Investment Growth Calculator: How to Project Your Portfolio
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Open the Investment Growth Calculator →An investment growth calculator is one of the most powerful financial planning tools available — but only if you feed it realistic inputs. This guide explains each input and how to choose numbers grounded in reality rather than optimism.
The Core Formula
Where P = starting balance, r = periodic rate, n = number of periods, PMT = periodic contribution.
Starting Balance (P)
Use your current investable assets — brokerage accounts, ISAs, TFSAs, RAs. Don't include home equity or pension entitlements you can't invest; these are separate calculations. If starting from zero, P = 0 and contributions alone drive the formula.
Choosing a Realistic Return Rate
| Asset Class | Historical nominal return | Conservative planning rate |
|---|---|---|
| Global equity index funds | ~9–10%/year | 7% |
| SA equity (JSE All Share) | ~12–14%/year nominal | 9% (inflation ~5%) |
| UK equity (FTSE All Share) | ~7–8%/year nominal | 6% |
| Bonds (diversified) | ~3–5%/year | 3% |
| 60/40 portfolio (equity/bond) | ~7–8%/year | 6% |
Use 7% for a mixed portfolio. Use 5% for a more conservative projection. Never plan on 12%+ unless you have a specific reason — that's the optimism trap that leaves people short.
Monthly Contribution (PMT)
Use your current contribution amount, not your aspirational one. Run two scenarios: your current contribution and what happens if you increase by $100–$200/month. The difference is often stunning.
Time Horizon (n)
Years until you need the money. For retirement: target retirement age minus current age. Be conservative — if you think you might retire at 62, model to 65. The extra cushion doesn't hurt.
Inflation Adjustment
A nominal 7% return with 3% inflation gives a real return of ~4%. For long-term planning (20+ years), always also run the real-return scenario to understand purchasing power:
| Nominal rate | Inflation | Real return | $500k today worth in 20yr real terms |
|---|---|---|---|
| 7% | 3% | ~4% | $1.1M in real purchasing power |
| 9% | 5% (SA) | ~4% | Similar in real terms |
Common Mistakes in Investment Projections
- Using gross returns, not net-of-fees: A fund charging 1.5% on a 7% return gives you 5.5%. Over 30 years this costs hundreds of thousands.
- Ignoring tax on withdrawals: Not all accounts are tax-free at withdrawal. Model the tax bill too.
- Assuming contributions never increase: Most people earn more over time. Even modelling 2–3% annual contribution increases makes projections significantly higher.
- Not accounting for investment gaps: Career breaks, parental leave, illness. A 2-year gap early in the investment horizon costs significantly at the end.
Project Your Portfolio Now
Enter your starting balance, monthly contribution, and expected return. See your balance at any future date.
Open Investment Growth Calculator →
Sensitivity Analysis: Run Three Scenarios
Always model three scenarios: bear (5% return), base (7%), and bull (9%). The range between bear and bull shows you the uncertainty in financial planning. If even your bear case gives you enough to retire on, your plan is robust.
Worked Example: $80,000 Starting Balance, 25-Year Horizon
Set up: starting balance $80,000, monthly contribution $600 (rising 3% per year with raises), 25-year horizon, expected nominal return 7.5%, expected inflation 2.5%. The investment growth calculator returns a final nominal balance of approximately $1.05 million. Real purchasing power in 2026 dollars: about $570,000.
Now run the sensitivity. Drop the return to 5.5% (bear): final balance falls to $720,000 nominal, $390,000 real. Bump to 9.5% (bull): final balance climbs to $1.55 million nominal, $840,000 real. The bear-to-bull range is more than 2x. That's not a flaw of the calculator — it's the inherent uncertainty of compound returns over multi-decade horizons. Plan around the bear case; treat the bull case as a tailwind.
Layer fees. A 1.0% expense ratio on the same portfolio drops the effective return by 1 percentage point. Bear case at 4.5% net: final balance $600,000 nominal, $325,000 real. That's $120,000 of lifetime real-dollar value transferred from your account to fund managers. Switching to a 0.05% expense ratio index fund recovers about $115,000 of that gap. Fees are the single most controllable variable in long-term investing. The calculator makes the difference visible in a way bank statements never do.
