How Investment Fees Compound: Platform Charges, Fund Costs and What You Really Pay

A fee of 1% a year sounds trivial, which is exactly why it is so expensive. Understanding how investment fees compound means looking past the headline number on a pricing page and adding up every layer you pay: the platform charge, the fund’s expense ratio (the ongoing charges figure in the UK), dealing charges, currency conversion, stamp duty and spreads. Over 20 or 30 years, the gap between a lean setup and an expensive one can reach a third of your final pot. This guide shows the arithmetic, explains where each cost hides, and ends with a checklist.

Key Takeaways

Question	Short, Practical Answer
Why do small fees matter so much?	They come off your whole balance every year and compound. In our illustration, 1% a year costs roughly a quarter of the final pot over 30 years.
What are the main layers of cost?	Platform fee, fund ongoing charges, transaction costs inside funds, dealing charges, FX fees, stamp duty and spreads.
Flat fee or percentage platform?	Percentage tends to suit smaller pots and flat fees larger ones. The break-even is the flat fee divided by the percentage rate.
Where do I find a fund’s true cost?	In its key information document and your platform’s MiFID II costs and charges disclosure, which totals everything in pounds.
Are commission-free apps really free?	No dealing charge, but they typically earn from FX fees, spreads, interest on cash and premium tiers.
What should I do first?	Work out your total annual cost as a percentage of your portfolio, then compare it with the alternatives once a year.

1. Why Small Percentages Become Large Sums

A percentage fee is taken from your whole balance every year, not just from your gains. Two consequences follow:

• The fee grows as your pot grows. 1% of £50,000 is £500. 1% of £200,000 is £2,000. You pay more in pounds precisely when investing has gone well.
• Every pound paid in fees loses its future growth. Money taken out in year three cannot compound for the next 27 years.

Fees behave like a negative return that compounds alongside your positive one. Market returns are uncertain, but costs are known in advance and largely within your control.

2. The Maths of Fee Drag: Illustrative Worked Examples

These figures are illustrative only. They assume a one-off £50,000 investment, a steady gross return of 6% a year and no further contributions. To keep the arithmetic transparent, the total annual cost is deducted from the return each year, so the formula is the starting amount × (1 + net return) to the power of the years. Over 30 years:

• No costs: £50,000 × 1.06 to the power of 30 (about 5.743) = £287,175
• 0.25% cost: £50,000 × 1.0575 to the power of 30 (about 5.351) = £267,535
• 1.00% cost: £50,000 × 1.05 to the power of 30 (about 4.322) = £216,097
• 1.75% cost: £50,000 × 1.0425 to the power of 30 (about 3.486) = £174,282

Total annual cost	After 20 years	After 30 years	Lost to costs by year 30
None (reference)	£160,357	£287,175	£0
0.25%	£152,960	£267,535	£19,640 (about 7%)
1.00%	£132,665	£216,097	£71,078 (about 25%)
1.75%	£114,945	£174,282	£112,893 (about 39%)

The damage accelerates: at 1.00%, the shortfall is about £27,700 after 20 years but £71,100 after 30. And the fees you actually pay are smaller than the total cost. At 1.00%, fees deducted add up to roughly £33,200 over 30 years, rising from £500 in year one to over £2,000 in year 30. The rest of the gap is growth those fees would have earned had they stayed invested.

A 1% annual charge does not cost you 1% of your money. Over 30 years, in this illustration, it costs you about a quarter of it.

3. The Layers of Cost You Actually Pay

Most investors pay several of these at once. The ranges are broad illustrations, not quotes from any provider.

• Platform fee: a percentage of assets (often around 0.15% to 0.45% a year, sometimes tiered or capped) or a flat monthly or annual fee.
• Fund ongoing charges (OCF or TER): from under 0.1% for the largest trackers to 1% or more for some active funds.
• Transaction costs inside funds: the fund’s own trading costs, excluded from the OCF.
• Dealing charges: from nothing to around £10 or more per trade.
• FX fees: charged whenever sterling is converted to another currency or back.
• Stamp duty or SDRT: 0.5% on most UK share purchases.
• Spreads: the gap between buying and selling prices.
• Extras: wrapper fees, transfer-out fees, inactivity fees and interest retained on cash.

