How to Read and Convert Betting Odds: American, Decimal, and Fractional

Odds look like a code until someone shows you the key. Once you have it, you can glance at -150, 2.50, or 6/4 and instantly know the payout and the probability behind it. This guide covers all three formats you'll encounter, how to convert between them by hand, and how to turn any price into an implied probability — the number that actually tells you whether a bet is worth making. Keep our odds converter open in another tab to check your work.

American odds

American odds (also called moneyline odds) are quoted with a plus or minus sign and centered on $100.

• A negative number is how much you must stake to win $100. At -150, you risk $150 to win $100.
• A positive number is how much you win on a $100 stake. At +200, you risk $100 to win $200.

The favorite always carries the minus sign, the underdog the plus. An even-money bet is +100 (or -100): risk $100 to win $100. You don't have to bet in hundreds — the ratio is what matters. At -150, a $30 bet wins $20; at +200, a $30 bet wins $60.

Decimal odds

Decimal odds, common in Europe and on exchanges, express your total return per $1 staked, including your stake. Decimal 2.50 returns $2.50 for every $1 — a $1.50 profit plus your $1 back. To find your profit, subtract 1: at 2.50, profit is 1.50× your stake.

Decimal is the easiest format for quick math and for comparing prices, which is why many sharp bettors prefer it. Anything above 2.00 is an underdog (positive American odds); anything below 2.00 is a favorite (negative American odds); exactly 2.00 is even money.

Fractional odds

Fractional odds, traditional in the UK and horse racing, show profit relative to stake. 6/4 ("six to four") means you win $6 profit for every $4 staked. 2/1 means $2 profit per $1; 1/2 means $1 profit per $2 staked (a heavy favorite). To get your total return, add the two numbers and divide by the second: 6/4 returns 10/4 = 2.5× your stake — the same as decimal 2.50.

Converting between formats

All three describe the same bet, so they convert cleanly. Here are the same five prices in each format:

American	Decimal	Fractional	Implied probability
-200	1.50	1/2	66.7%
-150	1.67	4/6	60.0%
+100	2.00	1/1	50.0%
+150	2.50	3/2	40.0%
+200	3.00	2/1	33.3%

The quick conversions to memorize:

• American to decimal: for positive odds, divide by 100 and add 1 (+150 → 2.50). For negative odds, divide 100 by the absolute value and add 1 (-200 → 1.50).
• Decimal to fractional: subtract 1, then express as a fraction (2.50 → 1.5 → 3/2).
• Decimal to implied probability: divide 1 by the decimal odds (2.50 → 1 ÷ 2.50 = 40%).

Implied probability: the number that matters

Implied probability is what the odds say about the chance of an outcome. The formula from decimal odds is simply 1 ÷ decimal odds. From American odds: a -150 favorite implies 150 ÷ (150 + 100) = 60%; a +200 underdog implies 100 ÷ (200 + 100) = 33.3%.

Here's the catch: if you add up the implied probabilities of both sides of a market, they total more than 100%. Two -110 sides sum to about 104.5%. That extra 4.5% is the sportsbook's vig — its built-in margin. Our vig calculator strips it out so you can see the true, no-vig probability the book really assigns each side.

Reading a real bet slip

Put it together on a concrete line. Say you see "Lakers -150" next to "Celtics +130". The -150 favorite implies about 60% (150 ÷ 250) and a $150 bet returns $100 profit. The +130 underdog implies about 43.5% (100 ÷ 230) and a $100 bet returns $130 profit. Notice the two implied probabilities add to roughly 103.5% — that 3.5% overage is the vig the book keeps on this market, and it's why both sides never simply sum to 100%.

One last habit: always check the odds format setting in your sportsbook app. Most let you toggle between American, decimal, and fractional. If a price ever looks unfamiliar or absurd, you're probably reading a format you didn't expect — switch the setting and the number will snap back into something you recognize.

Why this is the foundation of everything

Reading odds fluently isn't trivia — it's the prerequisite for finding value. Once you can convert any price to a probability, you can compare books instantly and take the best number every time, which is the single highest-leverage habit in betting (see how comparing odds saves you money). It also lets you spot a +EV bet — one where your estimated probability beats the implied probability in the price. That's the whole game, and we break it down in expected value betting explained.

Practice on real numbers: pull up the comparison page, pick any game, and convert a few prices to implied probability in your head. For a deeper treatment of formats and value, our how to read betting odds guide and the spread explainer go further. Within a week it'll be second nature.