A prop is a subject, a number and a price — at −110 a side, the price alone says 52.4%.
- The subject. A player and one stat — the claim's who and what.
- The number. The midpoint the book expects to split the outcomes in half.
- The price. What a side costs. The only part that states the odds.
- The other side. Priced too, so the two can be added together.
- What they add to. Over 100% — the overhang is the book's cut.
Favourite — −110 implies 52.4% before the vig comes out.
The number
The line is a forecast, not a coin flip — it is the level at which the book expects the result to land roughly half the time.A passing-yards line of 274.5 is not a guess that he will go for exactly 274 — it is the level the book believes splits the outcomes down the middle. Set it too high and everyone hammers the Under; set it too low and the Over gets pounded. The book’s incentive is balance, so the posted number is the most informed midpoint available at that moment.
The odds and the vig
Vigorish is the rent the book charges to take both sides; at -110 each way the house keeps about 4.8% no matter who wins. A −110 price implies a 52.4%break-even win rate, but the true probability of a coin-flip line is 50% — that gap on each side is the house’s cut. Across both sides the implied probabilities add to 104.8%, and that overage is what the book keeps no matter who wins. The converter above turns any price into its break-even rate instantly.
The half-point
A line ending in .5 exists to kill the push — there is no way to land exactly on 24.5, so every bet resolves Over or Under. A whole number like 24 leaves room for a push — land on exactly 24 rebounds and the bet refunds. The hook (.5) removes that. It also lets the book fine-tune: moving a line from 24.5 to 25.5 is a full point of demand management, while changing the juice from −110 to −120 nudges the same number without touching it. When the number holds but the price shifts, that is a juice move, not a real line move.
The true probability
Strip the vig off both prices and the implied probabilities add back to 100% — that no-vig number is the market's honest estimate. De-vigging is the move: take the implied probability of each side, divide each by their sum, and you get the no-vig fair probability. For a −110 / −110 market that is 50% a side. Once you have the fair number, your edge is your own estimate minus the fair line.
Worked example · illustrative · Mahomes passing yards, 274.5
Suppose the analysts who set the number looked at his season mean, the defense he faces, the game total and the weather, and landed on 274.5. The book posts it two ways and charges juice on both: Over 274.5 −110 and Under 274.5 −110. Here is what those prices say.
The fair line says 274.5 is a true coin flip. Your only job from here is to decide whether your own read on Mahomes beats 50% by enough to overcome that 4.5% rent. If your model says 278, the Over has value; if it says 272, the Under does. The number was never the question — the gap between your number and the fair number is.
The book's cut — the book keeps 4.55% of balanced action on this market.
As the book prices it
adds up to 104.8%Fair, cut removed
adds up to 100%4.76% over 100% is the book’s cut — 4.55% of everything staked, if the action is balanced.
Game context
Injuries, matchup rank, rest, and weather move a prop before the public ever sees it — the posted number already prices most of it in.A starter ruled out can swing a teammate’s usage line several points; a back-to-back saps minutes; a top-ranked defense caps a ceiling; wind and cold flatten passing and total markets. The book already knows all of this. By the time a prop is public, the obvious context is priced in — the edge lives in the context the market has mis-weighted, not the context it ignored.
The line move
When the number moves against the side the public is hammering, that reverse move is the sharpest signal a prop gives you. Measure it from the open; the sharp-money guide says what a move proves and what it cannot.
Putting it together
A prop is six things at once: a forecast, a price, a half-point that forces a decision, a hidden true probability, a context already half-baked in, and a line that moves when someone disagrees. Read them in order and the question stops being “will he go over?” and becomes “is the market’s number wrong, and by how much?” That second question is the only one with money in it.
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The Mahomes prices are illustrative, not a quote. Research and methodology only — Dynatyze is not a sportsbook and takes no wagers.