How a Trade Actually Gets Executed

What "Buy" Actually Triggers

You open your broker's app, pick a fund or a share, type in an amount, and press buy. From where you're sitting, that's the entire transaction — a tap, a confirmation screen, and shares appearing in your portfolio a moment later. Underneath that single tap, though, a short chain of distinct steps has to happen, each handled by a different party, each with its own rules and its own small chance of something going slightly differently from what you expected.

None of this is a reason to worry about buying shares — millions of trades execute cleanly every single trading day, and for a routine purchase of a diversified fund the whole process is genuinely a non-event. But understanding the chain matters for a few very practical reasons: it explains why the price you click on isn't always the exact price you pay, why some orders don't fill at all, why there's a delay between "trade" and "money actually moves," and why certain investments (illiquid small-caps, for instance) behave very differently from others (a large index fund) even though the buy button looks identical for both.

This chapter walks through that chain in order — from the moment you click, through to the point, a day or more later, where the trade is fully and formally settled.

Step Zero: The Order Type You Choose

Before anything else happens, your choice of order type decides how the rest of the process behaves. This is one of the most under-explained parts of investing for beginners, and getting it wrong is one of the easiest ways to pay more than you needed to, or to miss a trade entirely.

For most long-term, routine investing — a monthly contribution into a global index fund, say — a simple market order is entirely fine, because the fund is liquid enough that the price you get will be extremely close to the price you expected. Limit orders earn their place when you're trading a less liquid individual stock, placing a larger order, or trading during a period (the open, the close, extended hours) when prices move more erratically.

The Five Steps Between Your Click and a Finished Trade

Once you've chosen an order type and hit confirm, here's roughly what happens, usually within well under a second for a liquid stock or fund during normal trading hours:

1. Validation. Your broker's own system checks the order is legitimate before sending it anywhere — do you have enough cash (or shares, if selling), is the order within any limits, is the market even open for this instrument. This happens almost instantly and is where an "insufficient funds" rejection would appear, before the order ever reaches an exchange.
2. Routing. The broker decides where to send your order — directly to an exchange, to a market maker, or through a smart order router that compares multiple venues. In the UK and EU, brokers are legally required under MiFID II rules to seek "best execution" — broadly, the best overall outcome for you, considering price, speed, and the likelihood the order actually fills — rather than simply routing to whichever venue is most convenient for the broker.
3. Matching. The order arrives at an exchange's matching engine, a computer system whose entire job is pairing compatible buy and sell orders by price and, among orders at the same price, usually by whichever arrived first. Your buy order is matched against the best available sell order (or orders, if a large buy has to be filled by several smaller sellers at slightly different prices).
4. Execution and confirmation. The trade happens, a timestamped record is created, and both sides receive confirmation. This is the point at which you legally have a binding trade — the price and quantity are fixed — even though money and shares haven't formally changed hands yet.
5. Settlement. The back-office process of actually transferring money for shares. In the UK and US, standard settlement is currently T+1 — trade date plus one business day. If you buy on a Monday, settlement completes on Tuesday. Until settlement, you have a contractual right to the shares, but the formal transfer of ownership hasn't yet completed.

How a Matching Engine Actually Decides Who Trades With Whom

It's worth understanding the basic logic of a matching engine, because it explains a lot of behaviour that otherwise seems arbitrary. Every live order sits in what's called an order book — a running list of every current buy order (bids) and sell order (asks), ranked by price. The matching engine's rule, in almost all modern exchanges, is price-then-time priority: the best-priced orders get matched first, and among orders at an identical price, whichever was placed earliest gets filled first.

This means two investors placing an identical limit order at an identical price, a split second apart, are not treated identically — the earlier one has priority if there isn't enough opposing volume to fill both. It also means a market order, which by definition accepts whatever the best available price is, always jumps to the front and trades against the best-priced resting limit order in the book, regardless of how long that limit order has been waiting.

For a large order — more shares than are available at the best price alone — the matching engine works through the order book price level by price level, filling what it can at the best price, then moving to the next best price for the remainder, and so on. This is exactly why a very large market order in an illiquid stock can get a noticeably worse average price than the quote you saw before placing it: your order is "walking the book," consuming progressively worse-priced liquidity as it fills.

The Bid-Ask Spread: An Invisible Cost on Every Trade

Every tradable share effectively has two prices at any given moment: the bid, the highest price any buyer is currently willing to pay, and the ask (or offer), the lowest price any seller is currently willing to accept. You always buy at the ask and sell at the bid — never in between — and the gap between the two, the spread, is a real cost you pay on every single trade, separate from any commission your broker charges.

