Total
$19,671.51
Interest Earned
$9,671.51
Rates & sources
Compound growth assumes reinvested returns and no platform fees. Past performance is not a guide to future returns.
Source: FCA — Investment basics — figures refreshed at the start of each tax year.
When to use this calculator
- Before choosing between saving, investing, or increasing your monthly contribution.
- When you want to compare best-case, base-case, and cautious return assumptions.
- When you need a quick projection before making a longer-term portfolio decision.
- When you are deciding how many more years of contributions are needed to reach a specific target balance.
- When you want to see whether starting earlier versus contributing more each month produces a bigger outcome.
A realistic US planning example
Use these sample inputs as a quick scenario test, then change one variable at a time to compare outcomes.
Initial Amount ($)
$15,000
Annual Interest Rate (%)
5%
Years
10 years
Compounds Per Year
Annually (1x/year)
After entering these figures, review total and interest earned together rather than in isolation — each metric tells a different part of the story. Then rerun the tool with one input adjusted to see which variable has the biggest effect on all three outputs before you settle on a plan.
How to read your results
Total
This is the headline outcome of the calculation, but it is most useful when read alongside the supporting metrics below it rather than in isolation. Try changing one input at a time and watching how this total moves to understand which driver has the biggest impact.
Interest Earned
Use this to separate genuine growth from your original capital so you can compare savings or investment scenarios on an equal footing. It is also the figure you may need to declare on a self-assessment return if it exceeds the personal savings allowance.
Method & assumptionsAuthoritative sources
This calculator uses the standard compound interest formula: A = P(1 + r/n)^(nt), where P is your principal, r is the annual interest rate expressed as a decimal, n is the number of compounding periods per year, and t is the number of years. It assumes a constant interest rate throughout, which is rarely guaranteed in practice — savings rates change, and investment returns fluctuate. The results are nominal figures and do not account for inflation, tax, or product charges. UK savers should note that the AER figure on savings products already standardises for compounding frequency, making it the fairest basis for comparison. This tool is for illustrative purposes only and does not constitute financial advice.
Common mistakes
- !Assuming a constant return without checking a more conservative growth rate.
- !Forgetting to include 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 platform fees or fund charges, which can reduce the real compounded return significantly over ten or more years.
- !Comparing ISA and general investment account projections without adjusting for the tax treatment of interest, dividends, or capital gains.
What to do next
- Test a cautious, expected, and optimistic growth rate instead of relying on a single projection.
- Compare this result with related savings or retirement tools before committing more money.
- Use the linked guides to understand which assumptions matter most over longer periods.
- Consider running the same figures in an ISA and a general account scenario to see how the tax treatment changes the outcome over ten or more years.
- If the projected balance falls short of your target, use the tool to work backwards — increase the monthly contribution until the result meets your goal.
Frequently asked
Use arrow keys to navigate items, Enter or Space to expand/collapse.
End-of-article next steps
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