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An investment grows by FV = PV·(1+r)^n + PMT·((1+r)^n − 1) ÷ r. Enter a starting amount, monthly contribution, return and time to see the future value, how much is growth versus your own money, and the multiple on what you invested.
Your investment
The lump sum you start with
$
What you add each month — set to 0 for a pure lump sum
$
Long-run average — the S&P 500 has averaged roughly 10% before inflation
%
How long the money stays invested
yrs
Future value · 20 years
$225,974
2.76× your money in · 8.0% return
Principal $82,000Growth $143,974
Total invested
$82,000
Total growth
$143,974
Started with
$10,000
You contributed
$72,000
Estimate only. Investment returns vary year to year and are not guaranteed; this uses a steady average and ignores taxes and fees.
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The basics
What makes an investment grow?
Future value has two engines — the lump sum and the stream of contributions — and one force that multiplies both.
growth earns growth
Compounding is the multiplier
Returns are reinvested and then earn returns of their own. The effect is small at first and large later, which is why the balance curves upward the longer it is left alone.
$82,000 invested → $225,974, a 2.8× return
two sources of value
Lump sum plus contributions
A starting amount compounds for the full period; each monthly deposit compounds from the day it lands. The calculator grows both and shows how much of the result is growth, not deposits.
$10,000 up front and $300 a month, both compounding
Method
How to work out future value by hand
Four steps take a starting amount and a monthly deposit to a future balance.
- 1
Set the monthly rate and the periods
Divide the annual return by twelve for the periodic rate r, and multiply the years by twelve for the number of periods n.
8% ÷ 12 = 0.6667% · 20 × 12 = 240 - 2
Grow the lump sum
The starting amount compounds for the whole period: multiply it by (1+r) raised to the number of periods.
$10,000 × (1 + r)^240 = $49,268 - 3
Grow the contributions
Each monthly deposit compounds from the day it lands. The annuity term adds them all up at once.
PMT·((1+r)^n − 1) ÷ r = $176,706 - 4
Add them and read the multiple
The two pieces together are the future value. Divide by everything you put in to see the return multiple.
$225,974 ÷ $82,000 = 2.76×
Worked examples
The same plan over more time
$10,000 plus $300 a month at 8%. Watch the growth share climb as the years stretch out.
| Years | You invest | Future value | Growth | Multiple |
|---|---|---|---|---|
| 10 | $46,000 | $77,080 | $31,080 | 1.7× |
| 20 | $82,000 | $225,974 | $143,974 | 2.8× |
| 30 | $118,000 | $556,465 | $438,465 | 4.7× |
| 40 | $154,000 | $1,290,036 | $1,136,036 | 8.4× |
Where it shows up
What the projection answers
Three questions investors reach for this to settle.
lump sum today
Investing a windfall
See what a one-time amount becomes if you invest it and leave it — the case for putting a windfall to work rather than letting it sit.
A bonus or inheritance left to grow
drip-feeding
Regular investing
Model a steady monthly contribution to see where dollar-cost averaging lands over five, ten or thirty years.
A fixed monthly amount into a fund
what return matters
Testing assumptions
Small changes in the assumed return swing the outcome enormously over time — a reason to test a cautious rate, not just an optimistic one.
6% vs 8% over decades is huge
Reference
Return assumptions to test
General guidance, not advice. No return is guaranteed — these are long-run planning bands, before inflation.
| Mix | Planning return | Character |
|---|---|---|
| Cash / money market | ≈ 1–4% | Safe, low growth, moves with rates |
| Bond-heavy portfolio | ≈ 4–6% | Steadier, smaller swings |
| Balanced stocks & bonds | ≈ 6–8% | The common middle assumption |
| All-stock (S&P 500) | ≈ 10% long run | Highest growth, biggest swings |
Worth knowing
Five things that move the result
The return is not the only lever, and not the one you control most.
time is the big lever
Start sooner, not richer
Years in the market matter more than the amount. An early, small investment often beats a later, larger one because of the extra compounding.
fees compound against you
Keep costs low
A 1% fee looks trivial but compounds like a negative return, removing a real slice of the balance over decades. Low-cost index funds keep more of the gain.
reinvest everything
Let gains ride
Compounding only works if returns and dividends are reinvested. Spending them along the way breaks the curve that makes investing powerful.
volatility is the price
Expect down years
The long-run averages include crashes. Staying invested through them is what earns the return — selling in a dip locks in the loss.
think in real terms
Discount for inflation
The future value is nominal. To judge what it will buy, plan with a return net of 2–3% for inflation.
rule of 72
A doubling shortcut
Divide 72 by the return for a rough doubling time. It is a fast way to sanity-check any projection in your head.
