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What does −150 mean?

−150 marks the favourite: you risk $150 to win $100. The price implies a 60.0% chance, which is how often it has to win to break even.

What does +130 mean?

+130 marks the underdog: $100 wins $130. The price implies a 43.5% chance.

How do I convert odds to a percentage?

For a minus price, divide the number by itself plus 100 (−150 → 150 ÷ 250 = 60%). For a plus price, divide 100 by the number plus 100 (+130 → 100 ÷ 230 = 43.5%). For decimal odds, divide 1 by the price.

Why do both sides add up to more than 100%?

The price-setter builds in a margin (also called the vig or juice). In a −150 / +130 game the sides add to 103.5%, a 3.5% margin. Scale both back to 100% to get each side's fair chance.