Compound growth & FIRE Calculator
Project cash savings, index funds and ETFs, and anything else you hold as three separate pots, each compounding at its own return, then read them as one balance with contributions and growth drawn as separate layers. Anchor a return on what VOO, SPY, IVV or QQQ actually delivered, take platform and fund charges off the top, switch to real terms to strip out inflation, and set a withdrawal rate to find your financial independence number.
- Price:
- Free, no sign-up
- Data:
- Processed locally
- Reviewed:
Interactive compound growth
Where your money sits
Input DataCash interest, funds and ETFs, and anything else — each compounds at its own rate, all on one timeline
Savings interest
Easy-access, notice and fixed-rate accounts
Funds & ETFs
Index trackers and managed funds — VOO, SPY, IVV, QQQ below
1928 – 2025
Other investments
Individual shares, bonds, crypto, property equity
Spending
Parameters & SettingsTake money back out part way through — a career break, school fees, or simply stopping work
Shared assumptions
Parameters & SettingsApplied to every pot above
Contributions and growth
Results & OutputEvery pot combined, split into what you paid in and what the market added — in future money
Financial independence
Parameters & SettingsAnnual spending divided by your withdrawal rate
Reference returns
What these funds and indices actually delivered — pick one in any pot above, or type your own figure
| Fund / index | Annualised | Period | Ongoing charge | What it hides | Source |
|---|---|---|---|---|---|
| VOOVanguard S&P 500 ETF | 14.80% | Since 7 Sep 2010 | 0.03% | Launched after the 2008 crash, so its whole life is one bull market. The most quoted number on this list and the least representative. | stockanalysis.com |
| SPYSPDR S&P 500 ETF Trust | 10.80% | Since 22 Jan 1993 | 0.09% | The oldest US ETF. Thirty-three years covering 2000 and 2008 lands almost exactly on the century-long average. | stockanalysis.com |
| IVViShares Core S&P 500 ETF | 8.49% | Since 15 May 2000 | 0.03% | Same index as VOO and SPY, launched at the dot-com peak. Six points a year of difference, bought entirely by the start date. | stockanalysis.com |
| QQQInvesco QQQ (Nasdaq-100) | 10.75% | Since 10 Mar 1999 | 0.18% | Concentrated in large-cap tech. It fell roughly 80% from its 2000 peak and took 15 years to recover — the average hides that. | stockanalysis.com |
| S&P 500 long run — nominal | 10.00% | 1928 – 2025 | — | Compound annual return with dividends reinvested across 97 years, including 1929, 1973, 2000 and 2008. The defensible default for a long projection. | NYU Stern — Damodaran dataset |
| S&P 500 long run — after inflation | 6.90%real | 1928 – 2025 | — | The same 97 years in purchasing power. Already net of inflation, so set the inflation assumption to 0 if you use it. | NYU Stern — Damodaran dataset |
Figures read on and quoted in USD with dividends reinvested; follow the links for the live numbers. VOO, SPY and IVV all track the same index — the spread between them is start date alone, which is the strongest argument on this page against projecting any one of these rates forward for thirty years.
Year by year
Results & Output| Year | Contributed | Growth | Balance | Change on year |
|---|---|---|---|---|
| Year 0 | £15,000 | £0 | £15,000 | +£15,000 |
| Year 1 | £22,200 | £1,556 | £23,756 | +£8,756 |
| Year 2 | £29,400 | £3,937 | £33,337 | +£9,581 |
| Year 3 | £36,600 | £7,227 | £43,827 | +£10,490 |
| Year 4 | £43,800 | £11,517 | £55,317 | +£11,490 |
| Year 5 | £51,000 | £16,907 | £67,907 | +£12,590 |
| Year 6 | £58,200 | £23,508 | £81,708 | +£13,801 |
| Year 7 | £65,400 | £31,442 | £96,842 | +£15,134 |
| Year 8 | £72,600 | £40,846 | £113,446 | +£16,604 |
| Year 9 | £79,800 | £51,868 | £131,668 | +£18,222 |
| Year 10 | £87,000 | £64,672 | £151,672 | +£20,004 |
| Year 11 | £94,200 | £79,440 | £173,640 | +£21,968 |
| Year 12 | £101,400 | £96,373 | £197,773 | +£24,133 |
| Year 13 | £108,600 | £115,691 | £224,291 | +£26,518 |
| Year 14 | £115,800 | £137,639 | £253,439 | +£29,148 |
| Year 15 | £123,000 | £162,485 | £285,485 | +£32,046 |
| Year 16 | £130,200 | £190,527 | £320,727 | +£35,242 |
| Year 17 | £137,400 | £222,094 | £359,494 | +£38,767 |
| Year 18 | £144,600 | £257,548 | £402,148 | +£42,654 |
| Year 19 | £151,800 | £297,288 | £449,088 | +£46,940 |
| Year 20 | £159,000 | £341,756 | £500,756 | +£51,668 |
| Year 21 | £166,200 | £391,439 | £557,639 | +£56,883 |
| Year 22 | £173,400 | £446,876 | £620,276 | +£62,637 |
| Year 23 | £180,600 | £508,661 | £689,261 | +£68,985 |
| Year 24 | £187,800 | £577,450 | £765,250 | +£75,989 |
Estimate only — not financial advice. Investment projections assume a steady, annualized rate of return and constant inflation. Real asset markets fluctuate with volatility, dividend taxation, and sequence-of-returns risk during drawdown phases. Projections do not guarantee future investment performance or constitute financial advice. Full disclaimer · Site-wide terms
How much will £500 a month grow to?
