What this solves
Las Vegas sportsbooks post lines like "Chiefs -3.5 (-110)" or "Over 12 — 225 (-110)". A bettor sees the spread and price but usually doesn't translate it into "I need this team to win 52.4% of the time to break even, not 50%." This calculator does it: convert any American price into implied probability, calculate vig, model line-movement scenarios, and project total return on parlays of any leg count.
Two halves: a single-bet calculator (true probability needed, return, vig, line-movement scenarios), and a parlay calculator (compounding vig effect, true probability needed to break even at any leg count).
The calculator
Part A — Single bet
For vig calculation: the price the other side is paying
Part B — Parlay calculator
If you don't have an edge model, use the break-even price from Part A as a starting point.
How to read the result
- Implied probability converts American odds into the bookmaker's implied win rate. +150 = 40% implied; -110 = 52.4% implied.
- Vig (overround) = your implied % + the other side's implied %. If both sides are -110, total implied = 104.8%, meaning 4.8% vig. The bookmaker pockets this on a balanced book, even if their betting distribution is uneven.
- True probability needed to break even = your implied probability (since you lose your stake on a loss). At -110, you need to win 52.4% to break even. Anything below = the house takes the difference over time.
- Line movement: a 1.5-point line move before the game changes your effective edge. If you took -110 on the favorite and the line moves to -2.5 (1.5 points toward you), your true win probability increased by roughly 2.5–3% depending on sport. This is the sharp money signal.
- Parlay compounding vig: a 4-leg parlay at -110 each looks like +1237 (paying $1,237 on $100) but the true compounded break-even is roughly 57.5% per leg — not 52.4%. The bookmaker's true hold on a 4-leg parlay is roughly 12%, not 4.8%.
Limitations
- Sportsbook limits, line freezes, and "limit only" players are not modeled. The math is correct; the real-world execution isn't. Strip books and Circa have different limits and policies.
- Vig varies by market. NFL spreads are typically 4.5% vig (each side -110). NBA moneylines on -400 favorites may run 2–3% vig. NFL props can run 7–10% vig. This calculator assumes symmetric -110/-110; double-check vig on heavy favorites and parlays.
- Line-movement impact depends on point volatility per game. A 1.5-point NFL line move is meaningful; the same move in an MLB run line is smaller; in an NBA spread, larger. This calculator assumes NFL-style point value. Adjust for sport.
- The "win probability per leg" is the bettor's own model — the bookmaker has their own. The parlay break-even is the model's break-even, not the book's.
- Same-game parlays (SGPs) are a separate product with their own structure — bookmakers can correlate legs and price them at higher hold (8–12% effective) than multi-game parlays. This calculator doesn't separate SGPs.
FAQ
What's the real hold on a 4-leg parlay?
A standard 4-leg parlay at -110 each pays +1237 (about 12.4:1). The true compounded hold, given that you need 52.4% per leg, is around 12–14% — versus 4.8% on a single straight bet. Parlays are by far the highest-margin product for the bookmaker.
Does line-movement timing ever matter?
Yes — sharp bettors wait for the line to settle, then place bets after steam moves. Recreational bettors bet early. The book's ticket flow has to net out; if the public hammers one side, the book moves the line to attract balanced action on the other side. If you see the line move 1.5+ points against your pre-game position, you likely have a sharper opinion than the market.
Are -110 / -110 bets really 50/50?
No. Both sides at -110 implies 52.4% win each side. They are paying you as if the game is 50/50 but keeping 4.8% vig on a balanced book. The market expects one side closer to 55% / 45%, but the bookmaker quotes 52.4% / 52.4% to look balanced.
Should I avoid parlays entirely?
For sharp bettors, yes. For casual bettors, parlays convert small stakes into interesting payouts (which is the appeal), but the expected return is 10–14% worse than straight bets given the same model. Treat parlays as paid entertainment, not investment.
Sources
- Nevada Gaming Control Board sportsbook hold reports (statewide), 2024–2026
- Stanford Wong — "Sharp Sports Betting" (line-movement and vig methodology)
- Wizard of Vegas sportsbook parlay calculator reference, 2026 update
- Action Network / VSiN public line-movement data, 2024–2026