Odds basics · 5 min read
Every betting price hides a percentage. Here's the two-line formula for American odds, why -110 means 52.38%, and how to use it to see what a price is really asking you to believe.
Odds are just probabilities wearing a costume. A line like -150 or +220 tells you how much you win, but it also tells you how often the book thinks that outcome happens. Converting one into the other is the first skill that separates guessing from pricing.
implied % = |odds| ÷ (|odds| + 100). For -150: 150 ÷ 250 = 60%.implied % = 100 ÷ (odds + 100). For +220: 100 ÷ 320 = 31.25%.implied % = 1 ÷ decimal. For 2.50: 1 ÷ 2.50 = 40%.That's the entire conversion. +100 (even money) is 50%, -200 is 66.67%, +300 is 25%.
Standard spread and total bets are priced at -110 on both sides. Run the formula: 110 ÷ 210 = 52.38%. If both sides really were a coin flip, you'd expect 50% each. The extra 2.38% on each side is the book's cut.
At -110, you have to win 52.38% of your bets just to break even. That's the bar before you've made a dollar.
Add both sides of a -110/-110 market: 52.38% + 52.38% = 104.76%. Real probabilities can't exceed 100%, so the extra 4.76% is the sportsbook's margin, often called the overround. Expressed as a share of every dollar bet, it works out to about 4.5% (this is the "hold").
To strip the margin out, divide each side's implied probability by the total. Take a market at -150 / +130:
Those fair numbers are a better read of what the market actually believes. If you think the underdog wins more than 42% of the time, the +130 price may be worth a look; if not, it's just the favorite's tax in disguise.
What implied probability does -110 represent?
-110 implies a 52.38% chance (110 ÷ 210). That's also the win rate you need at -110 just to break even, because the book's margin is baked into the price.
Why do two sides of a bet add up to more than 100%?
The sportsbook builds in its margin (the vig). At -110 on both sides, the implied probabilities are 52.38% each, 104.76% total. The extra 4.76% is the overround.
Is implied probability the same as the true chance of winning?
No. It's what the price says after the book's margin, not the real probability. Removing the margin gives you a fairer estimate, but it's still the market's opinion, not a guarantee.
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