Quick chart
What a lump sum grows to
Future value of a one-time investment at 8%, with no further contributions — read the row for your amount and the column for your horizon.
| Lump sum | 5 yrs | 10 yrs | 20 yrs | 30 yrs |
|---|---|---|---|---|
| $1,000 | $1,490 | $2,220 | $4,927 | $10,936 |
| $5,000 | $7,449 | $11,098 | $24,634 | $54,679 |
| $10,000 | $14,898 | $22,196 | $49,268 | $109,357 |
| $25,000 | $37,246 | $55,491 | $123,170 | $273,393 |
| $50,000 | $74,492 | $110,982 | $246,340 | $546,786 |
Lump sum only, at 8% a year, no further contributions.
How we calculate this
The formula and our assumptions
The projection is future value with regular contributions, compounded monthly.
FV = PV·(1+r)^n + PMT·((1+r)^n − 1) ÷ r
Future value
PV is the lump sum, PMT the monthly contribution, r the annual return over twelve, and n the number of months.
multiple = FV ÷ total invested
Return multiple
Dividing the future value by everything you put in shows how many times your money grew — a plain way to feel the effect of compounding.
- The return is applied as a steady monthly average; real markets are volatile.
- Contributions are added at the end of each month and compound thereafter.
- Figures are nominal — before inflation, taxes and fund fees.
- Set the monthly contribution to zero to model a pure lump sum.
Sources
Where the method comes from
- U.S. Securities and Exchange Commission (Investor.gov) — compound interest and investing basics.
- Long-run S&P 500 average return, before and after inflation.
- Standard future-value-of-an-annuity formula for regular contributions.
FAQ
Common questions
Future value with contributions is FV = PV·(1+r)^n + PMT·((1+r)^n − 1) ÷ r, where PV is the lump sum, PMT the monthly contribution, r the annual return over twelve, and n the months. Investing $10,000 plus $300 a month for 20 years at 8% grows to about $225,974 — 2.8 times the $82,000 you put in.
Use a long-run average and stay honest. The US stock market (S&P 500) has averaged roughly 10% a year before inflation over the long term, or about 7% after it. A diversified mix of stocks and bonds is often modelled at 6% to 8%. Any single year can be sharply higher or lower, so test a cautious rate too.
Over a long horizon, most of it is growth. In the example you invest $82,000 and end with $225,974, so $143,974 — the majority — is what the returns added. The longer the money compounds, the more the growth share dominates. That is the whole case for investing early and leaving it alone.
It is earning returns on your past returns, not just on your original money. In year one you earn a return on your principal; in year two you earn it on the principal plus year one's gain; and so on. The balance grows faster and faster, which is why an investment curve bends upward rather than climbing in a straight line.
Both work; they answer different situations. A lump sum invested early has the longest time to compound, so historically it beats spreading the same amount out. But most people invest what they can each month from income — regular contributions — which also smooths out the price you pay over time. The calculator handles either, or both together.
No — it shows the nominal balance. Inflation lowers what the money buys, so subtract 2–3% from the return to see it in today's dollars. Taxes depend on the account and what you sell. Fund fees quietly reduce the return — a reason low-cost index funds tend to win over decades. Treat the figure as before-tax, before-inflation.
A quick way to estimate how long money takes to double: divide 72 by the annual return. At 8%, 72 ÷ 8 ≈ 9 years to double. It is an approximation, but a handy one for sanity-checking a projection in your head without a calculator.
Dramatically, because growth compounds. The same $300 a month at 8% grows to about $54,884 in 10 years but $447,108 in 30 — three times the years, far more than three times the result. Time is the most powerful input in the whole calculation.
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Ugo CharlesBuilt and fact-checked against our editorial standards · Last updated June 2026
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