MLB Parlay Builder

Why a parlay pays less than it looks like it pays

A single bet at −110 is priced so that both sides together imply about 104.8% of the outcomes, not 100%. That extra 4.8 points is the margin built into the price structure. Take the margin out and a −110 side is a coin flip: 50%, not the 52.4% its price implies.

Now multiply. A parlay's price is the product of its legs' decimal prices, and its true probability is the product of the legs' true probabilities. So the margin does not add up across legs, it compounds. If each leg returns 95.45 cents of a fair dollar, then two legs return 91.11 cents and three legs return 86.97 cents. In shorthand, a ticket of n legs each priced at decimal d against a true probability p returns (p × d)ⁿ of a fair payout. Every leg you add multiplies the shortfall by another factor.

The part that inverts

A +600 ticket breaks even at a 14.29% hit rate, and the three −110 legs above break even at 14.37%. A hit rate of 50% on tickets like that would be a spectacular result. But 50% of three-leg parlays is not a 50% skill level on the legs. It is 79.4% on every single leg, because 0.794 cubed is 0.50. The prices themselves imply 52.4% per leg. The payout looks like it is rewarding a long shot, while the hit rate needed to reach a coin-flip strike rate on the ticket is far above anything the individual prices describe. That is the inversion: the ticket gets easier to be paid well on and much harder to win, at the same time.

Why parlays get promoted

Not because anything is hidden. The prices are all posted in the open, and the arithmetic above uses nothing but them. It is that the margin per ticket grows with the number of legs while the stake does not. A book holding roughly 4.5% on a straight side holds roughly 13% on a three-leg version of the same sides. That is the whole mechanism, and it is why the tool above prints the offered payout and the fair payout side by side instead of just the payout.

What the compounding measures out to in our own archive

The arithmetic says margins multiply. Our odds archive says how much they multiply by in practice. The figures below come from a committed script over a read-only session on our own lake: every pre-game MLB moneyline our board captured, and then every pair and every triple of different games the same book was pricing at the same instant.

MLB moneylines, pre-game captures only, 2025-02-23 to 2026-08-14 · 11 books · 4,372 games · one slate per book per ET day (that day's earliest board run) · row 1 counts posted two-sided markets, rows 2 and 3 count every pair and every triple of different games a single book was pricing at one instant · median, not mean · committed script: seo/f78_parlay_hold_facts.py

Hold is the share of the payout the price structure keeps. Read down the column: 3.98% on one moneyline becomes 7.86% on two and 11.61% on three, which is what multiplying prices does. It is close to what the arithmetic predicts and not identical to it, because the median of a product is not the product of medians: raising the one-leg median to the third power predicts 112.94 and the measured three-leg median is 113.13.

For context, our hedge guide measured the same quantity across every sport we hold and found a median of 4.16% across 711,756 two-way quotes. MLB moneylines are tighter than that average, at 3.98%. Two other things worth stating plainly: 10.3% of the MLB price rows in this window were captured after first pitch, and every one of them is excluded here, because an in-play price is not a price you could have put in a pre-game parlay. And the worked example above uses a −110 pair, which holds 4.55%; real MLB moneylines hold less than that. The compounding is the same either way.