Open the calculator: it is at the top of our free tools page and runs entirely in your browser. Nothing is uploaded and no account is needed.
This article is about how to use it honestly, because a compound interest calculator with bad inputs will produce a confident, precise, completely fictional number.
The formula
Future value = P × (1 + r)^n + M × [((1 + r)^n − 1) / r]
P = starting balance
M = monthly contribution
r = monthly rate (annual rate ÷ 12)
n = number of monthsThe first term grows what you already have. The second grows what you add each month, with each contribution compounding for a different length of time — the earliest for the full period, the last for one month.
You do not need to calculate it. You need to understand what drives it, because the drivers are where people fool themselves.
What actually determines the outcome
Ranked by influence over a thirty-year horizon:
| Input | Effect of getting it wrong |
|---|---|
| Years | Enormous. 20 vs 30 years roughly halves or doubles the result |
| Monthly contribution | Large and linear. Doubling it nearly doubles the outcome |
| Return assumption | Large but unreliable — this is the one you cannot control |
| Starting balance | Moderate at long horizons, large at short ones |
| Inflation assumption | Changes the meaning of the number, not the number |
The critical insight: the two inputs that matter most are the ones you fully control. How long you stay invested and how much you add. The return rate is the one you control least and predict worst, yet it is the input people spend most time optimising.
Choosing a defensible return rate
This is where projections go wrong. Some guidance:
| Asset | Long-run historical (nominal) | Defensible planning assumption |
|---|---|---|
| US large-cap equities | ~10% | 6–8% |
| Total US stock market | ~9–10% | 6–8% |
| International developed equities | ~7–8% | 5–7% |
| US aggregate bonds | ~4–5% | 3–4% |
| High-yield savings | Tracks policy rates | Whatever it says today |
| A 60/40 portfolio | ~7–8% | 4.5–6% |
| Cash long-run | ~3% | 1–2% real |
Three adjustments to make:
1. Plan below the historical average. Long-run equity returns include periods that no investor would have held through. Published capital market assumptions from major investment managers have consistently been below historical averages for the coming decade, and they are produced by people with far more data than a blog. Using 10% because it happened historically is optimism, not analysis.
2. Subtract your fees. A 0.60% expense ratio and a 1.00% advisor fee reduce a 7% return to 5.4%. Over thirty years that gap is worth roughly a third of your final balance. Our index fund vs ETF vs mutual fund guide covers why this dominates fund selection.
3. Use real rather than nominal, and say which you are using. A "7% return with 2.5% inflation" projection and a "4.5% real return" projection give the same answer in today's money. Mixing them — quoting a nominal return and then treating the result as today's purchasing power — is the most common error by far.
Run every projection twice: once at a pessimistic 4% and once at an optimistic 8%. If the plan only works at 8%, it is not a plan, it is a hope. If it works at 4%, you have genuine margin for error — and margin for error is what makes long-term investing survivable.
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The inflation adjustment nobody makes
Our calculator outputs both the nominal balance and what it is worth in today's money. The second number is the one to plan with.
At 2.5% inflation over thirty years, prices rise by a factor of about 2.1. A projected $1.2 million in 2056 buys roughly what $570,000 buys today. At 3.5% inflation the factor is 2.8, and the same $1.2 million buys what $430,000 buys now.
This is not a reason to be gloomy — it is a reason to be accurate. Anyone planning retirement on a nominal figure is planning against a number that will not mean what they think it means.
The linearity trap
Compounding is not linear and human intuition treats it as though it is. Look at $300 a month at 6%:
| Years | Balance |
|---|---|
| 5 | $69,800 |
| 10 | $163,900 |
| 15 | $290,700 |
| 20 | $461,600 |
| 25 | $691,900 |
| 30 | $1,002,300 |
The first ten years add $164,000. The last ten add $540,000 — more than three times as much, for the same contributions. More than half the final balance arrives in the last decade.
