A percentage platform fee grows with your portfolio; a flat fee doesn’t. That’s the whole game: on small pots, percentage fees are usually cheaper — past a certain size, a flat fee wins by thousands. Enter your numbers below and see the 20-year difference, compounding included. Everything runs in your browser; nothing is uploaded.
Your investing setup
Simplified model: contributions at the start of each year, fees charged annually on the year-end balance, growth applied before fees, no fund OCF or dealing charges (they'd apply equally). Mathematical context only — not investment advice. Capital at risk.
Why percentage fees look tiny and cost fortunes
A “0.45% a year” platform fee doesn’t feel like money — but it’s charged on your whole portfolio, every year, forever. On £100,000 that’s £450 this year; as the portfolio grows, so does the fee — and every pound paid is a pound that stops compounding for you. Over 20 years, the gap between a percentage fee and a flat fee is rarely pocket change: it’s typically a five-figure sum.
Where is the breakeven?
Divide the flat fee by the percentage rate — that’s the portfolio size where the two cost the same. A £144/year flat fee against a 0.45% percentage fee breaks even at £32,000: below that, the percentage platform charges less; above it, the flat fee wins by a growing margin every year. The calculator does this properly — with your contributions and growth compounding year by year, not a one-year snapshot.
What this calculator deliberately doesn’t include
It isolates the platform fee — one clean comparison. Real platforms also differ on dealing charges, FX fees on non-GBP trades, and fund OCFs (which you pay whichever platform you use). Those matter — but mixing them all produces mush. Compare the platform-fee drag here, then check each platform’s dealing and FX schedule in our broker guides: the numbers for specific UK platforms live in our Hargreaves Lansdown deep-dive and the broker articles rolling out on the blog.
About this tool
- All computation happens locally in your browser using JavaScript — no data is uploaded, stored or sent anywhere.
- Default figures are illustrative examples of common UK fee structures — always check a platform’s current schedule before deciding. This calculator provides mathematical context only, not financial advice. Capital at risk.
It depends on portfolio size. Divide the flat fee by the percentage rate to find the breakeven — for example, £144/year vs 0.45% breaks even at £32,000. Below it, percentage platforms usually cost less; above it, flat fees win by an increasing margin.
Yes — because they compound. A fee taken this year also removes all the growth that money would have earned for the next 20 years. That’s why a “small” annual percentage often ends up costing more than a decade of flat fees.
No — deliberately. Fund OCFs follow the fund, not the platform, and dealing/FX charges depend on how often you trade. This tool isolates the platform fee itself, so you compare one thing cleanly.
Yes — the calculator runs entirely in your browser. Nothing you type is uploaded, stored, or sent to any server.