The Point Spread Calculator turns a spread, an odds quote, and an optional final score into the four answers sports bettors actually need: did the bet cover, what does it pay, what win rate would I need to break even, and how much of my bankroll should I risk? This guide explains the math behind each output, how American odds work, why a small move across a key number can flip the result of one in seven NFL games, and how Kelly Criterion sizing protects a bankroll from the variance that wipes out most casual bettors.

How spread betting works

A point spread is a margin-of-victory handicap that levels two unequal teams. The favorite (negative spread) must win by more than the spread for a bet on them to cover. The underdog (positive spread) wins the bet if they lose by less than the spread or win outright. Both sides of a standard spread are usually priced around -110, meaning you risk $110 to win $100. That -110 quote is the sportsbook's commission, the vig, and it's the single most important number for a long-term bettor to understand — it determines the win rate you must clear to stay even.

When the spread is a whole number (e.g., -7 or +3), the bet can push: the margin lands exactly on the spread and your stake is refunded. Sportsbooks deliberately use half-point spreads (-6.5, +8.5) on most lines to eliminate pushes, because pushes reduce the book's hold.

Reading American odds and implied probability

Negative American odds tell you how much you must risk to win $100. -110 means a $110 stake wins $100 in profit. -200 means $200 wins $100. Positive American odds tell you how much $100 wins. +120 means a $100 stake wins $120; +250 means $100 wins $250. The two formulas are mirrors of each other, and the calculator handles both automatically.

Every American odds quote also encodes an implied probability — the win rate at which the bet exactly breaks even. The shortcut formulas are |odds| / (|odds| + 100) for negative odds and 100 / (odds + 100) for positive odds. At -110 the implied probability is 52.38%, meaning you must win 52.4% of -110 bets just to break even after the vig. At -120 the break-even rises to 54.5%. Tracking this number across bets is how serious bettors decide whether a line is worth taking.

Key numbers and the value of half-points

NFL final margins do not occur uniformly. Margins of 3 and 7 are dramatically over-represented (roughly 15% and 9% of all games respectively), and 10, 6, 4, and 14 are also key. Buying a half-point that crosses one of these key numbers — moving a spread from -3 to -2.5, or from -7 to -6.5 — can shift the cover probability by several percentage points. Whether the move is worth the price depends on the new juice: moving -3 (-110) to -2.5 (-130) historically pays off, but the same buy off less-common numbers like -8 typically does not.

NBA key numbers cluster around 5, 6, and 7. College football has a flatter distribution but still favors 3, 7, and 10. The Compare tab lets you eyeball the trade-off between alt-line spreads and their associated juice.

Bankroll discipline and the Kelly Criterion

The Kelly Criterion, derived in 1956 by Bell Labs researcher John Kelly Jr., specifies the bankroll fraction that maximizes long-run logarithmic growth given a known edge. The formula is (b·p − q) / b where b is net profit per $1 risked, p is your honest probability estimate, and q = 1 − p. With a 55% win estimate at -110 odds, full Kelly recommends about 5.5% of bankroll per bet.

Most professional bettors use Half Kelly or Quarter Kelly instead. The reason is variance: full Kelly's expected drawdown — the peak-to-trough loss you should expect at some point — is dramatic. Half Kelly captures roughly 75% of full Kelly's growth rate while cutting variance roughly in half. If your edge estimate is at or below the implied probability of the line, Kelly correctly returns zero — the optimal action is to skip the bet, no matter how strong the team-loyalty pull.