A worked comparison makes the difference concrete. A large, heavily traded FTSE 100 share might show a bid of 500.0p and an ask of 500.2p — a spread of just 0.2p, or roughly 0.04% of the price. A small, thinly traded company might show a bid of 48.0p and an ask of 52.0p — a spread of 4.0p, or about 8% of the price. Buy and immediately sell that small-cap stock with no price movement at all, and you'd still be down roughly 8% purely from crossing the spread twice. That's not a fee your broker charges — it's the structural cost of trading something few other people want to trade at that moment.

Who's on the Other Side of Your Trade

A natural question most new investors eventually ask: when I buy, who is actually selling to me? The honest answer is usually "whoever happened to have a matching order in the book at that moment" — which, for a liquid stock, is very often not another individual investor at all, but a market maker.

A market maker is typically a bank or specialist trading firm that continuously quotes both a bid and an ask price for a security, standing ready to trade either side, in both size and speed that individual investors generally can't match. Their function is providing liquidity — making sure there's almost always someone to trade against, even when no other ordinary investor happens to want the opposite side of your trade at that exact instant. In exchange for taking on that risk (holding inventory they didn't necessarily want, hedging their own exposure constantly), they earn the spread as compensation.

In the US specifically, some brokers receive payment from market makers for routing retail orders to them — a practice called payment for order flow. It's controversial because it can, in some cases, result in a marginally worse price for the retail investor than a fully independent routing decision might achieve, even though the broker itself charges no commission. Payment for order flow is banned in the UK and EU under MiFID II, which is one reason "best execution" obligations are taken particularly seriously by brokers operating here.

Settlement: Why the Trade and the Money Don't Move at the Same Moment

The gap between "trade executed" and "trade settled" surprises a lot of new investors, mainly because it's almost entirely invisible in everyday use. The trade itself — the agreement on price and quantity — is legally binding the instant it executes. But the actual mechanics of transferring money out of your account and shares into your name (and vice versa for the seller) take a short additional time to process through the clearing and settlement infrastructure that sits behind every exchange.

The current standard in both the UK and US is T+1: trade date plus one business day. Buy shares on a Monday, and settlement completes on Tuesday. The US moved from the older T+2 standard to T+1 in 2024, specifically to reduce the amount of time, and therefore risk, sitting in the system between trade and settlement; the UK has signalled it intends to follow with its own move to T+1, though timing for that transition has shifted and is worth checking against current news rather than assuming it has already happened by the time you're reading this.

For almost all everyday investing, this gap is invisible — you see your holding appear in your portfolio and your cash balance update, and the broker handles settlement in the background without you needing to do anything. It becomes practically relevant mainly in two situations: trying to sell shares you only just bought (some brokers restrict this, or restrict using unsettled proceeds, until settlement completes) and corporate actions like dividend eligibility, which are determined by a specific "record date" tied to settlement timing rather than simply the date you clicked buy.

Slippage: When the Price You Click Isn't the Price You Pay

Slippage is the difference between the price displayed when you place an order and the price you actually receive once it executes. It's most relevant for market orders, and it happens for a simple reason: in the fraction of a second between you seeing a quoted price and your order reaching the matching engine, the best available price in the order book can have moved — because someone else's order got there microseconds before yours, or because volatile conditions mean the book is simply shifting faster than quotes can update on your screen.

For a liquid, heavily traded fund or large-cap stock during normal conditions, slippage is usually negligible — a fraction of a penny, immaterial against any real investment amount. It becomes genuinely significant in three situations: illiquid stocks with thin order books, fast-moving markets during major news events, and unusually large orders relative to the stock's typical trading volume.

Why Stop-Loss Orders Don't Always Do What You Expect

Stop-loss orders are popular precisely because they sound like a guarantee — "sell automatically if the price falls to X, so I never lose more than that." In calm markets, they generally do behave this way. In fast-moving or illiquid conditions, the mechanism behind them produces a result many investors don't expect.

A standard stop-loss order isn't actually a promise to sell at your stop price — it's an instruction that converts into a plain market order the instant the price touches your trigger level. If the price is falling quickly, by the time that newly created market order reaches the matching engine, the available price may have already fallen well below your stop level, and the order fills there instead — sometimes significantly below what you intended, especially around a sudden gap-down on bad news. A stop-limit order fixes this specific problem by converting into a limit order instead of a market order, but introduces the opposite risk: if the price gaps straight through your limit without pausing at it, the order may not fill at all, leaving you holding the falling position you were trying to protect against.

