap is only three percentage points.
- Can I use this to model a retirement account with regular contributions?
- Only for the lump sum already in the account. This formula assumes one deposit and no further additions or withdrawals; a retirement projection that includes ongoing contributions needs a savings-goal or future-value-of-an-annuity calculation, which sums the growth of each contribution separately.
- What annual return is realistic to assume?
- There is no single right answer, but long-run US equity market returns have historically averaged in the high single digits before inflation over multi-decade periods; savings accounts and short-term bonds have earned far less. Use a rate consistent with the actual asset you are modeling, not a blended figure.
Sources
- Federal Reserve Economic Data (FRED) — S&P 500 total return series — Historical index level data used to gauge realistic long-run equity return assumptions (series updated daily).
- Internal Revenue Service — Traditional and Roth IRAs — Confirms that growth inside these accounts is not taxed annually, unlike a standard taxable brokerage account (guidance current for 2025 tax year).
- HM Revenue & Customs — Individual Savings Accounts (ISA) — States the annual ISA subscription limit and confirms growth within an ISA is free of income and capital gains tax (2025-26 tax year).