Calculate the expected value of any parlay (accumulator). See how bookmaker margin compounds across legs and whether the combined bet is +EV or -EV.
Parlays (also known as accumulators or combos) are one of the most popular bet types in sports betting — and one of the most misunderstood. A parlay combines two or more individual selections into a single bet, where all legs must win for the bet to pay out. The combined odds are the product of each leg's decimal odds, which creates the illusion of enormous payouts. But behind those eye-catching odds lies a mathematical reality that most bettors overlook: the bookmaker's margin compounds with every leg you add.
Consider a single bet at odds of 1.90 where the true probability is 55%. The bookmaker's implied probability is 52.6%, so there's a 2.4% edge in your favour. That's a solid +EV bet. Now imagine you add a second leg at the same odds and probability. Your combined true probability is 0.55 x 0.55 = 30.25%, and the combined parlay odds are 1.90 x 1.90 = 3.61. The combined EV is (0.3025 x 3.61) - 1 = +9.2%. Sounds great, right? But the combined implied probability is 1/3.61 = 27.7%, which means the compounded margin has grown from 2.4% to 2.55 percentage points. The margin doesn't just add — it multiplies.
Each bookmaker embeds a margin (also called vig, juice, or overround) into their odds. On a standard -110/-110 market, the margin is about 4.5%. When you parlay two such markets together, the effective margin becomes roughly 9%. Three legs push it to about 13.5%. By the time you reach a 6-leg parlay, the house edge can exceed 25%. This is why bookmakers actively promote parlays — they are significantly more profitable for the house than singles.
The mathematical explanation is straightforward. If a single leg has an implied probability that's inflated by margin m, then for n legs the compounded margin factor is approximately (1 + m)^n - 1. With a 4.5% margin per leg and 4 legs, the compounded margin is (1.045)^4 - 1 = 19.3%. The bookmaker's expected profit on a 4-leg parlay is nearly four times their profit on a single bet.
Despite the margin compounding effect, parlays can still be profitable under the right conditions. If each individual leg has sufficient positive expected value, the combined parlay can overcome the compounded margin. The key insight is that the edges also compound. A 2-leg parlay where each leg has +5% EV individually will have a combined EV greater than +5% — because you're multiplying probabilities that are each above the implied probability. The parlay amplifies both your edge and the bookmaker's margin; the question is which one is larger.
The rule of thumb for professional bettors is simple: only parlay bets that are individually +EV. Never parlay bets just because the combined odds look attractive. And be aware that the more legs you add, the more edge you need per leg to overcome the compounding margin. A 2-leg parlay with +3% EV per leg is reasonable. A 6-leg parlay requires significantly stronger edges on each leg to remain profitable in the long run.
There is one scenario where parlays offer a genuine structural advantage: correlated outcomes. When two outcomes are positively correlated (for example, a team winning and the game going over the total, because the team's win probability increases when they score more), the true combined probability is higher than the product of the individual probabilities. Most bookmakers price parlay legs as if they're independent, which means correlated parlays can contain hidden edge that doesn't exist in the individual legs.
However, many sharp bookmakers have become aware of this and now restrict or reprice correlated parlays. The opportunity still exists at softer books, but it requires careful analysis. Our recommendation is to use this calculator to evaluate every parlay before placing it. If the combined EV is negative, the parlay is destroying value regardless of how appealing the individual legs look. Stick to singles unless you have a clear mathematical reason to combine.