£500 a month for 30 years grows to roughly £610,000 at a 7% annual return, or about £1.13m at the S&P 500’s long-run 10%, of which £180,000 is money you paid in either way. Growth overtakes total contributions around year 15 to 17 at 7% and closer to year 12 at 10% — which is why starting earlier generally beats saving more later, and why the return you assume deserves a sourced figure rather than a guess.
How to use the compound growth
- Fill in each pot: Enter the balance and monthly contribution for cash savings, for funds and ETFs, and for anything else you hold.
- Set a return per pot: Drag the slider, type an exact figure, or apply a published reference such as VOO, SPY, IVV or QQQ. Deduct platform and fund charges once in the shared assumptions.
- Adjust for inflation: Toggle real terms to see the balance in today money instead of future pounds.
- Find the crossover: The chart marks the year growth across all pots first exceeds the money you put in.
How the compound growth works
How compounding is computed
With a lump sum only, the future value is FV = P(1 + r/n)^(nt). Regular contributions add an annuity term, but rather than use a closed form the tool simulates month by month — which keeps contribution timing, changing compounding frequency and inflation adjustment all exact and consistent.
Nominal, real and the inflation adjustment
Real balances divide the nominal balance by (1 + i)^t. Note that this is not the same as subtracting inflation from the return: a 7% return with 2.5% inflation is a real return of 1.07/1.025 − 1 ≈ 4.39%, not 4.5%. The difference looks trivial and compounds to a great deal over thirty years.
The two curves that matter
The chart separates what you paid in from what the market added. Early on the contribution line dominates and the growth line looks almost flat — this is the discouraging phase that causes people to stop. Growth is exponential while contributions are linear, so the lines must eventually cross, and after they do the gap widens quickly.
Three pots, one timeline
Cash savings, funds and ETFs, and anything else are projected separately and summed month by month, because averaging their rates by hand gives the wrong answer: the pot with the higher return grows to dominate the balance, so its rate should carry more weight every year. The blended figure shown beside the totals is the weighted average that actually applied by the end, not the one you started with.
Fees come off the return, not the contribution
The platform and fund charge is subtracted from each pot's rate before compounding, which is how charges on assets under management genuinely work. That is why the effect grows over time: the fee is levied on a balance that the fee itself keeps shrinking.
What this model leaves out
Sequence-of-returns risk above all: a constant return is a smooth curve, while real markets deliver the same average through crashes and rallies, and the order matters enormously once you start withdrawing. Taxes on gains and dividends, currency movement on a US-listed ETF held in sterling, and rebalancing between the pots are also outside the model.
Compound Growth — frequently asked questions
The S&P 500 compounded at about 10% a year nominal, or 6.9% after inflation, across 1928 to 2025 — that is the defensible default for a long projection. A projection is a planning tool, not a forecast: run an optimistic and a pessimistic case and see whether your plan survives the pessimistic one.
No. VOO launched in September 2010, so its entire history is one bull market, and the reference table shows what that does: VOO, SPY and IVV all track the same S&P 500 index, yet their since-inception returns run 14.8%, 10.8% and 8.49% purely because of the year each one launched. IVV started at the dot-com peak. The 97-year figure of roughly 10% nominal already includes 1929, 1973, 2000 and 2008, which is why it is the safer planning number.
Only by the return you give each one. Cash savings interest, funds and ETFs, and other holdings each compound at their own rate on a shared timeline, and the tool sums them — so you can model 4% on an easy-access account alongside 10% on a tracker without averaging them by hand. The per-pot cards show what each grows to and its share of the final total.
Because it is charged on the whole balance every year, not on your contributions. Fees are deducted from every pot’s return before compounding, so the drag grows with the portfolio: over thirty years, half a percent typically costs a low-double-digit percentage of the final balance. A mainstream tracker’s ongoing charge is 0.03% to 0.2%; the platform holding it usually adds more than the fund does.
It deflates every future balance by your inflation assumption, so the figure is expressed in the purchasing power of today. A 1m balance in 30 years at 2.5% inflation is about 477,000 in today money — the same number, a very different retirement.
The month when cumulative investment growth first exceeds cumulative contributions. It is the moment the portfolio starts doing more work than you do, and for typical assumptions it arrives somewhere between year 12 and year 18 — which is precisely why starting early dominates saving more.
The FIRE target shown is your annual spending divided by your withdrawal rate — 4% implies 25x annual spending. It originates in the Trinity study of historical US portfolios and assumes a 30-year horizon and a stock-heavy allocation; longer retirements or lower expected returns argue for 3-3.5%.
Official resources & government references
Verified references, primary standards specifications, and official publications governing the rules and calculations implemented in this tool:
Official Compound Interest Calculator & Investor Guide
US Securities and Exchange Commission guide to compounding mechanics, investment fees, and asset allocation.
Investing Basics & Long-Term Savings Guidance
Independent guidance on investment risk, index tracker funds, and tax-sheltered ISAs and pensions.
VOO — Vanguard S&P 500 ETF Profile & Performance
Official fund page for VOO: current performance history, holdings and the 0.03% expense ratio used as a reference return here.
Historical Returns on Stocks, Bonds and Bills (1928 onward)
The academic dataset behind the long-run S&P 500 figures, nominal and inflation-adjusted, refreshed each January.
Historical UK Inflation & Purchasing Power Calculator
Official CPI and RPI historical inflation records for assessing real versus nominal portfolio returns.
Important Disclaimer
Investment projections assume a steady, annualized rate of return and constant inflation. Real asset markets fluctuate with volatility, dividend taxation, and sequence-of-returns risk during drawdown phases. Projections do not guarantee future investment performance or constitute financial advice.