Compound interest, regular savings, pension, ISA, inflation and fee calculators: what each one tells you and the inputs to use.
Looking for more detail? Our full guide covers this topic in depth, with step-by-step advice and sources.
Investment calculators answer different questions. Using the right one, with sensible inputs, matters more than the tool itself.
Compound interest calculator
Use for a lump sum. Inputs: starting amount, annual rate, years and how often interest is added. Good for comparing savings accounts using their AER.
Regular savings calculator
Use when you pay in monthly. Check whether it assumes payments at the start or end of each month; the difference is small but real.
Pension calculator
Inputs: current pot, monthly contributions (including employer and tax relief), expected growth after fees and retirement age. Use your State Pension forecast from GOV.UK alongside it.
Inflation calculator
Shows what a sum of money is worth in today's prices. Essential for long-term plans, because £1,000 in 20 years will buy much less than £1,000 today.
Fee calculator
Shows how platform and fund charges reduce growth. Try the same investment at 0.2% and 1% a year over 20 years to see the gap.
Good inputs
- Use modest growth rates and model a range rather than one figure.
- Subtract fees from the growth rate.
- Think in today's money by using a rate after inflation.
Treat every result as an illustration. Real returns vary year to year.
Sources and further reading
- GOV.UK: check your State Pension forecast
- Bank of England: inflation calculator
- FCA: consumer information
Facts checked on 20 October 2026. Spotted an error? Tell us and we will correct it.