Spreads, fund transaction costs and FX margins are absorbed into prices and never appear as a line item, so they need hunting.

4. Platform Fees: Percentage vs Flat, and the Break-Even Point

A percentage fee scales with your pot, sometimes tiered so the rate falls on larger balances, and sometimes capped for shares and ETFs. A flat fee is fixed regardless of how much you hold, sometimes charged per account.

The break-even is a simple division: flat annual fee ÷ percentage rate. As an illustration, compare a 0.25% platform with a flat £120 a year:

• £120 ÷ 0.0025 = £48,000 break-even.
• At £20,000, the percentage platform costs £50 against £120 flat, so percentage wins.
• At £150,000, it costs £375 against £120. Flat wins by £255 a year, and the gap widens as the pot grows.

Dealing charges move that line. If the flat-fee platform also charged £5 per trade and you bought two funds monthly, that adds £120 a year (24 trades × £5), so the real flat cost is £240 and break-even doubles to £96,000. Always compare your total annual bill under your own trading pattern, and check for caps and transfer-out fees.

5. Fund Costs: OCF, TER and the Transaction Costs You Don’t See

Funds deduct running costs internally, so you never receive a bill. The UK headline figure is the ongoing charges figure (OCF), covering the management fee plus administration, custody and similar costs. The total expense ratio (TER) is a broadly similar measure widely quoted for ETFs.

The OCF leaves out the fund’s own trading. Transaction costs (commissions, spreads, and taxes such as stamp duty when the fund buys UK shares) are disclosed separately. They are usually small for broad trackers but can add a tenth of a percent or more for high-turnover active funds. Occasionally the figure is negative, a quirk of the measurement method, and is best treated as roughly zero.

Illustratively, Investor A holds a tracker with a 0.10% OCF on a 0.25% platform: 0.35% in total. Investor B holds an active fund with a 0.85% OCF and 0.20% transaction costs on a 0.40% platform: 1.45%. With the same £50,000 at 6% gross over 30 years, A ends with about £260,000 and B about £190,000. To finish level, B’s fund must beat the tracker by around 1.1 percentage points a year, every year. Our comparison of index funds and ETFs covers the two main low-cost options.

6. Dealing Charges, Spreads and Stamp Duty

Dealing charges hurt most on small amounts. An illustrative £5 charge on a £100 monthly purchase is 5% of the money invested; on £5,000 it is 0.1%. Many platforms offer cheaper or free regular investing on a fixed monthly date.

The spread never appears on a statement, but you pay it on every round trip. For large FTSE 100 companies and major ETFs it is typically a small fraction of a percent. For smaller companies, thinly traded AIM shares or niche ETFs it can be 1% or more.

Stamp duty applies when you buy UK company shares: electronic purchases incur Stamp Duty Reserve Tax (SDRT) at 0.5%, and paper transfers over £1,000 pay stamp duty at the same rate. The gov.uk guide to tax when you buy shares has the details. In practice:

• It applies to purchases only, and inside an ISA or SIPP just as in a general account.
• Investment trusts are UK-listed companies, so their shares usually incur 0.5% SDRT.
• AIM shares (not also on a main market) are generally exempt, as are ETFs and unit trust or OEIC units.
• Overseas shares escape UK stamp duty, though some countries levy their own transaction taxes.

Investing £1,000 a month into UK investment trusts costs £60 a year in SDRT, which is modest. Buying back into the same £10,000 position four times a year costs £200, or 2% of the position, before any other charges.

7. FX Conversion Fees on Overseas Shares

Buying US or European shares from a sterling account means converting currency, charged as an explicit percentage or a margin on the exchange rate. Fees range roughly from 0.15% to 1% or more per conversion, in both directions. Illustratively, you invest £10,000 in US shares and later sell at £20,000:

• At 0.15%: £15 in plus £30 out = £45.
• At 0.75%: £75 in plus £150 out = £225.

Dollar dividends are usually converted too, and for active traders FX can become the largest single cost. A sterling-priced global fund or ETF moves conversion inside the fund, at institutional rates. Conversion fees are separate from currency risk, covered in our guide to currency risk in a global portfolio.

