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Useful next calculations
Rates & sources
Compound growth assumes reinvested returns and no platform fees. Past performance is not a guide to future returns.
Source: FCA — Investment basics — check the linked guidance and any live quote before acting.
When to use this calculator
- Before choosing between saving, investing or changing a contribution.
- When you want to compare cautious, base and optimistic return assumptions.
- When you need a projection before making a longer-term decision.
- When you want to see whether starting earlier or contributing more changes the outcome more.
Example: projecting a steady index-fund plan
Use these sample inputs as a quick scenario test, then change one variable at a time to compare outcomes.
| Input | Value |
|---|---|
| Starting balance | £12,000 |
| Monthly contribution | £350 |
| Expected annual return | 6.5% |
| Investment horizon | 15 years |
This example helps because it turns a broad investing idea into a contribution path you can compare with more active alternatives using the same timeline.
How to read your results
Portfolio Value
The figure produced by this output for the inputs you entered. Change one variable at a time if you want to see what drives it.
Total Invested
The headline outcome of this calculation. It is most useful when read alongside the supporting metrics rather than in isolation.
Growth
The figure produced by this output for the inputs you entered. Change one variable at a time if you want to see what drives it.
Method & assumptionsAuthoritative sources
This calculator models the growth of index fund investments by applying a net annual return to a starting lump sum and regular contributions. The return rate you enter should reflect expected market growth minus the fund's Ongoing Charges Figure (OCF) and your platform fee — this gives a realistic net return rather than a gross headline figure.
The calculator assumes reinvestment of all dividends and a constant annual growth rate. In practice, index fund returns fluctuate significantly year to year. Many UK financial planners use 5–7% annually as a long-term real return assumption for global equity index funds, though past performance is not a guarantee of future results. The model does not account for tax on dividends or capital gains above annual allowances if held outside an ISA or SIPP wrapper. Holding index funds within an ISA or SIPP eliminates most ongoing UK tax considerations.
Common mistakes
- !Assuming a constant return without checking a more conservative growth rate.
- !Forgetting ongoing contributions, fees or tax wrappers where relevant.
- !Focusing only on the final balance instead of the path required to reach it.
- !Ignoring the drag of charges over a long period.
What to do next
- Test a cautious, expected and optimistic growth rate.
- Compare this result with related savings or retirement tools before committing more money.
- Consider charges and any tax wrapper that applies.
- If the projected balance falls short, increase the contribution until the result meets your goal.
Frequently asked
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