CalculatorsSwitch calculator
How this works
Each month the calculator grows your pot by the yearly return, turned into a monthly rate, takes off a small slice for fees, and then adds your monthly amount. It repeats that for every month you invest.
It runs the same plan a second time with no fees, so you can see what fees cost you in pounds. Then it divides the final value by inflation over the years to show what it would buy in today’s money.
What it assumes
- Returns are smooth. Real investments rise and fall, and you can get back less than you put in.
- Fees are a yearly percentage of the pot, taken a little each month.
- Tax isn’t included. An ISA or pension keeps growth free of tax.
Worked example
£5,000 plus £200 a month for 20 years
- You expect 6% a year, pay 0.5% a year in fees and allow for 3.1% inflation.
- After 20 years the pot is worth £99,994 from £53,000 paid in.
- With no fees it would be £106,723, so fees cost you £6,729 over the period, even though 0.5% sounds small.
- In today’s money the £99,994 would buy what £54,300 buys now.
Worked out with the same maths as the calculator above.
What to do next
Common questions
What return should I expect?
Nobody knows. Try a few rates, for example 4%, 6% and 8%, and plan around the lower ones. Shares have tended to beat cash over long periods, but with big falls along the way.
Why do small fees matter so much?
Fees are taken from the whole pot every year, including past growth. Over 20 years a 0.5% fee in the example above costs more than £6,700. Comparing platform and fund charges is one of the easiest wins.
Should I invest or save in cash?
Keep money you may need within five years, and your emergency fund, in cash. Money you can leave longer has a better chance of growing faster than prices if invested.
Do I pay tax on investment growth?
Not inside an ISA or pension. Outside them, gains and dividends above your yearly allowances can be taxed. The ISA allowance is £20,000 for 2026/27.