How Sportsbooks Price Alternate Run Lines

An alternate run line is priced by working out how often a game finishes inside each extra run you add or take away. MLB margins bunch up at one, two and three runs, so each half-run step buys or sells a slice of a different size, and the book's margin is not spread evenly across the menu.

You meet this every time you open the alt run line tab and see -2.5 at +375 sitting next to -1.5 at +260. The concept decides whether that bigger number is a fair trade or a worse bet with better marketing. My position is plain: the standard 1.5 is usually the cheapest number on the board. Leave it only when a specific rung survives the arithmetic below.

Start with the fixed point. The standard run line is 1.5 runs and, as Covers explains, that number essentially never moves. Books adjust only the price. Everything on the alt menu is a repricing away from that anchor.

The fair price comes from two inputs. The first is the no-vig moneyline. The second is the game total, because a 10-run environment produces wider margins than a 7-run one. Michael Shackleford's run line tables at Wizard of Odds turn those two numbers into a fair run line using a logistic regression on MLB games from 2000 to 2009. They also keep home and away underdogs separate, because a home team can win on a walk-off and never bat in the bottom of the ninth when already leading. In his words, "It is more valuable to get the extra 1.5 runs on an away team."

Next is the margin distribution. Betting for Baseball estimates that about 28% of MLB games are decided by one run, about 18% by two, about 14% by three, and the remaining 40% or so by four or more. Moving a favorite from -1.5 to -2.5 throws away its two-run wins. Moving an underdog from +1.5 to +2.5 adds back its two-run losses. Because the slices shrink as you move outward, the first half-run you buy or sell is worth far more than the third. That is the nonlinearity, and it is why a one-run step can swing the odds so hard.

The last input is the vig. Implied probability is |odds| / (|odds| + 100) for a negative price and 100 / (odds + 100) for a positive one. At -110 on both sides the two probabilities add up to more than 100%, and that excess works out to a hold of roughly 4.8%, per the Wizard of Odds pricing series. The same series puts exotic props at a 10 to 20% hold. Alternate run lines tend to drift toward that end, for a reason the Betting for Baseball team states directly: "Alternate lines carry wider margins than standard markets because fewer sharp bettors trade them."

The obvious defense of the deep rungs is the one Griffin Murphy makes at Doc's Sports: at long odds, "A miniscule 28% win rate will still result in an overall profit." That is true. But it is only true if your real probability beats the break-even. A low hurdle that the book has set even lower is not an edge.

The second ladder shows where the hold goes. SportsCapping gives illustrative prices between two evenly matched teams of about +350 at -3.5 and about -700 at +3.5. Those convert to 22.2% and 87.5%, which sum to 109.7%. The -110/-110 standard sums to about 104.8%, so the margin on that alt number is roughly double. The same thing happens one level up: in a Eutaw Street Report example, a 10-cent moneyline of -130/+120 becomes a 20-cent run line of -170/+150 for the same game.

The third case is a fair-value check. In the Wizard of Odds Yankees example, a +130 fair moneyline with a 9.5 total gives a fair +1.5 price of -152. A book offering -170 instead is holding 4.20% on that bet. You can only see that number once you have a fair price to set against the posted one.

An alternate run line is a separate market you choose. It is not the run line shifting. When Covers describes +160 moving to +150, that is the book rebalancing juice on the fixed 1.5 to manage handle. The alt menu is a different product with its own margin.

A big cushion is not cheap insurance either. At -700 on +3.5 you risk $700 to win $100. One loss wipes out seven wins.

Run four checks before you bet any rung.

1. Convert every rung to a break-even percentage and look at the steps. They should shrink as you move away from 1.5, roughly in line with the 28, 18 and 14% slices. A step that is out of line points to the rung worth questioning.
2. Where a book posts both sides of the same alt number, add the two implied probabilities. Anything well above the standard market's 104.8% tells you what that rung costs.
3. Benchmark against a fair price built from the no-vig moneyline and the total, and remember the home and away difference.
4. Check the matchup tier. ProComputerGambler's data shows -1.5 covering only 39.0% for -110 to -130 favorites, against a 52.9% straight-up win rate. As that piece puts it, "A bet can look attractive at one number and become poor at another."

Alt menus differ from book to book more than standard lines do, and that is exactly the case for comparing prices across sportsbooks before you commit to a rung.