Investment Growth & Inflation: 10 Questions Answered
Real numbers, honest answers, and no jargon.
Q1What rate of return should I use in an investment calculator?
For long-term stock market investments (10+ years), 7% nominal or 4–5% real (after inflation) is a common conservative estimate based on global stock market history. For bonds, use 3–4% nominal. For cash savings, use your current account rate. Avoid using recent high returns (like the 2019–2021 period) as they are not representative of long-run averages.
Q2What has the S&P 500 returned on average?
The S&P 500 has returned approximately 10% per year nominally and 7–8% after inflation since 1928. However, individual decades vary enormously — the 2000s ("lost decade") produced near-zero real returns, while the 2010s produced exceptional returns. Always model long-term averages, not recent performance.
Q3How does inflation affect my ISA or 401(k)?
Inflation reduces the purchasing power of your savings regardless of the account wrapper. A stocks and shares ISA or 401(k) invested in equities typically beats inflation over the long run. A cash ISA typically does not. The account type affects tax treatment but not the inflation impact on the underlying assets.
Q4What is pound-cost averaging?
Pound-cost averaging (or dollar-cost averaging in the US) means investing a fixed amount at regular intervals regardless of market price. When prices are high, you buy fewer units. When prices are low, you buy more. This smooths out the impact of volatility over time and removes the pressure of trying to time the market.
Q5What is a real return and why does it matter?
The real return is your investment growth minus inflation. A 7% nominal return with 3% inflation is a 4% real return — that's what your money can actually buy in the future. Real returns matter because your future expenses (retirement, education, etc.) will cost more in nominal terms due to inflation. Planning with nominal returns alone overstates your future purchasing power.
Q6How much do I need to save monthly to retire comfortably?
A rough rule: aim for 25x your expected annual retirement expenses (the "4% rule"). To accumulate £500,000 in 30 years at 7% nominal return, you'd need to invest approximately £430/month from scratch, or less with a lump sum head start. Our calculator helps you model your specific target.
Q7Is it better to invest a lump sum or spread it out?
If you already have the money, lump sum investing beats monthly averaging approximately two-thirds of the time based on historical data, because markets trend upward over time. However, the psychological benefit of averaging in during a falling market is real. If you receive money monthly (salary), monthly investing is both practical and optimal.
Q8What is the Rule of 72?
Divide 72 by your annual return rate to find how many years it takes to double your money. At 7% return, your money doubles every ~10 years. At 4%, it takes 18 years. This works for both nominal and real returns. It's also useful in reverse: divide 72 by years to find the required return rate to double your money in that time.
Q9How does compound interest differ from simple interest?
Simple interest earns a fixed amount each period on the original principal only. Compound interest earns returns on both the principal AND the accumulated interest — creating exponential rather than linear growth. All investment accounts use compound growth. The longer the period, the more dramatic the difference: £10,000 at 7% simple for 30 years = £31,000. Compound = £76,000.
Q10What inflation rate should I use in the UK vs US?
The Bank of England targets 2% inflation; the US Fed also targets 2%. Long-run actual averages are slightly higher: UK ~2.5%, US ~2.5%, Australia ~2.8%, India ~5–6%. Use historical averages for planning rather than current rates, which can be temporarily high or low. Our calculator pre-fills sensible defaults per currency.