Investment Platform Fee Calculator: Flat vs % Fees
Investment platform fee calculator: see what a percentage charge or a flat monthly plan costs your ISA over the years, and the pot size where flat wins.
Two investing apps can hold the exact same funds and still charge in completely different shapes: one takes a slice of your pot every month, the other takes a fixed sum whatever you hold. That difference decides which app is cheaper for you, and it flips at a specific pot size. A £5.99 monthly plan costs £71.88 a year, which a 0.45% platform charge matches at a pot of about £16,000, so below that the percentage wins and above it the flat plan does. Put your own numbers in below and this tool works out the fee bill, the effect on your final pot, and the exact pot size where the answer changes.
Compare two platforms over the years you plan to invest
Copy the numbers from each app's own fees page. Percentage charges on UK platforms commonly sit between 0.15% and 0.45% a year, fixed plans are usually billed monthly, and a few apps charge neither and earn from currency conversion and interest on cash instead. Leave the cap blank if the percentage is uncapped.
Fees are taken monthly from the pot, contributions go in at the start of each month, and any yearly cap is applied to the percentage charge only. This compares platform charges, so it does not include fund or ETF ongoing charges (you pay those wherever you hold the fund), dealing commission, currency conversion charges on overseas shares, regular investing discounts, exit fees or interest earned on uninvested cash. Check each provider's current fee schedule before you move anything.
Why the shape of the fee matters more than the size of it
A percentage charge is gentle on a small pot and unpleasant on a large one, because the bill grows with the balance while the service stays the same. A fixed monthly plan does the opposite: it is a heavy toll on £2,000 and close to a rounding error on £150,000. This is why the "cheapest investing app" question has no single answer, and why the honest version is "cheapest for a pot of this size, at this contribution rate".
Three details change the sum, and all three are in the calculator:
- A yearly cap. Some percentage platforms cap the charge on shares, ETFs and investment trusts. A cap turns a percentage into a flat fee once your pot is big enough, so a capped 0.45% can beat an uncapped 0.25% on a large balance.
- Contributions. Paying in every month drags your average balance up over the term, which quietly raises a percentage bill and does nothing to a fixed one.
- Lost growth. Money taken as fees is money that never compounds. Over a long term, the gap in your final pot is usually bigger than the gap in the fees themselves, which is the figure the tool separates out for you. It can also run the other way: a platform that takes its cut late, once the pot is large, can charge more in total and still leave you ahead, and the tool says so when that happens.
What to check before you switch platform for the fee
A lower platform charge is easy to measure and easy to overrate. Before moving, look at the dealing charge on each purchase (a fixed commission ruins a small monthly investment), the currency conversion charge if you buy US shares or funds, whether your existing funds can be transferred in stock or have to be sold and rebought, and how long the transfer takes. Check too that the fee you are comparing is the one that applies to what you actually hold, since several platforms charge different rates for funds and for shares in the same account.
If you are still choosing an app rather than costing one you already use, start with the best investment apps in the UK or the narrower list of stocks and shares ISA apps, and see Trading 212 against Freetrade for how two different charging models look side by side. If some of the money is better off in cash, the cash ISA versus savings calculator answers that half of the question.
