How Sportsbooks Set Lines

Understanding where a number comes from tells you when to trust it and when to disagree. The common belief — that books set lines to attract equal money on both sides — is mostly a myth, and acting on it leads to bad conclusions.

Where An Opening Number Comes From

Most sportsbooks do not originate their own numbers. A small group of market-making operations post first, at low limits, and everybody else copies with adjustments.

The originating number comes from a power rating model — a numerical rating for each team, adjusted for home field, rest, personnel and situation. The difference between two ratings plus adjustments produces the projected margin, which becomes the opening spread. See power ratings.

Openers are deliberately posted at low limits, because the book knows its model is imperfect and wants informed money to help correct it cheaply. Early bets from respected accounts move the number quickly, and by the time limits rise the number reflects both the model and the market's information.

Shading, Not Balancing

The textbook description of a sportsbook has it taking equal action on both sides and collecting the margin risk-free. Real books rarely operate that way.

They know recreational money is systematically biased: towards favourites, towards overs, towards popular teams, towards parlays. Rather than moving numbers until both sides are even, they shade prices towards that flow and carry the exposure, because the exposure is profitable in expectation.

A game where 80% of tickets are on the favourite may have a spread half a point worse than fair, with the book happy to hold the other side. That is not a mistake; it is the business model.

The consequence for a bettor: the unpopular side is systematically slightly cheaper than fair. Not enough to bet blind, but enough that it should tilt marginal decisions. See public betting percentages.

Market Makers Versus Retail Books

The practical implication is that a market maker's closing number is the best available estimate of true probability, which is why closing line value is measured against it rather than against a retail book. See closing line value and account limits.

What Moves A Number After It Opens

Respected money. Bets from accounts the book considers informed move numbers disproportionately to their size. A five-figure bet from a known winner moves a line more than the same amount spread across a thousand recreational tickets.

Information. Injury news, weather forecasts, lineup announcements and starting pitcher changes.

Public volume. Moves numbers, but less than most people assume, and books often prefer to shade the price rather than the handicap.

Other books. Most operations follow the market. When a market maker moves, the rest follow within minutes, which is why a stale number at a slow book is a genuine if short-lived opportunity.

Distinguishing informed movement from public movement is the skill described in steam moves and reverse line movement.

Frequently Asked Questions