How Bookmaker Margins Work
A bookmaker margin (also called the vig, juice or overround) is the cut baked into the odds before you place a bet. It is the gap between the true probability of an event and the probability implied by the price offered. Across a sample of US markets we track, measured average margins run from roughly 4.2% at the sharpest books to 6.3% at the widest — a spread that directly changes your long-run payout. This guide shows the math, works a full numeric example, and ranks the operators by the margin data we actually recorded.
The core mechanic: implied probability and overround
Every price converts to an implied probability. In American odds, a favorite at -150 implies 150 / (150 + 100) = 60.0%. An underdog at +130 implies 100 / (130 + 100) = 43.5%.
Add those two implied probabilities together: 60.0% + 43.5% = 103.5%. In a fair market with no margin, the two outcomes would sum to exactly 100%. That extra 3.5 percentage points is the overround — the bookmaker's margin on that market.
The margin exists because the book prices each side slightly worse than its true chance. Over thousands of settled bets, that structural edge is what pays the operator regardless of individual results. Your job as a bettor is to find the markets where that overround is thinnest.
Worked example: how margin changes your payout
Take a two-way market — say an NBA moneyline. Assume the true, fair probabilities are 55% (Team A) and 45% (Team B).
A zero-margin book would price these at: - Team A: -122 (55% implied) - Team B: +122 (45% implied)
Now apply a 4.25% margin (the level we measured at DraftKings) versus a 6.35% margin (the level at BetRivers).
At a 4.25% margin, the implied probabilities are scaled up so they sum to ~104.25%. Team A prices near -133, Team B near +115. A $100 stake on Team B returns $215 total.
At a 6.35% margin, the two sides sum to ~106.35%. Team A prices near -140, Team B near +108. That same $100 winning bet on Team B returns only $208 total.
Same outcome, same stake, same true probability — but a $7 difference in payout on a single bet purely from the margin. Compound that across a season of hundreds of bets and the margin gap is the single largest controllable cost in betting.
Implied Probability Calculator
Measured margins across US sportsbooks
These are the average margins from our sampled US markets, not estimates. Lower is better for the bettor.
- Fanatics Sportsbook: 4.25% avg margin (466 markets sampled), 41.8% best-price share
- DraftKings: 4.25% avg margin (1,068 sampled), 54.0% best-price share
- FanDuel: 4.46% avg margin (596 sampled), 36.1% best-price share
- BetMGM: 5.21% avg margin (438 sampled), 19.9% best-price share
- Caesars Sportsbook: 5.27% avg margin (906 sampled), 36.2% best-price share
- BetRivers: 6.35% avg margin (480 sampled), 28.3% best-price share
DraftKings and Fanatics sit at the sharp end near 4.25%. BetRivers, at 6.35%, is roughly 210 basis points wider — meaning it holds back more of the true price on the average market. Note that bet365, ESPN BET and Hard Rock Bet are not shown here because we did not have sampled margin data for them; their editorial odds ratings (bet365 8.8, ESPN BET 7.3, Hard Rock Bet 7.3) reflect our scoring model, not the same measured overround.
Best-price share: margin isn't the whole story
Average margin tells you how a book prices the typical market. Best-price share tells you how often that book actually posts the top available number across the field.
DraftKings leads on best-price share at 54.0% — meaning in our sample it offered the best available price more often than any other book measured. Fanatics follows at 41.8%, with Caesars (36.2%) and FanDuel (36.1%) clustered behind.
Why the two metrics can diverge: a book can carry a slightly higher headline margin but still top the market on specific outcomes where it prices aggressively. That is why line shopping across two or three accounts beats loyalty to one book. If you hold accounts at the two operators with the highest best-price share — DraftKings and Fanatics — you capture the top posted price on a large majority of the markets we tracked.
Why margins vary by sport and market type
Margins are not uniform. Two-way markets like NBA and MLB moneylines, NHL puck lines and point spreads carry the thinnest overrounds because they are high-volume and easy to balance. Popular NFL and NBA spreads are typically the tightest a book offers.
Margins widen as markets get more granular. Same Game Parlays, player props, and multi-leg futures stack an overround on each individual leg, so the combined margin compounds fast — a four-leg parlay effectively multiplies four separate margins together. That is structurally why parlays are the most profitable product for sportsbooks and the most expensive for bettors.
Low-liquidity events — smaller tennis draws, niche soccer leagues, deep futures markets — also carry wider margins because the book has less balancing volume and prices in more protection. When you see a heavily juiced number, it is usually one of these market types.
How to use margin data when you bet
Three practical rules follow directly from the numbers:
1. Shop the line. Hold accounts at more than one licensed book and compare before every bet. With DraftKings (54.0% best-price share) and Fanatics (41.8%) covering most top prices, two accounts capture most of the available edge.
2. Concentrate on thin-margin markets. Main NFL/NBA/MLB/NHL lines carry lower overrounds than props and parlays. If you bet the highest-margin products, even the sharpest book keeps more of your money.
3. Convert every price to implied probability before betting. A number is only good if its implied probability is lower than your own estimate of the true chance. That single habit is what separates value betting from guessing.
US sports betting is regulated state by state — an operator licensed in New Jersey is not automatically legal elsewhere, and the minimum age is 21. Which of these books you can actually access, and the exact lines they post, depend on your state regulator.
FAQ
What is a bookmaker margin?
It is the built-in cut a sportsbook takes, also called the vig, juice or overround. It is the amount by which the implied probabilities of all outcomes in a market sum above 100%. In our US sample, average margins ran from about 4.25% to 6.35% depending on the operator.
How do I calculate the margin on a market?
Convert each price to an implied probability and add them together. For a two-way market, a -150 favorite implies 60.0% and a +130 underdog implies 43.5%; summed, that is 103.5%, so the margin is 3.5%. Anything above 100% is the book's overround.
Which US sportsbook has the lowest margin?
In our sampled markets, DraftKings and Fanatics Sportsbook tied for the lowest measured average margin at 4.25%, followed by FanDuel at 4.46%. BetRivers carried the widest at 6.35%. These are measured figures from tracked markets, not every book had sampled data.
Does a lower margin always mean a better price?
Not on every single bet. Average margin describes the typical market; best-price share shows how often a book posts the top number. DraftKings led best-price share at 54.0%, so combining low margin with high best-price share is what identifies the strongest pricing.
Why are parlays and props more expensive than straight bets?
Each leg of a parlay or Same Game Parlay carries its own margin, and those margins compound when combined. Player props and niche markets also have less balancing volume, so books price in extra protection. Main two-way lines on NFL, NBA, MLB and NHL carry the thinnest overrounds.
Can I bet on any of these sportsbooks in my state?
Only if that operator holds a licence in your state. US betting is regulated state by state, the minimum age is 21, and availability and exact odds depend on your state regulator such as the NJ DGE, NYSGC or PGCB.