Parlay Boost Calculator
Books love to dangle a '+20% boost' on parlays. Add your legs and the boost and see the truth: each leg's margin compounds, so most boosts still leave the bet -EV.
e.g. 20
margin baked in (≈4.5)
A parlay multiplies each leg's odds — and each leg already hides the book's margin, so the vig compounds with every leg you add. Four legs at a routine 4.5% margin put you about 16% behind fair value before the promo even applies. That is why a "+20% boost" is so seductive and so rarely enough: it has to out-run the stacked vig just to reach break-even, and most advertised boosts (10–15%, often on longer-odds slips where the margin is even fatter) don't. Boosts aren't gifts; they're the most profitable product on the menu, engineered to feel like value while staying -EV.
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