Betting Odds Explained: American, Decimal, and What They Really Mean

Open any sportsbook and you’ll see two numbers stacked next to a matchup: something like -150 on one team, +130 on the other. Most bettors know instinctively that the minus number is the favorite and the plus number is the underdog. Far fewer could tell you what -150 actually costs, what probability it implies, or why -150 and +130 don’t describe a fair coin flip at all. They’re not supposed to — the gap between what a fair bet would look like and what the book actually offers is called the vig, and it’s the single most important concept in sports betting math. Understand it and every other number on the board starts making sense.

American odds: reading the minus and the plus

American odds tell you two different things depending on the sign, and the sign is the whole trick.

A negative number is how much you must risk to win $100. -150 means: risk $150 to win $100 profit, for a total return of $250 if the bet hits.

A positive number is how much you win on a $100 risk. +130 means: risk $100 to win $130 profit, for a total return of $230 if the bet hits.

Notice what’s happening: the favorite (-150) has to risk more than it can win, because it’s expected to win more often. The underdog (+130) risks less than it stands to win, because it’s expected to win less often. The size of the number scales with how lopsided the book thinks the game is — a -400 favorite is a much bigger favorite than a -150 one.

Converting to decimal odds

Decimal odds (common outside the US, and increasingly shown alongside American odds on US books) collapse both cases into one formula: multiply your stake by the decimal to get your total return, not just your profit.

Convert American to decimal:

• Favorite (negative): decimal = 1 + (100 / |odds|). For -150: 1 + (100/150) = 1.667
• Underdog (positive): decimal = 1 + (odds / 100). For +130: 1 + (130/100) = 2.30

Check the math against the American version: risk $100 at 1.667 decimal odds returns $166.70 total — but at true -150 American odds you’d have risked $150 to get $250 back, which is also a 1.667x multiple ($250/$150 = 1.667). Same bet, same payout ratio, different notation.

Implied probability: what the book thinks will happen

Every price implies a probability. The formula is simple: implied probability = risk / (risk + win).

For -150: 150 / (150 + 100) = 60.0%

For +130: 100 / (100 + 130) = 43.5%

You can get the same number from decimal odds with 1 / decimal: 1/1.667 = 60.0%, 1/2.30 = 43.5%.

This is the number that matters. The odds themselves are just a payout schedule; implied probability is the book’s actual claim about how often that outcome happens. If you think a team is a true 65% favorite and the market only implies 60%, you’ve found a bet worth making — assuming your 65% is right, which is the entire skill of betting.

The vig: why the two sides never add to 100%

Add the two implied probabilities above: 60.0% + 43.5% = 103.5%. That’s already more than certainty — one of those two teams will win, so a fair market would sum to exactly 100%. The extra 3.5% is the vig (also called the juice or the overround): the book’s built-in margin, baked into the price on both sides of every bet.

The cleanest place to see it is a pick’em game, priced -110 on both sides — the most common number on point spreads and totals (see our point spread guide for how that market works):

• -110 decimal odds: 1 + (100/110) = 1.9091
• -110 implied probability: 110 / (110 + 100) = 52.38%
• Both sides: 52.38% + 52.38% = 104.76%

You’ll often hear “the vig is about 4.5%” as a rule of thumb — and it’s a reasonable one as a blended average across markets, some of which run tighter. But the exact number on the most common bet in sports betting, a standard -110/-110 line, is 4.76%, not 4.5%. That gap between the number people repeat and the number the math actually produces is the whole reason it’s worth doing the calculation yourself instead of trusting folklore.

That 52.38% figure is also your breakeven win rate at -110: pick winners less often than that, long-term, and you lose money even if you’re “right more than wrong” in some looser sense.

Why “both sides look good” is impossible

Every bettor has felt it: you look at a matchup and both teams’ prices seem reasonable. That feeling is designed into the product. Because the two implied probabilities always sum to more than 100%, the book has already collected its cut from whichever side wins — it doesn’t need you to be wrong, it just needs the market’s combined price to exceed fair value, and it always does by construction. A two-sided market where both sides carry genuine positive expected value for the bettor doesn’t exist within a single sportsbook; the only way to find a mispriced side is to believe the true probability differs from the market’s implied one, in your favor, on at least one side.

How odds move, and what a move actually means

Odds aren’t static. A game might open at -150/+130 and drift to -170/+150 by game time. Convert the new number: -170 implies 170/270 = 63.0% — up from 60.0%. The market now believes the favorite is more likely to win than it did when the line opened.

Line moves happen for two broad reasons: money (enough bets came in on one side that the book shifts the price to balance its liability — often called “sharp action” when it’s driven by well-informed bettors rather than public volume) and information (an injury, a lineup change, weather). A number moving toward your side after you bet it is one of the few objective, in-the-moment signals that you got a better price than the closing market — a concept called closing line value, and it correlates with long-term profitability better than any single result does.

What this means for your betting

• Always compute implied probability before judging a bet — the odds alone don’t tell you what the book thinks will happen; the formula does.
• Don’t trust the “~4.5% vig” rule of thumb as gospel — run the actual numbers on the specific price you’re being offered.
• A bet is only worth making if your estimate beats the implied probability, not if the team just seems likely to win.
• Track how the line moved after you bet — it’s one of the few free signals about whether you got a good number.

All formulas above are standard, publicly available sportsbook math and apply to any book or market — they’re not specific to any single site’s pricing. For how SharpBetz turns model probabilities into published picks, see how our model works; for the actual results those picks have produced, see our results page.