What People Get Wrong With Projections
- Assuming smooth annual returns. The S&P 500 has averaged 10% nominal but has produced double-digit losses 11 times in the last 50 years. Projections show the mean; real life has variance.
- Compounding aspirational contributions. Run with the contribution you actually make, not the one you intend. The calculator is honest only if the inputs are.
- Treating future tax brackets as today's brackets. Pre-tax 401(k) projections need a haircut for retirement-bracket income tax. Roth balances don't.
- Forgetting reinvested dividends. S&P 500 price return is about 7% nominal; total return (with dividends) is closer to 10%. Always use total-return assumptions.
- Ignoring withdrawal sequence risk. A calculator showing a $1.5M endpoint doesn't tell you whether the path got there smoothly or via a 40% drawdown in year 27. Monte Carlo tools fill that gap.
Regional Differences: USA, UK, South Africa
USA. Historical S&P 500 total return averaged about 10% nominal / 7% real from 1928–2024 per Damodaran/NYU data. Long-term bond index returned about 5% nominal. A typical 60/40 portfolio modelled at 7% nominal / 4.5% real is reasonable. Vanguard's 10-year forward return forecasts (early 2026) sit around 5–7% for global equity, 4–5% for bonds — more conservative than historical.
UK. FTSE 100 total return has averaged closer to 7% nominal long-run. Global tracker (Vanguard FTSE Global All Cap, HSBC FTSE All-World) gives broader exposure. Bank of England long-run inflation target is 2%, but CPI has averaged closer to 3% over the last decade. Plan with 6.5% nominal / 4% real for a diversified equity allocation.
South Africa. JSE All Share long-term return is around 12% nominal in rand terms, but rand depreciation of about 6%/year against the dollar erodes real-world purchasing power. Most SA planners use 9% nominal for local equity, 7% nominal for offshore equity in rand terms. Inflation target 3–6%, real returns of 4–5% are typical planning assumptions. Two-Pot system since September 2024 changes liquidity assumptions for RA projections.
Actionable Next Steps
- Open the Investment Growth Calculator and enter your real current balance and contribution.
- Run three scenarios: bear (5% nominal), base (7%), bull (9%). Plan around the bear.
- Compute your real return (nominal minus inflation) and use that to assess future purchasing power.
- Check the expense ratio of your funds. Anything above 0.30% for broad-market index exposure needs justification.
- Re-run the projection every year on the same date. Adjust contributions to close any gap.
FAQ
Should I use historical returns or forward-looking estimates? Forward-looking. Vanguard, Blackrock, and JP Morgan publish 10-year capital-market expectations each year. The post-2020 versions suggest equity returns of 5–7% nominal, well below the long-run historical average. Using historical 10% returns risks over-projecting.
How do I account for dollar-cost averaging? Monthly contributions buy more shares when prices are low and fewer when high, which slightly improves average cost basis over a lump-sum at the peak. Calculators with monthly contribution inputs implicitly model this — there is no separate adjustment needed.
What return should I assume for bonds and cash? Cash typically returns 0.5–2% real, bonds 1–3% real depending on duration and quality. Most retirement calculators default to a blended 60/40 equity/bond return of 5–6% real, which is a defensible long-run planning figure.
How often should I re-project? Annually for routine planning, quarterly if you are within five years of a goal date. More frequent recalculation often leads to overreacting to short-term market noise rather than improving the plan.
Sources and Methodology
Long-run US equity returns reference Aswath Damodaran's NYU Stern data series (1928–2024). UK FTSE returns are from FTSE Russell historical data and Barclays Equity Gilt Study. South African JSE returns are from the Old Mutual Investment Group long-term studies. Forward-looking capital-market assumptions are taken from Vanguard's annual Economic and Market Outlook (early 2026 edition). Fee-impact calculations follow standard expense-ratio drag mathematics confirmed in academic papers including Bogle's 2014 cost-vs-return analysis.