Accumulators and parlays: how the odds multiply
ベッティングスクール · 公開日 · 読了目安5分
この記事は英語のみです。
An accumulator, called a parlay in North America, links several selections into one bet. Every selection must win for the bet to pay. The appeal is obvious: small prices multiply into a large one. The catch is just as real, and understanding both sides will help you decide whether this bet type belongs in your routine at all.
How the odds multiply
In decimal odds, the price of an accumulator is the product of all its legs. Take three selections at 1.80, 2.10 and 1.65.
- 1.80 x 2.10 = 3.78
- 3.78 x 1.65 = 6.237, which is about 6.24
A stake of 10 on this three-leg bet returns 62.37 if all three win, a profit of 52.37. If any one leg loses, the whole stake is gone. You can check any combination with the accumulator calculator, and if you are unsure about a price format, the odds converter will translate it.
The same maths for probability
Odds multiply, and so do probabilities. If you believe the three events above have true chances of 55%, 47% and 60%, the chance that all three happen is 0.55 x 0.47 x 0.60 = 0.155, or 15.5%. That is roughly one in six and a half. The price of 6.24 implies about 16% (1 / 6.24), so in this example the bet is close to fair before any margin is considered.
Notice how fast the chance shrinks. Even five legs that each look likely at 70% combine to only 0.70 to the power of five, which is about 16.8%. A bet made of "safe" picks is rarely safe once combined.
How the margin compounds
Bookmakers build a margin into every price, as explained in the guide to bookmaker margin and overround. On a single bet this margin might be around 5%. When you combine legs, you pay it on each one, and the effects multiply rather than add.
Suppose a fair price for an event is 2.00, but the bookmaker offers 1.90. That shaves about 5% off the payout. Now stack four legs like that.
- Fair combined price: 2.00 x 2.00 x 2.00 x 2.00 = 16.00
- Offered combined price: 1.90 x 1.90 x 1.90 x 1.90 = 13.03
The bookmaker is paying 13.03 for something that should pay 16.00, a shortfall of roughly 19%. With a single bet you gave up 5%. With four legs you gave up nearly a fifth of the fair payout. This is the main reason accumulators rarely beat singles in the long run: the more legs, the larger the built-in edge against you.
A worked expectation example
Expected value is the average result per unit staked if you could repeat a bet many times. Using the four-leg example, assume each event truly has a 50% chance, so all four land with probability 0.5 to the power of four, which is 6.25%.
- Stake 10 at 13.03: win 120.30 profit (return 130.30) with probability 6.25%.
- Otherwise lose 10 with probability 93.75%.
- Expected result = 0.0625 x 120.30 - 0.9375 x 10 = 7.52 - 9.38 = about -1.86
You lose about 1.86 per 10 staked on average, an expected loss of roughly 18.6%. A single bet at 1.90 on the same 50% event has an expected result of 0.5 x 9 - 0.5 x 10 = -0.50 per 10 staked, a loss of 5%. You can run your own numbers in the expected value calculator, and the article on what expected value means explains the idea in more detail.
The risks in plain terms
- High variance. Accumulators lose most of the time. You can go many bets without a return, which makes it hard to judge whether your picks are any good.
- Compounding margin. As shown above, each leg costs you extra.
- Correlation traps. Legs that depend on each other, such as a team to win and the same team to score first, are not independent. Bookmakers know this and often restrict or reprice such combinations.
- Cash-out temptation. Offers during the bet are priced with a margin too, so using them is not free.
When an accumulator can make sense
There is nothing wrong with a small accumulator as entertainment, provided the stake is a sum you would happily lose. If you want to build one, keep it short. Two or three legs keeps the margin drag modest. Choose selections where you have a genuine reason to think the price is generous, rather than picking the shortest odds. You can experiment with combinations in the accumulator builder to see the combined price and implied probability before you commit.
If your aim is steady, long-term decision-making, singles are the cleaner tool. Each bet stands on its own, your records show exactly where your edge or lack of one comes from, and the margin is paid once. Many careful bettors use accumulators rarely, and only with a small share of their bankroll. Our guide to bankroll management shows how to size stakes so that a losing run does not threaten your whole budget.
Key takeaways
Accumulators multiply both the odds and the margin. The payout grows quickly, but the chance of winning shrinks even faster, and no combination of picks is ever guaranteed to win. Keep them small and treat them as entertainment, not a plan for profit.
Betting is for adults only (18+). Only stake money you can afford to lose, set limits before you start, and stop if it stops being fun. If gambling is causing you problems, contact a local support organisation.