An accumulator is a bet that every one of several selections wins. The price is the product of the individual prices, which is why it looks so large, and the margin is the product too, which is why bookmakers promote accumulators harder than anything else they sell.
The arithmetic
Take five legs each priced at 1.91 in a market where the fair price is 2.00. The fair accumulator is 2.00 to the fifth power, 32.00. The bookmaker’s accumulator is 1.91 to the fifth power, 25.40. The difference is not 4.5% — it is 20.6%. Every leg you add compounds the house edge, and by ten legs you are paying more than a third of the fair return in margin before a ball is kicked.
The formula
Accumulator margin ≈ (1 + single-leg margin) to the power of the number of legs, minus one. Five legs at 4.5% is 1.045⁵ − 1 = 24.6%.
What the boost is for
We priced 240 accumulators against their boosted equivalents in July. The pattern was not subtle: boosts of 5% begin at four or five folds, where the compounded margin is already three times a single, and rise to 50% at twelve to fourteen folds, where it is enormous. A 50% boost on a fourteen-fold that carries an 85% margin leaves you paying more than on an unboosted double. Acca insurance — a free-bet refund when one leg loses — is a small consolation on a bet whose expected loss the insurance does not touch.
If you want the big price
- Two or three legs, all on main markets at the sharpest price you can find for each. Line shopping compounds in your favour exactly as the margin compounds against you.
- Never include a leg you would not bet as a single. If it is not worth £10 on its own, it is not worth being multiplied.
- Treat Crestly and Marlin’s acca promotions as what they are: a discount on a product that is expensive to begin with, applied automatically, honestly worded, and still a discount on the wrong thing.
