The most popular bet in the world and the most profitable for the bookmaker. What an accumulator costs at each number of legs, when acca insurance is worth something, and the four rules that make one less expensive.
| Legs | Margin per leg 3 percent | Margin per leg 5 percent | Margin per leg 8 percent |
|---|---|---|---|
| 1 | 3.0 percent | 5.0 percent | 8.0 percent |
| 2 | 5.9 | 9.8 | 15.4 |
| 3 | 8.7 | 14.3 | 22.1 |
| 4 | 11.5 | 18.5 | 28.4 |
| 6 | 16.7 | 26.5 | 39.4 |
| 8 | 21.6 | 33.7 | 48.7 |
The margin on an accumulator is 1 minus the product of (1 - margin) over the legs. An eight-fold of 1.30 favourites at a European brand, where the favourites carry 8 percent margins, hands the bookmaker nearly half the stake in expectation. It is the bet the industry advertises most, and the table is the reason.
A small stake for a large return is the product; the price of the product is in the table. There is nothing wrong with paying it as entertainment. There is a great deal wrong with paying it in the belief that it is a way to make money, or with paying more of it than necessary.
Insurance refunds the stake, usually as a free bet, when one leg of a five-fold or more loses. Worth roughly the probability of exactly one leg losing times the free bet’s value; on a five-fold of 1.50 legs that is a few percent of the stake, which offsets part of the margin but not most of it. Boosts add a percentage to the winnings by number of legs, ten percent for five legs and so on; a ten percent boost on a five-fold with a 15 percent margin leaves a 6.5 percent margin, still worse than a single. Both are worth taking when the legs would have been bet anyway and are priced at the best available odds; neither turns the bet into a good one.
Singles, at the best price, sized by the bankroll rules. Less exciting on a Saturday; considerably richer by May.
The margins compound: four legs at 5 percent cost 18.5 percent against 5 percent for each single.
It offsets a few percent of the margin; take it when you would bet the accumulator anyway.
Not in one bet; place singles at each and the average result is the same with less variance.