Nobody knows what markets will return over the next 30 years — but your fees are certain, and they compound just like returns do. This calculator shows what a fee difference costs in pounds across a whole range of market outcomes, and finds the pot size where a flat-fee platform starts beating a percentage one. Nothing you enter leaves your browser.
Compounding. Fees are subtracted from the growth rate: a portfolio growing at 5% paying 1.32% in charges is modelled as compounding at 3.68% a year, converted to a monthly rate and applied month by month, with your contribution added at the end of each month. This is the standard approximation — in reality OCFs are deducted daily inside the fund price and platform fees are billed monthly or quarterly, but over decades the difference is negligible.
The growth band. The comparison table reruns the identical calculation at growth rates from 4 points below to 4 points above your central figure. The pound cost of fees moves a lot with growth (bigger pots pay more in fees); the share of your final pot that the cheaper portfolio adds barely moves — that stability is the point. Fees are the one input you control with certainty.
Smooth returns. Growth arrives smoothly in the model, which real markets never do. Volatility changes the path and the final pot, but it changes both fee scenarios nearly identically, so it largely cancels out of the comparison.
Platform crossover. The percentage platform charges its rate on the whole pot, up to the annual cap if you set one; the flat-fee platform charges the fixed amount regardless. Both are deducted monthly (one-twelfth of the annual figure). Fund charges are assumed identical on either platform and are left out of this mode; the crossover pot size is simply the flat fee divided by the percentage rate. If the cap is at or below the flat fee, the percentage platform can never cost more and there is no crossover. The "year you cross" projection grows your pot at your central growth figure with the percentage platform's fees deducted.
What is left out. All figures are nominal — inflation erodes every scenario equally and is deliberately ignored. No tax: the comparison is wrapper-agnostic and works the same inside an ISA, SIPP or taxable account. No dealing charges, FX fees or fund spreads in the fee-drag mode (fold known dealing costs into the flat-fee figure in the crossover mode). Transfer costs and exit fees are not modelled — most mainstream platforms no longer charge them, but check before switching.
This tool provides general information and education, not financial advice or a personal recommendation. Investment returns are not guaranteed and you may get back less than you put in; fee tariffs change and platforms differ in service, not just price. Check each provider's own charges page before acting. See the investment fees guide for the full picture.