Platform Fee Basics: Percentage Fees vs Flat Fees
A plain-English guide to when percentage platform fees beat flat fees, where the crossover point sits, and how UK investors should compare charges.
Platform fees look simple until you price them in pounds at your own portfolio size. The fee structure that feels cheap on a small balance can become the expensive option once the portfolio grows.
The Short Version
- Percentage platform fees usually feel lighter on smaller balances because the pounds charged stay small while the portfolio is small.
- Flat platform fees often become more competitive as the portfolio grows because the annual charge stays fixed instead of rising with the balance.
- The real comparison is never the headline alone. You need the platform fee, the fund charge and any dealing costs in the same view.
- For UK investors, the useful habit is to calculate costs at today’s balance and at a plausible future balance before choosing a platform.
How percentage platform fees work
A percentage platform fee is charged as a slice of the assets you hold. If the platform takes 0.25% a year and your portfolio is worth GBP 5,000, the platform charge is about GBP 12.50. If the same portfolio later reaches GBP 50,000, the pounds charged rise to about GBP 125 because the percentage is being applied to a bigger balance.
That shape often suits smaller portfolios because the fee grows gradually with the account. It can become less attractive later because the platform keeps taking a larger cash amount even if the service itself has not changed much.
This is also why percentage fees can feel invisible at first. A quarter of a per cent sounds small in abstract terms, but the pounds matter more once the balance becomes meaningful.
How flat platform fees work
A flat platform fee is a fixed annual amount such as GBP 48, GBP 60 or GBP 120. The charge does not rise just because the portfolio grows. That makes the fee feel heavy on a small balance and lighter on a larger one.
If a platform charges GBP 60 a year, that is a 1.2% drag on a GBP 5,000 portfolio but only 0.12% on a GBP 50,000 portfolio. The fee has not changed, but the impact has.
This is why flat-fee platforms are often worth closer attention once the portfolio has moved beyond the starter stage. The investor is not buying better returns, but they may be keeping more of them.
Where the crossover point usually sits
The crossover point is where the pounds charged by the percentage model roughly match the pounds charged by the flat-fee model. Using 0.25% versus GBP 60 a year, the breakeven point sits around GBP 24,000. Below that, the percentage fee is cheaper. Above that, the flat fee starts to look better.
That does not make GBP 24,000 a universal rule. Real platforms may have caps, tiered charging bands, separate dealing fees or wrapper-specific pricing. The point is not the exact number. The point is that every fee structure has a balance where the economics flip.
A quick spreadsheet or calculator is often enough. Plug in the annual platform fee in pounds at two or three portfolio sizes and the pattern becomes obvious.
That exercise also helps investors avoid a common mistake: choosing a platform as if today’s balance will be permanent. Fee structure is a moving decision because the account itself is supposed to change. A platform that suits a new ISA or SIPP can become the wrong shape later if contributions and growth push the balance far beyond the original assumptions.
Why headline fees can mislead
Fee pages are usually designed to look simple, which means they often lead with one neat number. That number can be technically correct while still being unhelpful on its own. A low percentage can become expensive on a large portfolio, and an apparently chunky flat fee can be perfectly reasonable once the balance grows.
It also helps to separate the platform charge from the cost of the investments you hold on the platform. Fund ongoing charges, foreign exchange costs and dealing fees can all change the real annual drag. A platform can look cheap at the top line and still be expensive once the rest of the stack is included.
For a wider look at what platforms do and do not show clearly, Cristoniq’s guides to average purchase price and risk ratings on funds help explain why the numbers on the screen are not always the whole story.
What belongs in the total-cost comparison
A fair platform comparison usually includes four layers: the platform fee itself, the ongoing charge on the fund or ETF, any dealing costs, and any currency conversion costs if overseas assets are involved.
- Platform fee: the charge for using the wrapper or account
- Fund charge: the ongoing cost inside the investment product itself
- Dealing cost: any charge when you buy or sell holdings
- Foreign exchange cost: relevant when assets or dividends involve another currency
MoneyHelper’s investing charges overview and MoneySavingExpert’s stocks and shares ISA comparison guide are useful practical references because they show how the visible platform charge fits into the wider cost picture.
A Worked Example
Imagine two investors comparing the same index fund on two platforms. Platform A charges 0.25% a year. Platform B charges GBP 60 a year. On a GBP 8,000 portfolio, Platform A costs about GBP 20 and Platform B costs GBP 60 before fund charges are considered, so the percentage model is clearly cheaper.
Now imagine the portfolio grows to GBP 80,000. Platform A now costs about GBP 200 while Platform B is still GBP 60. Nothing magical happened. The account simply moved through the crossover point and exposed the real shape of each pricing model.
That is why a platform choice that looked rational on day one can become expensive a few years later if the investor never revisits it.
What This Means For You
Start with your actual balance, not the marketing headline. Work out the platform fee in pounds at the size you have now, then repeat the same check at a realistic future balance. If you are contributing regularly, that second number matters more than many investors assume.
If the platform offers a fee cap, include that in the calculation as well because a capped percentage fee can behave more like a flat-fee model once the account is large enough. The practical goal is not to find a universally best platform. It is to find the least wasteful structure for the account you are actually building.
In Plain English
Percentage fees often suit smaller portfolios. Flat fees often suit larger ones. The right answer depends on how much money is in the account and how quickly it might grow.
The useful comparison is always in pounds per year, not in slogans. If you can see the platform fee, the fund charge and any dealing costs together, the best option usually becomes much easier to spot.
This article is for general financial education only. It is not financial advice or personal investment advice. Investments can fall as well as rise, and you may get back less than you invest.