What this estimator works out
This estimator projects a starting investment plus regular monthly contributions forward at an assumed annual return. It shows the projected value, the total you will have contributed, and the growth on top.
Separating contributions from growth is the useful part. It shows how much of the outcome depends on your saving and how much depends on markets — and over shorter periods, the answer is almost entirely the former.
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Before you rely on the projection
- Deduct charges from the return before entering it. Platform and fund fees are taken every year and compound against you.
- Choose a return appropriate to what you are actually invested in. Cash, bonds and equities behave very differently.
- Decide whether you want the result in today's money or future money. This projection is in future money.
- Run it two or three times across a range of returns rather than trusting one figure.
The projection applies the same return every year. Real investment returns arrive unevenly, and the order matters — a poor first decade with regular contributions can end better than a strong one, because you buy more units when prices are low. A single line expresses an average, not a path.
How this calculator works
The lump sum and the contribution stream are grown separately, then added:
i = annual return ÷ 12 ÷ 100, n = years × 12
Future value = start × (1 + i)n + monthly × [ ((1 + i)n − 1) ÷ i ]
Contributions = start + (monthly × n)
Growth = future value − contributionsThe second term assumes contributions are made at the end of each month, so the final one earns no return. It takes no account of charges, tax or inflation, all of which reduce the real outcome.
Worked example: £250 a month for fifteen years
Using the default figures — a £10,000 starting investment, £250 a month, a 6% annual return over fifteen years:
- Projected value: £97,245.61
- Total contributed: £10,000 + (£250 × 180) = £55,000
- Projected growth: £42,245.61
Growth is 43% of the final value — a large share, and it took fifteen years to get there. Reduce the return to 4% and the projection falls to about £77,000; raise it to 8% and it rises to about £124,000. That spread of nearly £50,000 comes entirely from an assumption nobody can verify in advance, which is why running a range matters more than choosing a number.
Common mistakes
- Using a headline return without deducting charges. A 0.75% annual fee turns 6% into 5.25%.
- Treating the projection as a forecast. It is one scenario among many.
- Ignoring inflation. £97,000 in fifteen years will buy considerably less than £97,000 today.
- Investing money needed within five years. Short horizons leave no time to recover from a fall.
- Investing outside a tax wrapper unnecessarily. An ISA shelters growth from tax within the annual allowance.
Frequently asked questions
What return should I assume?
There is no correct answer, and that is why the field is editable. Long-run global equity returns before inflation have historically been somewhere in the region of 5% to 8% a year, with enormous variation between decades. Regulated projections typically use low, medium and high assumptions rather than one figure. Run 4%, 6% and 8% and treat the spread as the answer.
Should I invest or keep it in cash?
It depends on the time horizon and what the money is for. Money needed within about five years generally belongs in cash, because there is no time to recover from a fall. Money for a decade or more has historically done better invested, though there is no guarantee, and an emergency fund should stay in cash regardless of horizon.
How much difference do charges make?
More than most people expect, because they compound. On the example above, an extra 1% of annual charges reduces the projected value by roughly £12,000 over fifteen years. Comparing the total ongoing cost — platform fee plus fund charges — is one of the few things within your control that reliably improves outcomes.
Should I use an ISA or a pension?
A pension gives tax relief on the way in and is inaccessible until at least 55, rising to 57. An ISA gives no relief but is tax-free on the way out and accessible any time. For higher rate taxpayers the pension advantage is substantial. Many people use both, and capturing an employer pension match comes before either.
Is what I enter stored?
No. Amounts and assumptions are processed in your browser and never transmitted or retained.
Related tools
References
- Financial Conduct Authority — rules on investment projections, charges disclosure and the regulated firm register
- MoneyHelper — impartial guidance on investing, risk and choosing a tax wrapper
- GOV.UK — ISA allowances and the tax treatment of investments
Sources are checked at publication and can change — how I choose and check references.