Where the number comes from
A parlay price is the product of its legs. Four selections at 2.00 give 2.00 Γ 2.00 Γ 2.00 Γ 2.00 = 16.00, and a β¬10 stake returns β¬160. That much is familiar.
What is less familiar is that the margin multiplies too. Every leg has the bookmaker's cushion baked into its price, and multiplying the prices multiplies the cushions along with them.
The compounding, with numbers
Take a book running a 5% margin per leg β respectable for football. On a single bet you are giving up about 5% of expected value.
On a four-fold, the effective margin is roughly 1 β (0.95)β΄ β 18.5%. On a six-fold it is around 26.5%. You have not made six 5% bets; you have made one 26.5% bet.
Check it directly: price the parlay with the parlay calculator, then strip the vig from each leg with no-vig fair odds and multiply the fair prices instead. The gap between the two totals is what the accumulator costs you, and on longer slips it is startling.
Correlation cuts both ways
Multiplying prices assumes the legs are independent. Real sporting outcomes often are not. Backing a team to win and the match to go over 2.5 goals are related events β a dominant favourite makes both more likely at once.
When legs are positively correlated, the true probability of them all landing is higher than the product suggests, which means the standard parlay price is too short for the bookmaker's comfort. This is exactly why books restrict or reprice obvious same-game combinations. Where correlation genuinely exists and is not priced in, it is one of the few structural arguments for a multiple β the correlated parlay calculator handles that case.
Negative correlation is the trap: two legs that pull against each other make the slip far less likely than the multiplication implies.
Round robins do not solve it
Splitting six selections into a set of doubles and trebles feels like risk management, and in one sense it is: you no longer need all six.
But every one of those combinations still carries compounded margin, and you are now paying it many times over. A round robin calculator will show the full combination count and total stake. Smoother variance, same underlying leak β often a larger one.
When a multiple is defensible
Three honest cases. Genuine correlation that the book has not priced. A bonus or price boost whose value exceeds the compounded margin β worth checking with the bet boost comparison before assuming. And entertainment, staked at a size you would be content to lose.
What does not work is the common reasoning: that a long parlay is a cheap route to a big return. It is a large return with a large, multiplied fee attached, and the fee is why bookmakers advertise accumulators rather than single bets on their front page.