8% Return Calculator
Use this version if you want to test a stronger return assumption. It can be useful for comparison, but it should not replace a conservative base case when you are planning serious goals.
Year-by-year breakdown
| Year | Ending balance | Total contributed | Total interest |
|---|
Why this page should not be your only plan
An 8% annual return can make a plan look attractive, but it is easy to become overconfident when the spreadsheet cooperates. Good planning compares optimistic, base, and cautious scenarios instead of falling in love with the highest number.
Use it as a comparison case
- Keep your starting amount and contribution fixed.
- Compare 5%, 7%, and 8% outputs side by side.
- Ask whether your goal still works if returns come in lower than expected.
Good next comparisons
What 8% looks like over different horizons
8% is at the optimistic end of a defensible long-run equity assumption. It is worth modelling to see the upside, and worth treating with suspicion when it is the only rate a plan works at. Every row uses this page's inputs — $10,000 to start and $300 a month — with 8% compounded monthly and contributions at the end of each month.
| After | You contributed | Balance | Interest earned |
|---|---|---|---|
| 5 years | $28,000 | $36,942 | $8,942 |
| 10 years | $46,000 | $77,080 | $31,080 |
| 15 years | $64,000 | $136,881 | $72,881 |
| 20 years | $82,000 | $225,974 | $143,974 |
| 25 years | $100,000 | $358,710 | $258,710 |
| 30 years | $118,000 | $556,465 | $438,465 |
Over the 25-year horizon this page defaults to, $100,000 of contributions becomes about $358,710. Extending to thirty years reaches $556,465 — the extra years are worth far more than the extra deposits that fund them.
How much the 8% assumption is actually carrying
The honest way to use a rate-specific calculator is to check what happens if the rate is wrong. Over 25 years, on the same contributions:
| Annual return | Final balance | vs 8% |
|---|---|---|
| 6% | $252,548 | -30% |
| 7% | $300,276 | -16% |
| 8% (8% — this page) | $358,710 | — |
| 9% | $430,421 | +20% |
| 10% | $518,619 | +45% |
Two points either side of 8% moves the 25-year result from about $252,548 to $518,619 — a spread of $266,072 on identical contributions. That spread is the uncertainty in the plan, and no calculator can narrow it. What you can do is decide which end of it your plan needs to survive.
- Fees come straight off the rate. A 1% annual charge on 8% behaves like 7%, costing about $58,434 over 25 years here.
- Inflation is not deducted. At 2.5%, $358,710 in 25 years buys about what $193,485 buys today.
- A constant rate is a modelling device. Real returns arrive out of order, and the order matters once you start withdrawing.
Questions about 8% returns
How much does $10,000 grow at 8%?
On its own, with no contributions, $10,000 at 8% compounded monthly reaches about $73,402 in 25 years. Adding $300 a month takes it to roughly $358,710.
How long does money take to double at 8%?
About 8.7 years with monthly compounding. The Rule of 72 estimates 72 ÷ 8 = 9.0 years, which is close enough to do in your head.
Is 8% a realistic assumption?
8% is at the optimistic end of a defensible long-run equity assumption. It is worth modelling to see the upside, and worth treating with suspicion when it is the only rate a plan works at. Whatever rate you choose, run the plan at one or two points lower as well — if it only works at the optimistic figure, it is not yet a plan.