How sportsbooks actually set their lines

Where an opening number comes from

Most books do not originate their numbers. A small number of sharp, low-margin operations price the market first and accept large bets early, and the rest of the industry follows within minutes, applying its own margin. That is why lines across a dozen sportsbooks look so similar: they are largely the same underlying number wearing different amounts of vig.

The originators build openers from models, injury and roster information, scheduling and rest, and market history. The opener is deliberately posted with lower limits, because it is the number they are least confident in — an opener is a first offer, not a forecast.

Why lines move, and whose money moves them

Lines move when new information arrives — an injury, a lineup, weather — and when money arrives. But books do not weight all money equally. Bets from accounts with a demonstrated history of winning move a number far more than the same amount from accounts without one, and a large bet from a respected account can move a line where a hundred small bets do not.

This is the practical reason the closing line is so accurate. By the time a market closes it has absorbed everything the informed side of the market knows, expressed as money. Beating that number consistently is difficult precisely because it aggregates the judgement of everyone who was willing to back their opinion.

The balanced-action myth

The familiar explanation is that books move lines to balance money on both sides and collect the vig risk-free. That describes a small part of reality. Books routinely carry large unbalanced positions when they believe their number is right, and on most markets the action never balances anyway — the public has persistent preferences for favourites, overs and popular teams, and books are content to take the other side.

The more accurate description is that a book manages risk against its own estimate of the true price, tightening or moving when informed money says its number is wrong and happily absorbing imbalance when it thinks the number is right. The margin in the price is the business model; the line management protects it.

What this means for reading a line

Treat the sharpest available price as the market’s best estimate and everything else as that estimate plus margin and lag. A price that differs materially from the sharp number is usually a slower book rather than an opportunity, and the difference tends to disappear before it can be used at any size.

It also means the useful question about any bet is not "will this win" but "is this number wrong, and wrong enough to survive the margin". Most of the time it is not, which is the honest reason most bettors lose.

Common questions