Where a long investing horizon ends up
This calculator projects an investment account rather than a savings account, which mainly means a higher assumed return and a much wider range of possible outcomes around it.
The single figure it produces is the middle of a distribution, not a promise. Treat it as the centre of a range and use the sensitivity section below to see how wide that range is.
The formula behind the number
Two things grow at once: the sum already invested, and each new contribution from the moment it arrives. Together they give:
Here P is the opening balance, M the monthly contribution, i the monthly return and n the number of months. The second term is why contributions made early matter more than contributions made late: each one is multiplied by growth for every month it remains invested.
Worked example: $10,000 plus $300 a month for 25 years
The calculator opens on a starting balance of $10,000 plus $300 every month, at 8% a year for 25 years.
- Total paid in: $100,000
- Ending balance: $358,710
- Growth: $258,710, which is 259% of what you contributed
Growth is nearly three times what you contributed. That ratio is what a long horizon buys, and it is unavailable at shorter ones.
Why the second half does the heavy lifting
At the halfway point, after 12 years, the balance is $104,010 — around 29% of the final figure, not half of it.
| Year | Paid in | Growth | Balance |
|---|---|---|---|
| 5 | $28,000 | $8,942 | $36,942 |
| 10 | $46,000 | $31,080 | $77,080 |
| 15 | $64,000 | $72,881 | $136,881 |
| 20 | $82,000 | $143,974 | $225,974 |
| 25 | $100,000 | $258,710 | $358,710 |
More than half the final balance is created in the last eight years. Selling early does not just forfeit future contributions — it forfeits the steepest part of the curve.
What starting late costs
Delay by 5 years and, contributing at the same rate, you end with $225,974 instead of $358,710. That is $132,736 less for $18,000 of skipped contributions — the gap is the growth those early payments would have earned.
How sensitive is this to the return
The rate is an assumption, not a fact, so it is worth seeing the range. Two points higher gives $518,619; two points lower gives $252,548. The spread between those two figures is larger than the entire amount you contributed. Any projection at this horizon should be read as a range.
What this calculator leaves out
Fees, tax, inflation, and sequence risk — the fact that returns arrive in an order, not as an average. A portfolio that earns 8% on average but loses heavily near the end can finish well below this figure.
What the average return hides
An 8% average can be produced by very different sequences. Steady 8% years and a mix of +25% and −15% years reach similar averages but not similar balances, because losses need larger gains to undo them.
A 30% fall requires a 43% rise to break even. This is why the drawdown you can tolerate matters more than the return you hope for, particularly in the years just before you need the money.
Related calculators
- Expense ratio calculator — what fund fees remove from this result
- Real return calculator — the same growth after inflation
- Drawdown calculator — the fall you would have to sit through