Neither version is "wrong" — they're trading off different risks (a worse price versus no execution at all), and which one suits a given situation depends on whether you'd rather guarantee you're out of the position or guarantee the price you're out at.

Does This Work Differently for Funds Than for Individual Shares?

Everything above describes exchange-traded instruments — individual shares and exchange-traded funds (ETFs) — which trade continuously throughout the session exactly like a share does, with their own live bid-ask spread and their own matching-engine execution. Most of what people think of as "buying a fund" in the UK, though, is actually buying units in an open-ended fund (an OEIC or unit trust), and that process works differently.

Open-ended funds are typically priced and traded just once a day, at a single valuation point (often midday or end of day in the UK), rather than continuously. When you place an order to buy during the day, you're not trading against a live order book at all — you're queued to transact at the next valuation point, at the fund's official Net Asset Value calculated from the closing or midday prices of everything it holds, plus or minus a small spread the fund itself applies. There's no slippage in the exchange-trading sense, no matching engine, and no bid-ask spread moving minute to minute — but there's also no way to know your exact execution price at the moment you place the order, since it's set later that day based on the valuation point.

ETFs sit in between conceptually — they hold a basket of underlying assets like an open-ended fund, but trade continuously on an exchange like a share, complete with their own live spread and the same five-step execution chain described above. This is one of the most genuinely useful practical distinctions in UK investing: know whether what you're buying is exchange-traded (continuous pricing, live spread, instant execution) or a traditional open-ended fund (once-daily pricing, queued execution, no intraday spread to worry about).

Partial Fills: Why an Order Can Execute in Pieces

Not every order fills in one clean transaction. If you place an order larger than what's immediately available at the price you want, it can execute as a partial fill — some of your shares trade immediately, and the rest either continue working in the market (for a day order) or get cancelled, depending on the specific instruction attached to your order.

Most retail brokers default to allowing partial fills automatically, which is almost always the sensible setting for routine investing — getting 80% of your intended purchase immediately and the rest a few minutes later is rarely a problem for a long-term holding. Some order types let you specify "all or none," rejecting any fill that wouldn't complete the entire order at once; this is mainly relevant for larger trades in less liquid stocks, where an investor specifically doesn't want to end up holding an oddly-sized partial position.

For context on when this actually comes up in practice: a routine purchase of a liquid, large fund or blue-chip share almost never partially fills, because there's typically ample volume available at or near the quoted price. Partial fills become more common with larger orders, less liquid small-cap shares, or orders placed right at the open or close when the order book is thinner and more fragmented across multiple price levels.

Currency Conversion: The Extra Step Hiding Inside Foreign Trades

Everything described so far assumes you're trading in the same currency your account holds. Buy a US-listed share through a UK broker, though, and there's an additional step tucked inside the process: your pounds need to become dollars before the trade itself can happen, and need to convert back to pounds whenever you eventually sell (or when a dollar dividend arrives).

Brokers handle this in different ways. Some convert automatically at the time of each trade, applying their own exchange rate and conversion fee, often disclosed as a percentage spread over the live interbank rate rather than a flat fee. Others let you hold a separate dollar balance, converting only when you choose to, which can be more efficient if you're making several US trades and want to avoid converting back and forth on every single transaction. Either way, the conversion rate used is a genuinely separate cost from the trade itself, and a weaker pound at the time of conversion can meaningfully affect your effective entry price on a foreign holding, independent of anything the share price itself did.

How Corporate Actions Interact With a Pending Trade

Occasionally, a company does something — pays a dividend, splits its shares, gets acquired — right around the time you've placed an order, and it's worth knowing how this interacts with the settlement process described earlier. Dividend eligibility specifically depends on owning the shares by a cutoff called the record date, which is itself set relative to an earlier "ex-dividend" date; buy on or after the ex-dividend date and you won't receive that particular upcoming payment, even though your trade executes and settles completely normally otherwise.

A stock split (where, say, each existing share becomes two, each worth roughly half as much) doesn't affect the mechanics of an order you've already placed and had fill — your holding simply adjusts automatically to reflect the new share count and price, handled entirely by your broker and the exchange without any action needed from you. The same is generally true of mergers and acquisitions, where your shares convert into cash, new shares in the acquiring company, or some combination, according to the terms of the deal, again processed automatically rather than requiring you to do anything at the point of settlement.