Bet Builders Explained: Correlation, Margin and Void Legs

Why the price is not the legs multiplied together

Take a favourite at 1.60 to win and over 2.5 goals in the same match at 1.85. Multiplied, the two give 2.96. Most bettors expect a number close to that, and most sites quote something well short of it.

The reason is correlation. Suppose a fair model of the match gives these chances. The figures are illustrative, chosen to keep the arithmetic easy to check.

The favourite wins 60% of the time and the match has three or more goals 52% of the time. If the two were unrelated, both would happen 0.60 × 0.52 = 31.2% of the time. In the table they happen together 38% of the time, because a side that wins often wins by scoring two or three. The fair price of the pair is 1 ÷ 0.38 = 2.63, not 2.96.

Now imagine the site paid the product anyway. At 2.96, a £1 stake returns on average 0.38 × 2.96 = £1.12, a profit of about 12.5p for the bettor on every pound. No sportsbook can offer that, so the builder price has to come in below the fair 2.63, not just below 2.96.

Correlation also runs the other way. Favourite to win and under 2.5 goals happen together 22% of the time in the table, less than the 28.8% (0.60 × 0.48) that independence would suggest. The fair price is 1 ÷ 0.22 = 4.55. With the under quoted at 2.00, the product of the two prices is 3.20, and a builder might offer 4.10. That is longer than the product, which looks generous, yet it still sits below the fair 4.55.

Where the margin goes: builder against single and double

Using the same table, here is what each bet returns on average per £1 staked. The singles are the prices above; the double assumes two separate matches with legs at the same prices and chances.

The double already costs about twice what a single does, because the two margins compound. The builder costs more again. Three things push it up. Each leg carries its own margin. The site adds a buffer for the uncertainty in its correlation model, since getting that wrong is expensive. And there is no easy way to compare the combined price across sites, so competition on it is weaker than on the match result.

A simple check before placing one: run the legs through the odds calculator to get each implied chance, then ask whether the builder price is close to your own view of how often everything lands together. For margins on single markets, the best odds betting sites page compares sites on the result market itself.

When one leg is void: the player who does not play

Player legs are where voids happen. A striker named in your slip is left on the bench, or pulls out in the warm-up, and his shots leg cannot be settled. Rules differ by site, and two approaches are common.

Take a three-leg slip at 3.75 for a £10 stake: favourite to win, over 2.5 goals and the striker to have at least one shot on target. On its own the shot leg was 1.80, and the first two legs as a builder were 2.40.

1. Re-price the remaining legs. The site settles the slip as if it had been the two-leg builder all along, at 2.40. If both land, the return is £24.00.
2. Divide the void leg out. The site removes the 1.80 from the combined price: 3.75 ÷ 1.80 = 2.08. If both land, the return is £20.83.

Some sites void the whole slip instead and return the £10. If the player comes off the bench and plays any part, the leg normally stands, even if he only had five minutes to take a shot. Read the builder section of the site's rules before you add a player who is a doubt, because late team news is the most common reason a builder settles differently from what the bettor expected.

Player markets: shots, cards, yards and points

Player legs are where the builder is most popular, and where the margin is usually highest, because each player market is thinner than the match result and the site has less information to price it from.

• Football shots and shots on target. Strongly linked to the team result and the goal total. A striker's shots and his side winning are positive partners, so the combined price is shortened. More on the match markets is on the football betting page.
• Football cards. A player to be carded pairs naturally with an over on total cards in the match. Check whether cards shown to players on the bench or after the final whistle count, because sites settle this differently.
• NFL yards and touchdowns. A quarterback's passing yards and his main receiver's receiving yards rise and fall together. Sites shorten that pairing sharply. The spreads and props behind them are covered on the NFL betting page.
• Basketball points, rebounds and assists. A star's points line and his team winning are linked, and so is his points line with the game total. Minutes are the risk: a blowout can sit starters in the last quarter. The basketball betting page explains the main lines.

When a bet builder makes sense, and when it does not

The product suits a bettor who has a specific view of how one match will go and wants a small stake on that whole story. It suits nobody as a way to get a long price cheaply. The common mistakes:

1. Assuming the legs multiply. They do not, as the table shows. Compare the quoted price with the product to see how much correlation the site has priced in.
2. Adding a leg that says the same thing twice. Favourite to win plus favourite to score adds nothing: a side cannot win without scoring, so the price should not move. Combining a correct score with the result does the same.
3. Stacking legs for a big number. Every extra leg brings its margin with it. At about 4p in the pound per leg, six legs compound to 0.96 multiplied by itself six times, or about 78p back per pound, a loss near 22p before any correlation buffer.
4. Ignoring the goal lines. A view on goals alone is often cheaper as a single on over 2.5 goals or both teams to score, where the margin is low.
5. Building on a doubtful player. If the team sheet is not out, the slip can be re-priced on settlement.

If your view is really about the margin of victory, a handicap carries a much lower margin than a result-and-goals builder. The Asian handicap guide and the handicap betting guide show how. Builders are also offered in-play on some sites, with the extra margin that live pricing carries, as the live betting guide explains. Where a site allows cash out on a builder, the offer is calculated from the site's own price for the legs still running, so it carries the same wide margin.

Big builder returns and document checks

A builder is one of the few bets where a small stake can return several hundred pounds from a single match. £20 at 25.00 returns £500. That is where the document question comes back. Signing up without documents does not mean they are never requested: a site's terms usually allow a check once a withdrawal, or a total of withdrawals, passes a set amount, or when a win looks out of line with the account's history. A large builder return can trigger both.

Before placing a long builder, know the site's withdrawal ceiling and the point at which it can ask for identity documents. The thresholds and payout speeds of each site are compared on the fast withdrawal betting sites page, and the way we read those terms is set out in our methodology. If builders are becoming a way to chase a big return, the responsible gambling page has the tools and contacts. All the market guides in this series are listed on the betting guides hub.

Sportsbooks that take these bets with only an email and a wallet are ranked among the no verification betting sites we have read.

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