Betting odds explained: how to read American, decimal and fractional odds

Betting odds tell you two things: how much a bet pays if it wins, and, hidden inside that, the chance the bookmaker is giving the outcome. There are three common formats. American odds show what you win on $100 (+200) or what you must stake to win $100 (-150). Decimal odds show the total you get back for each $1 staked, stake included (1.67). Fractional odds show profit against stake (4/6). All three can describe the same price: -150, 1.67 and 4/6 are each a 60% chance, and a $10 bet at any of them returns about $16.67.

This page is about reading odds. To convert between formats, or into a prediction market price, use the odds converter.

American odds: the plus and the minus

• Minus odds are for the favourite and show what you stake to win $100. At -150, you stake $150 to win $100 profit, so $10 wins $6.67. At -110, you stake $110 to win $100.
• Plus odds are for the underdog and show what a $100 stake wins. At +200, $100 wins $200 profit, so $10 wins $20. At +130, $10 wins $13.
• +100 is even money: you win what you stake.

To find the chance, use one of two sums. For minus odds, divide the number by itself plus 100: 150 ÷ 250 = 60%. For plus odds, divide 100 by the number plus 100: 100 ÷ 300 = 33.3% for +200.

Decimal odds: the total you get back

Decimal odds are the simplest. Multiply your stake by the odds and that's what you get back if you win, stake included. $10 at 1.67 returns $16.70; $10 at 3.00 returns $30.00, of which $20 is profit. The chance is 1 divided by the odds: 1 ÷ 1.67 = 60%, and 1 ÷ 3.00 = 33.3%.

Fractional odds: profit against stake

Fractional odds show profit over stake. At 4/6 you win $4 for every $6 staked, so $10 wins $6.67. At 2/1, often read "two to one", you win $2 for every $1, so $10 wins $20. Evens is 1/1. A fraction with the bigger number on the bottom, like 4/6, is a favourite; bigger on top, like 13/10, an underdog. The chance is the bottom number divided by the sum of both: for 4/6, 6 ÷ 10 = 60%.

The same prices in every format

The last column is a prediction market price for the same chance. On a market such as Polymarket or Kalshi, a share that pays $1 costs its chance in cents, so the price is the probability. How tennis prices work explains why.

Why the chances add up to more than 100%

Convert a bookmaker's odds for both players into chances and add them. At -110 and -110, a common line, each side is 52.4%, which adds up to 104.8%. The extra 4.8% is the bookmaker's margin, also called the vig, juice or overround. It's how the bookmaker is paid whichever player wins. The odds you're offered are always a little worse than the bookmaker's own view of the chances.

To see the fair chance underneath, take the margin out: divide each chance by the total. At -110 and -110 that gives 50% each. The no-vig calculator does it for any pair of odds, in any format. On a busy prediction market the two prices usually add up close to 100¢, and you pay a trading fee instead; Polymarket fees explained shows how much.

Four common mistakes

1. Mixing up return and profit. Decimal odds include your stake; fractional and American odds show profit only. Decimal 3.00 and fractional 2/1 are the same price, even though the numbers look different.
2. Reading a bigger number as more likely. In decimal and fractional odds, the bigger the number, the less likely the outcome. 5.00 is a 20% chance; 1.25 is 80%.
3. Treating -110 as a fair coin flip. It's a 52.4% chance, not 50%. You need to win more than half your -110 bets just to break even: 52.4%, or about 11 bets in every 21.
4. Comparing odds with the margin left in. Two bookmakers with the same view of a match can show different odds simply because one takes a bigger margin. Strip the margin from both before deciding which price is better.

Reading odds before a match

• The favourite is the shorter price. Minus in American odds, the smaller decimal, the fraction with the bigger bottom number.
• A chance is not a forecast. A 60% favourite still loses about 4 times in 10 if the odds are right.
• Compare like with like. To compare a bookmaker with a market, or two bookmakers, strip the margin from each first.
• Odds move. Before and during a match, odds change as money comes in and as the score changes. In tennis, a break of serve moves them most, as why the price jumps on a break of serve shows.

For the trading side of the same numbers, read tennis trading vs tennis betting.

Sources

This page is arithmetic: every figure above comes from the standard conversion sums shown, which you can check in the odds converter. The fee reference is from Polymarket's fee page, as set out in Polymarket fees explained, checked on 31 October 2026.

FAQ

What does -110 mean in betting?

You stake $110 to win $100 profit, or $10 to win $9.09. It's a 52.4% chance. When both sides are -110, the chances add up to 104.8%, and the extra 4.8% is the bookmaker's margin.

What does +200 mean in betting?

A $100 stake wins $200 profit, so you get $300 back in total. It's a 33.3% chance, the same as decimal odds of 3.00 or fractional 2/1.

How do you convert odds to a probability?

For decimal odds, divide 1 by the odds. For minus American odds, divide the number by itself plus 100; for plus odds, divide 100 by the number plus 100. For a fraction, divide the bottom number by the sum of both.

Are decimal odds of 1.67 good?

They mean about a 60% chance and a $6.70 profit on $10. Whether that's good depends on whether you think the real chance is higher than 60%, after the bookmaker's margin is taken out.

What does 4/6 mean?

You win $4 for every $6 staked, so $10 wins $6.67. It's a 60% chance, the same as -150 or 1.67.