Compare the current spread/total price to an alternate line offered at a different price. The calculator shows the implied "cost" of the half point in cents and flags NFL key numbers (3, 7, 10, 14) where half-point moves carry disproportionate value.
Sportsbooks offer alternate spreads and totals at different prices. Moving the spread in your favor by half a point makes the bet easier to win but costs more (worse odds). Moving against you makes it harder but pays better odds. The calculator measures the implied cost of the move in cents and as an implied-probability delta — the swing in win probability that the price shift represents.
The decision rule: a half-point buy is +EV when the actual probability gain (how often the new line wins vs the old one) exceeds the implied-probability delta the book charges. Most half-point buys away from key numbers are slightly −EV but small; the book's cushion is small enough that it can be worth the certainty bump on close lines.
NFL games end on certain margins much more often than others. About 15% of NFL games end with a margin of exactly 3 (one field goal). Margins of 7, 10, and 14 are also disproportionately common. A half-point buy that crosses 3 (e.g. -3 to -2.5) is materially more valuable than one that doesn't cross a key number (e.g. -2 to -1.5), because the new line wins on every game that finishes exactly at 3 — a real, measurable share of outcomes.
Sportsbooks know this and charge premium prices to buy off 3 or onto 3 in either direction. Disciplined bettors only buy half points when crossing key numbers (or when the offered price under-charges for the move).