Two consequences:
- Starting early beats starting big. $300 a month from age 25 produces more at 65 than $600 a month from age 35. The ten-year head start outweighs double the contribution.
- Stopping near the end is catastrophic. Withdrawing or pausing in the final five years removes exactly the period that contributes most. This is the mechanism behind sequence-of-returns risk in retirement, covered in our asset allocation guide.
Three inputs that produce fiction
A steady return. Markets do not deliver 7% a year; they deliver something like +22%, −14%, +28%, −20%, +12%. The average over thirty years may be 7%, but the experience is not, and the experience determines whether you stay invested. Anyone who has sold during a 30% fall has not earned the long-run average — they have earned whatever the market did between their purchase and their panic.
Contributions that never stop. Real careers have gaps: unemployment, parental leave, a business start, caring for a relative, a house purchase. Model at least one multi-year pause and see what it does. Usually the answer is that you need to work two or three years longer, which is worth knowing at 30 rather than at 62.
No tax. In a taxable account, dividends are taxed annually and gains on sale are taxed at withdrawal. A 7% gross return in a taxable account is closer to 5.8% net for a moderate earner. In a Roth IRA the projection is accurate. In a traditional 401(k) the whole balance is eventually taxed as income. Which account the money sits in changes the answer by tens of thousands. See investing beyond retirement accounts for the mechanics.
How to use it properly
- Enter your real starting balance, not a round number you like
- Enter the monthly contribution you will actually sustain for five years — not the aspirational one
- Set the return to 6% for a mostly-equity portfolio. It is conservative and defensible
- Set inflation to 2.5–3%
- Set the horizon to when you will need the money, not to age 65 by default
- Read the inflation-adjusted figure, not the nominal one
- Re-run it at 4% and at 8% and check the plan survives both
- Re-run it with a five-year contribution pause somewhere in the middle
That is eight runs and about fifteen minutes. What you get is a range rather than a number, which is the honest form of any long-term projection.
What the calculator cannot tell you
- Whether you will stay invested through a 35% fall. This determines more of your outcome than any input.
- What your expenses will be in retirement. The target matters more than the projection.
- Whether your asset allocation is right for your risk capacity. See asset allocation by age.
- Tax treatment, which varies by account and by future law.
- Sequence risk, which a smooth compound curve completely hides.
Use it to understand the shape of the problem and the value of time and consistency. Do not use it to predict your future, because nobody can, and the people who claim to are selling something.
What is a realistic compound interest rate to use?
Six to seven percent nominal for a portfolio that is mostly equities, which is roughly four to four and a half percent after inflation. Historical US equity returns are higher — around ten percent nominal — but planning at the historical average leaves no margin for fees, taxes or a below-average sequence.
How much do I need to invest to reach $1 million?
At a 6% real return over 30 years, roughly $1,000 a month. Over 20 years, about $2,200 a month. Over 40 years, about $500 a month. The horizon matters more than the amount, which is the entire argument for starting early.
Does compound interest work on savings accounts?
Yes, but slowly, because the rate is low and often below inflation. A high-yield savings account compounds at whatever APY it offers, which is fine for money you need within a few years and inadequate for a thirty-year goal. Compounding is a function of rate and time; with a low rate you need an implausible amount of time.
Why does the last decade add so much?
Because every earlier contribution has had longer to compound. A dollar invested in year one grows for thirty years; a dollar invested in year twenty-five grows for five. The contributions are identical but their growth periods are not, which is why more than half of a thirty-year balance typically appears in the final ten years.
- Ibbotson Associates / Yale — Stocks, Bonds, Bills and Inflation long-run historical return series.
- Federal Reserve — historical inflation and interest rate data.
- Vanguard and BlackRock — published capital market assumptions.
- Dimson, Marsh & Staunton — long-run global equity return dataset.
Reviewed for accuracy against our editorial guidelines. Figures quoted are illustrative and reflect publicly available rates at the time of the last update; always confirm current terms with the provider.