8. How “Free” Commission-Free Apps Make Money

“Commission-free” means no dealing charge, not no cost. Common revenue sources include:

• FX fees on every conversion, often a major revenue line for apps popular with US share buyers.
• Spreads and execution: the price you receive may not be the best available, especially outside main hours.
• Interest on uninvested cash, of which only part may be passed on.
• Subscriptions and premium tiers.
• Securities lending on some services, with income shared between you and the platform.
• Other products, such as CFDs or margin lending.

One source you will not find at UK-regulated brokers is payment for order flow, where market makers pay brokers to route orders to them. Common in the US, it is regarded by the FCA as incompatible with its rules on conflicts of interest and inducements, so it is effectively banned here. For small buy-and-hold portfolios these apps can be genuinely cheap. Just price the whole package.

9. How to Find a Fund’s True Cost

UK rules require firms to disclose costs, if you know where to look:

1. The key information document. A KIID shows the OCF and any entry or exit charges, while a PRIIPs KID also breaks out transaction costs. The UK is moving to a new Consumer Composite Investments regime, so names and layouts may differ, but the cost figures are what matter.
2. The factsheet, which usually repeats the OCF or TER.
3. Your platform’s costs and charges disclosure. Under MiFID II rules, firms must estimate all costs before you invest and send an annual statement afterwards, combining platform, fund and transaction costs in pounds and as a percentage, showing their cumulative effect on your return.
4. Contract notes, which show dealing charges, stamp duty and FX fees per trade.

The annual costs and charges statement is the most useful document most investors never open.

10. A Practical Fee Checklist

1. List every account and how its platform fee works: percentage, tiered, capped or flat.
2. Note each fund’s OCF or TER, plus transaction costs where shown.
3. Count last year’s trades and what each cost.
4. Check your FX rate and roughly how much you converted.
5. Add up stamp duty on UK shares and investment trusts.
6. Calculate your total cost as a percentage and compare it with your costs and charges statement.
7. Price your actual activity on one or two alternatives, including transfer-out fees.
8. Repeat yearly, and whenever your pot crosses a break-even point.

A simple index fund portfolio can often run for well under 0.5% a year in total. If you pay over 1%, know why.

11. Tracking What You Actually Pay With InvestInsight

Disclosure documents show what a provider charged, not what costs did to your returns across several accounts. InvestInsight’s portfolio tracker can sync a Trading 212 account using a read-only API key, or take transactions from other platforms by manual entry or CSV import. Fees are recorded as their own transaction type alongside buys, sells and dividends, so the dealing charges, FX costs and stamp duty you log stay visible in your history. Returns are shown against your cost basis, dividends are tracked separately, analytics compare your performance with the S&P 500, and multi-currency display helps with overseas holdings.

Charges deducted inside a fund are already reflected in its price, so they show up in returns but not as a separate line. Other tools do similar jobs, including Sharesight and getquin, and a disciplined spreadsheet works too. Whichever you choose, our guide to measuring your real investment returns explains the method.

Common Mistakes to Avoid

• Comparing headline platform fees only. Dealing, FX and wrapper fees can outweigh the difference.
• Treating the OCF as the whole fund cost. Transaction costs sit on top.
• Drip-feeding small sums with flat dealing charges. Use regular investing or invest less often.
• Forgetting stamp duty on investment trusts. 0.5% applies on every purchase.
• Choosing on cost alone. A slightly dearer fund you will hold can beat a cheaper one you abandon in a downturn.

Conclusion

Fees compound exactly as returns do, only in the wrong direction. One percentage point a year looks negligible on a pricing page, yet in our illustration it costs about a quarter of a 30-year pot, and over half of that loss is growth that never happened rather than fees you consciously paid.

Add up every layer, including those built into prices. Match your platform to your pot size and trading pattern, keep fund costs low unless you have a clear reason to pay more, and read your costs and charges statement. Repeat each year, because the cheapest setup for £20,000 is rarely the cheapest for £200,000.

Further reading

These independent sources explain charges, tax and fund costs in more detail.

• MoneyHelper: stocks and shares ISAs and the charges to compare
• FCA InvestSmart: guidance for people investing for themselves
• justETF: compare ETF costs and TERs
• Investopedia: the bid-ask spread explained

This article is for general information only and does not constitute financial advice. All figures are illustrative and historical patterns are not reliable indicators of future results. Consider your own circumstances or speak to a qualified adviser before making investment decisions.