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ETF mechanics deep dive

NAV vs market price — the bid-ask spread mechanics

An ETF's "price" isn't one number. There's the Net Asset Value (NAV) of the underlying portfolio. There's the iNAV (intraday indicative NAV) that updates every 15 seconds. There's the bid you can sell at, the ask you can buy at, and a spread between them. And for the retail investor pressing the buy button, all of this happens behind the scenes. Knowing how it works saves you 0.02–0.10% per trade and tells you when to avoid trading entirely.

Educational only. Spread varies through the day and across funds. Always check the spread before submitting a market order, especially on larger trades.

An ETF has three "prices"

  1. Net Asset Value (NAV): the fair value of one share, calculated by summing the value of every underlying holding (at last traded prices on the underlying exchanges) and dividing by total shares outstanding. Calculated once a day, end of the LSE day, by the fund administrator.
  2. iNAV (intraday indicative NAV): an estimated NAV updated continuously during trading hours, typically every 15 seconds, by an independent calculation agent. Reflects the changing value of underlying holdings during the day.
  3. Market price: what the ETF actually trades at on the LSE order book. Driven by supply and demand. Should be close to iNAV but isn't identical — the gap is where spreads, premiums and discounts live.

For a UK retail investor at the screen, the only price you see is the market bid and ask. The iNAV is published (usually with delay) on the fund's factsheet page. The NAV is published end-of-day. The relationship between all three is what determines whether you got a fair fill.

The bid-ask spread, properly explained

On the LSE order book, market makers (Flow Traders, Optiver, IMC, Jane Street and others) post two prices for the ETF: a bid (the price they'll buy from you) and an ask (the price they'll sell to you). The ask is always higher than the bid — the difference is the spread.

The market makers earn the spread as compensation for:

Typical UK retail ETF spreads on liquid funds (S&P 500 trackers, FTSE 100 trackers, MSCI World):

For thinly-traded funds (small caps, niche EM, thematic), spreads of 0.20% to 0.50% are normal even mid-session.

When to trade for the tightest spread

By time of day (UK)

If you can choose, target 10:00–14:00 UK time for the cleanest fill.

By day of week

Less impact than time of day, but:

Premium and discount — when market price diverges from NAV

On most trading days, the market price of a liquid ETF trades within a few basis points of iNAV. When it diverges meaningfully, you're seeing a "premium" (market price above NAV) or "discount" (market price below NAV).

Causes of premium/discount:

For UK retail investors holding for years, premium/discount on the day you trade is a one-off cost — the equivalent of paying a wider spread. The long-term return is determined by the index, not by entry timing.

Anatomy of a typical ETF trade

You decide to buy £10,000 of VWRL at 11:30am on a Tuesday. Step by step:

  1. You enter the order on your broker's app. The broker sends the order to the LSE.
  2. The LSE order book shows: bid 108.00, ask 108.06. Spread = 0.06p, or about 0.056% of the mid-price.
  3. Market order fills at ask: you buy at 108.06p. The market maker on the other side just sold to you and now needs to hedge.
  4. Market maker rebalances: they either buy the underlying basket directly, or they wait until they have a meaningful inventory and approach the fund's authorised participant for a "creation" (giving the fund a basket of underlyings in exchange for new ETF shares).
  5. Total cost to you: £10,000 + £5.60 (one half-spread) + your platform's dealing fee (if any). Spread cost on this trade: £5.60. Spread cost on the eventual exit trade: another £5.60ish at the prevailing spread on the day.

Should you use a limit order or a market order?

For UK retail buying liquid ETFs in normal market hours: market orders are fine 90% of the time. The spread is small, you get an immediate fill at the prevailing ask. The risk is during the open / close / news events when spreads widen unexpectedly.

For larger trades (>£25,000), in thinly-traded ETFs, or near the open / close, a limit order at iNAV + half-spread is safer:

This avoids the worst-case where a market order fills at a temporarily-wide spread during a brief liquidity gap.

The effective cost of spread over a holding period

For a buy-and-hold investor making one entry and one exit over a 10–30 year horizon, spread is a one-off cost. On a 0.05% spread, you pay it twice (entry + exit) = 0.10% total. Divided across 20 years, that's 0.005% per year. Tiny.

For a regular contributor (e.g. monthly DCA into an ISA), you pay the spread on every contribution. 12 contributions per year × 0.05% spread = 0.60% extra cost per year of contributions. On a steady-state portfolio where contributions are small relative to total value, this dilutes — but in the early years when contributions are most of your pot, spread matters.

For a frequent trader (rebalancing monthly, swapping between ETFs), spread can quickly become the dominant cost — multiples of the OCF. This is one reason buy-and-hold beats frequent rebalancing for retail investors.

How to check the current spread before trading

  1. Your broker's order ticket usually shows live bid/ask. If not, check the "order book" view.
  2. Calculate spread = ask − bid; spread % = spread / mid-price × 100
  3. Compare to "normal" for that ETF (factsheets often publish typical spread under "trading characteristics")
  4. If spread is > 2x normal, consider waiting or using a limit order

Frequently asked questions

Is the bid-ask spread the same as a platform's dealing fee?

No. The spread is a market cost paid to the market maker; it's not the broker's commission. Some brokers charge zero dealing fees but you still pay the spread. Some charge both (e.g. ii £5.99/trade plus the spread). Trading 212 has no dealing fees but still has the underlying market spread.

What is iNAV and how is it calculated?

iNAV (intraday indicative NAV) is calculated by an independent agent (e.g. SIX, Solactive, Bloomberg) by valuing the fund's underlying portfolio in real time during trading hours, updated every ~15 seconds. It's an estimate, not a guarantee. For funds with closed underlying markets (Japan ETFs traded in London), iNAV is "stale" until those markets reopen.

Should I worry about ETF premium / discount?

For liquid UK retail ETFs in normal market conditions, premiums and discounts are tiny (a few basis points) and self-correct quickly via the creation/redemption mechanism. For thinly-traded ETFs or in stressed markets (March 2020) they can be larger but still don't persist for long. Buy-and-hold investors should largely ignore them.

Why is my Trading 212 fractional ETF buy showing a different price than the LSE quote?

Trading 212 (and most fractional platforms) aggregate retail orders and execute on the LSE at the prevailing market price. They may execute as a market order at the ask or use VWAP-style execution. The "price you see" may include the spread differently than viewing the order book directly. Read your platform's execution policy.

Can I trade ETFs at the close auction?

Some brokers route orders to the LSE 16:30 closing auction. This typically gets a single closing price (the auction match price) rather than a market order across the spread. For larger trades, closing auctions can give a tighter effective spread than mid-session. Smaller retail orders rarely benefit.

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