How to Calculate Odds in Gambling: Formulas + Examples

How to Calculate Odds in Gambling

By the CasinosCanadaGuide research team · Updated August 3, 2026

The short answer

To calculate gambling odds, convert the price into implied probability. For negative American odds, divide the number by itself plus 100: -110 gives 110/210 = 52.4%. For positive odds, divide 100 by the odds plus 100: +130 gives 43.5%. For decimal odds, divide 1 by the price. Compare that percentage to your own estimate of the true chance to judge the bet.

The Three Conversion Formulas

American negative odds: implied probability = odds / (odds + 100), using the number without the minus sign. So -150 is 150/250 = 60%. American positive odds: 100 / (odds + 100), so +200 is 100/300 = 33.3%. Decimal odds are the simplest: 1 divided by the price, so 2.05 implies 48.8% and 1.50 implies 66.7%.

Converting between formats is just as mechanical. Negative American to decimal: 1 + 100/odds, so -110 becomes 1.909. Positive American to decimal: 1 + odds/100, so +170 becomes 2.70. Once everything is in decimal, payouts are one multiplication: stake times odds equals total return.

Run a real example. The Canadiens are +130 against the Bruins at -150. Implied probabilities: 43.48% and 60%. Notice they sum to 103.48%, not 100%. Both prices are inflated because the sportsbook charges a margin on each side.

Removing the Vig to Find True Odds

To estimate the market's real opinion, normalize the percentages. Divide each implied probability by the total. Boston: 60 / 103.48 = 57.98%. Montreal: 43.48 / 103.48 = 42.02%. These no-vig numbers are the fair probabilities the prices actually encode, and they are the correct benchmark for any handicapping model.

The no-vig line also exposes what you are paying. Fair odds on Boston at 57.98% would be about -138, but you are being charged -150. That gap, roughly 1.7 cents on the dollar, is the cost of the bet. Sharp bettors compare no-vig closing prices across books and only bet when their number beats the fair line, not just the posted one.

Calculating Expected Value

Expected value ties it together. EV = (true probability x profit if won) minus (losing probability x stake). Say you believe an underdog priced at +120 wins 50% of the time. Per C$100 bet: 0.50 x 120 minus 0.50 x 100 = +C$10, a 10% edge. If your true estimate is only 42%, the same bet is 0.42 x 120 minus 0.58 x 100 = minus C$7.60.

Every gambling decision reduces to this calculation. Casino games have fixed negative EV, about minus C$0.50 per C$100 on good blackjack and worse on slots. Sports betting is the rare format where the true probability is debatable, which means positive EV is possible, though only if your probability estimates are genuinely better than the market's.

Frequently asked

What is the fastest way to convert -110 in my head?

Memorize the anchors. -110 is 52.4%, -120 is 54.5%, -150 is 60%, -200 is 66.7%, +100 is 50%, +150 is 40%, +200 is 33.3%. Most prices you see are within a few points of an anchor, so you can interpolate quickly. For anything precise, the formula takes ten seconds on a phone.

Why do implied probabilities add up to more than 100%?

Because the sportsbook shortens both prices to build in its margin, called the vig or overround. A -110/-110 market sums to 104.76%. The amount above 100 is the book's expected cut of the total money wagered, about 4.5% at standard pricing.

How do I calculate odds for a parlay?

Convert every leg to decimal odds and multiply them, then multiply by your stake for the total return. Two legs at -110 give 1.909 x 1.909 = 3.64, so C$50 returns C$182.20. Subtract the stake for profit. The implied probability of the ticket is 1 divided by the combined odds.