ETF due diligence: what to check before comparing funds

Choose an ETF to check

Choose a fund to see its fact file, then use the checklist below to confirm the details in the fund’s own documents before comparing performance, yield or cost.

Types of exposure in the fact file

The fact file labels each fund by the kind of exposure it gives. The labels describe the fund; they are not ratings or judgements about who it suits.

Small funds are also more likely to be closed by their provider. See what happens when an ETF closes.

What due diligence on an ETF actually means

Due diligence is not picking last year's best performer. It is verifying three things before the fund earns a place: that it tracks an index whose rules you actually want, that it does so cheaply and faithfully, and that it is structured so you can get in and out at a fair price for years. A fund that fails any of those is a problem no headline return makes up for. The job here is elimination, not selection — most of the work is finding reasons to say no early, so the shortlist that survives is small and boring on purpose.

It also helps to be clear about the role a fund would play. Broad, low-cost index funds are often used as the main holding in a portfolio, and narrower funds as smaller, deliberate additions. If the job a fund would do that existing holdings do not cannot be put in one sentence, that is worth resolving before going further.

Start with the index, not the ticker

An index fund is only as good as the rules it tracks, so the index methodology is the actual product. Read what it includes and excludes, how holdings are weighted (market-cap, equal, fundamental), how often it reconstitutes, and how concentrated it is — a "global" or "tech" label can hide the fact that a handful of companies drive most of the return. Two funds with similar names can hold very different things; the index document, not the ticker or the marketing, is where you find out. If you are still deciding between fund structures entirely, fund vs ETF vs investment trust and the broader UK ETF guide set the context for everything below.

Replication: physical or synthetic?

Physical funds actually hold the underlying securities (fully, or a representative sample for very broad indices). Synthetic funds use a swap with a counterparty to deliver the index return, which can track tightly and reach awkward markets, but adds counterparty risk that must be collateralised and understood. Neither is automatically wrong; what matters is that you know which one you are buying and why. Sampling is normal and fine for huge bond or global indices — it is only a concern when tracking quality suffers. The mechanics of how units are actually created and redeemed, and why that keeps price near value, are covered in how ETF creation and redemption works.

Cost is more than the headline OCF

The ongoing charge (OCF/TER) is the number everyone quotes and the least complete. The figure that actually decides whether a fund did its job is tracking difference: how far the fund's return drifted from its index after all real-world frictions, which can be larger or, with securities-lending income, occasionally smaller than the OCF alone. On top of that sit the costs you pay to trade: the bid-ask spread, any currency conversion, and the platform's own fee. A cheaper OCF that comes with a wide spread and poor tracking is not cheaper. The relationship between an ETF's market price and the value of its holdings, and why spreads matter, is set out in NAV vs market price and the bid-ask spread.

Domicile, currency and the UK-specific tax bit

This is where UK investors most often trip up. Most ETFs on UK platforms are UCITS funds, often domiciled in Ireland; for a UK investor, domicile affects the withholding tax the fund pays on its income and which tax rules and documents apply to it, as explained in UCITS vs US-domiciled ETFs. Decide deliberately between accumulating and distributing share classes — it changes admin and how income is handled. And separate two things people conflate: the currency a fund is denominated in (largely cosmetic for a globally diversified fund) versus whether it is currency-hedged (a deliberate, costed decision). Holding the fund inside a Stocks & Shares ISA removes most of the ongoing tax friction in the first place.

Liquidity and size: can you exit at a fair price?

A fund you cannot leave cleanly is not a good fund, however good the index. Look at assets under management and how tightly it trades: on-screen liquidity matters less than the liquidity of the underlying holdings, because authorised participants create and redeem units against that. Very small funds tend to have wider spreads and a higher chance of being closed and liquidated — usually returning your money, but on the manager's timetable and possibly with an unwanted taxable event. Plan the exit before the entry; what happens when an ETF closes and how securities-lending revenue works cover the two areas most people skip.

Red flags that should slow you right down

• Leveraged or inverse. These are short-horizon trading tools whose returns decay over time — almost never a buy-and-hold position. See the leveraged and inverse ETF warning.
• Ultra-niche thematic. A fund built around a recent story often launches near the top of the hype and is heavily concentrated.
• Tiny AUM with a wide spread. Closure risk plus a real cost every time you trade.
• An index you cannot get a clear rulebook for. If the methodology is opaque, you are buying a black box.
• Structure you cannot explain. Overlay, commodity, crypto, AT1/CoCo or unusual exposure must be understood before headline yield or past returns are even looked at.

Is a cheaper ETF always better?

No. Cost is one of the few variables you control with certainty, so it matters a lot — but the right comparison is total cost of ownership and tracking difference, not the headline OCF in isolation. A marginally pricier fund that tracks its index faithfully, trades on a tight spread and is large and durable beats a cheaper one that tracks poorly or might be wound up. Cheap is a strong tiebreaker between two otherwise sound funds, not a reason to overlook structure.

How do I know if an ETF is UK-friendly?

Three quick gates clear most of the doubt: it is a UCITS fund (typically Ireland-domiciled), it has UK reporting-fund status, and it is available in the wrapper you intend to use. Get those right and the bulk of the UK-specific tax and reporting friction disappears before you even reach the index and cost questions above. A broad, large, low-cost UCITS fund is usually the simplest version of a given exposure to check, and a portfolio of two or three funds is a common structure, as set out in how to build a 3-fund portfolio.