See how your investments could grow with compound interest. Enter your starting amount, monthly contribution, expected annual return and number of years to get a projected future value — and see how much of it comes from your contributions versus growth.
Enter your initial lump sum, the amount you plan to contribute each month, your assumed annual return rate, and how many years you intend to invest. The calculator applies the compound interest formula, compounding monthly, to project your portfolio's value at the end of the period.
Future Value — your projected total at the end of the period.
Total Contributed — the money you actually put in (lump sum + all monthly payments).
Investment Growth — the return generated by the market on top of your contributions.
Growth Multiple — future value ÷ total contributed. A 2× multiple means every pound contributed became two pounds.
£10,000 starting amount + £300 per month at 7% annual return over 20 years → total contributed £82,000 → projected future value approximately £165,000 → growth multiple 2.01×.
The investment calculator models the compound growth of a portfolio over time. Compound interest is the mechanism by which investment returns generate their own returns — often described as one of the most powerful forces in personal finance. The earlier you start, the more compounding cycles your money goes through, and the greater the proportion of your final value that comes from growth rather than contributions.
The growth multiple figure is a particularly useful way to understand this effect. A 2× growth multiple over 20 years at 7% means that for every pound you contributed, you received two pounds back. The time and return rate inputs have an outsized effect on this multiple — extending the period from 20 to 30 years can push a 2× multiple to a 4× multiple at the same return rate.
Be conservative with the annual return assumption. Financial advisers typically use 5–7% for diversified global equity portfolios as a long-run planning assumption, acknowledging that actual returns in any given year will vary significantly — including negative years. Running the same scenario at 5%, 7% and 9% gives you a useful range of possible outcomes rather than false precision from a single estimate.
Remember that the longer the time horizon, the less meaningful the precision of the estimate. A 20-year projection is far less certain than a 5-year one. Use this tool for directional insight — to see whether you are roughly on track — rather than a precise forecast.
For regulated investment advice — including advice on choosing funds, managing pension drawdown, or planning for retirement income — you will need to speak to a Financial Conduct Authority (FCA)-regulated financial adviser. The FCA's Financial Services Register (register.fca.org.uk) lets you check whether an adviser or firm is authorised.
For free impartial guidance on saving and investment basics, MoneyHelper (moneyhelper.org.uk) is the UK government-backed service combining the former Money Advice Service, Pension Wise and The Pensions Advisory Service.
Common questions about this calculator and how to use it.