Back and lay odds, explained for builders

Backing is betting for an outcome; laying is betting against it. You can back at a bookmaker or an exchange, but you can only lay at an exchange, because laying means taking the bookmaker's side of the bet. Matched betting and arbitrage run on a pair: a back at a bookmaker and a lay at an exchange, on the same selection. This post explains both sides plainly, then shows the data the pair is built from.

What does it mean to back a bet?

Backing means staking on an outcome to happen. It is the ordinary bet everyone knows: you pick Arsenal to win, you put down a stake, and if Arsenal win you collect at the odds you took. A bookmaker like bet365 offers back prices, and so does an exchange. If the outcome does not happen, you lose your stake. That is the whole of it, and it is only half of what a matched position needs.

What does it mean to lay a bet?

Laying means betting that an outcome will not happen. You are the one offering the odds now, so you win the other backer's stake if the outcome fails, and you pay out if it succeeds. Only an exchange lets you do this, because an exchange matches your lay against someone else's back. In the UK the three that matter are Betfair, Smarkets and Matchbook, and for a fuller picture of the lay odds definition it helps to hold both sides in view at once.

This is why the exchange is the missing half. A bookmaker can only take your back. To bet against an outcome, and so to cover a back you placed elsewhere, you need the lay side that only an exchange provides. That is the mechanic underneath exchange and lay coverage.

Lay stake, liability and commission

Three terms decide what a lay actually costs and returns. They are simpler than they sound once you name them:

• Lay stake: the amount you would win if the outcome does not happen. It is your reward for taking the bookmaker's side, and roughly mirrors the backer's stake on the other side of the bet.
• Liability: the amount you would pay out if the outcome does happen. It grows with the lay odds, so a lay at high odds ties up far more than a lay at short odds. This is the number that governs how much you must hold at the exchange.
• Commission: the exchange's cut, taken only on net winnings. Because you keep less of a winning lay than the raw odds imply, the effective lay price is slightly worse than the quoted one.

A small illustration makes the shape clear. Suppose you back a selection at a bookmaker at 2.10, then lay the same selection at an exchange at 2.14. Your lay stake is sized so the two bets roughly cancel whichever way the result goes: if the selection wins, the back collects and the lay pays its liability; if it loses, the back stake is lost and the lay wins its stake, net of commission. The gap between 2.10 and 2.14, and the commission, are what leave you with a small qualifying loss rather than a clean wash. This is arithmetic, not a promise: the point is only that the two prices and the commission fully determine the outcome.

What data does a back/lay pair need?

A usable pair needs three facts, not one price. You need the back price at the bookmaker, the current lay price at the exchange for the same selection, and the liquidity available to lay at that price. The first two set the relationship between the bets; the third decides whether the lay is real or just a headline number you cannot actually get matched. Miss any one and the pair is incomplete.

Liquidity is the fact most people forget. An attractive lay price with nothing behind it cannot be filled at any meaningful size, so the position you modelled never exists. We go into this in why lay liquidity matters; for now, treat the available lay amount as part of the price, not a detail beside it.

How the feed pairs back and lay for you

OddsRelay delivers the pair already assembled, so you never compute the relationship yourself. Each row carries the bookmaker back price, the exchange lay price with its liquidity, and two derived fields: a rating for how good the pair is, and the qualifying_loss it implies. Here is the shape of a single row (illustrative, not live data):

{
  "event": "Arsenal vs Chelsea",
  "market": "match_odds",
  "selection": "Arsenal",
  "back": { "bookmaker": "bet365", "odds": 2.10 },
  "lay":  { "exchange": "betfair", "odds": 2.14, "liquidity": 1840 },
  "rating": 98.1,
  "qualifying_loss": -0.12
  // ... region, feed_type and freshness fields elided
}

The back block is the bookmaker price; the lay block is the paired exchange price with the liquidity you can actually match against. The rating and qualifying_loss are computed from that pair, so your oddsmatcher or scanner renders results without building a matcher of its own. For what the two derived numbers mean, see qualifying loss and ratings.

That pairing is the work the feed does. It covers 60+ UK books with bet365 included, each back price matched against Betfair, Smarkets and Matchbook lay prices, refreshed by pre-match polling on roughly a few-second cycle. You receive the relationship, not two separate numbers to reconcile.

The short version

Back is for, lay is against, and only an exchange lets you take the against side. A matched position is a back at a bookmaker paired with a lay at an exchange, governed by the lay stake, the liability and the exchange commission. Getting there yourself means integrating an exchange, tracking liquidity, and computing the pair for every selection. A matched feed hands you the pair ready to read. You can see it live on the coverage dashboard, or take a free trial and read real back/lay rows against your own code. It powers a leading UK matched-betting platform today.