How Do Betting Odds Work? Decimal, Fractional and American Odds Explained

A price like 11/8 on a betting slip can feel like the wrong maths exam. So how do betting odds work? Here’s the good news: there’s one idea underneath all three formats, and it takes about ten minutes to learn.

Odds are a price. Like any price, they tell you what you get and what the seller thinks the thing is worth. Once you can read both messages, you can compare bookmakers (sportsbooks, if you’re in the US), check whether a tip makes sense, and spot when you’re being charged too much.

How do betting odds work? The two jobs every price does

Every set of odds, in any format, answers two questions.

1. What do I get back if I win? This is the payout side. It’s the only part most beginners look at.
2. How likely does the bookmaker think this is? This is the probability side. It’s hidden inside the number, and it’s the part that decides whether a bet is any good.

Short odds (small numbers like 1.20) mean a likely outcome and a small payout. Long odds (big numbers like 11.00) mean an unlikely outcome and a big payout. A favourite is the side with the shorter odds; the underdog has the longer odds.

The three formats below are just different notations for that same price, the way 0.5, 1/2 and 50% are the same number.

Decimal odds: stake times odds

Decimal odds are the standard in Europe, Australia, Canada and on many prediction apps. They show your total return per 1 unit staked, stake included.

The rule: stake × decimal odds = total return.

• 10 at odds of 2.50 returns 25.00. That’s your 10 stake plus 15.00 profit.
• 10 at odds of 1.50 returns 15.00. That’s 5.00 profit.
• 10 at odds of 11.00 returns 110.00. That’s 100.00 profit.

To get profit alone, subtract your stake, or multiply the stake by (odds − 1). Anything below 2.00 is “odds-on”: you win less than you stake. Exactly 2.00 is even money.

Fractional odds: profit over stake

Fractional odds are the traditional UK and Irish format, and they’re still common in horse racing. They show profit relative to stake, and they don’t include the stake.

Read 6/4 as “six to four”: you win 6 for every 4 you stake.

• 10 at 6/4 makes 15.00 profit (10 × 6 ÷ 4). You also get your 10 back, so the return is 25.00.
• 10 at 1/2 (“two to one on”) makes 5.00 profit.
• 10 at 10/1 makes 100.00 profit.

To turn a fraction into decimal odds, divide it out and add 1. So 6/4 = 1.5, plus 1 = 2.50. Evens, written 1/1 or EVS, is 2.00.

The awkward ones are fractions like 11/8, 13/8 or 10/11. They’re traditional prices that sit between the round numbers, and they look worse than they are. 11/8 is simply 2.375 in decimal.

American odds: the plus and the minus

American odds (also called moneyline odds) are what US sportsbooks show. They’re built around the number 100, and the sign tells you which way to read them.

• Plus odds (+150) show how much profit you make from a 100 stake. +150 means stake 100, win 150.
• Minus odds (−200) show how much you must stake to make 100 profit. −200 means stake 200, win 100.

You don’t have to bet 100. It’s just the reference point. A 10 bet at +150 makes 15.00 profit. A 10 bet at −200 makes 5.00 profit.

Converting to decimal:

• Plus odds: divide by 100 and add 1. +150 → 1.5 + 1 = 2.50.
• Minus odds: divide 100 by the number (ignore the sign) and add 1. −200 → 0.5 + 1 = 1.50.

The favourite carries the minus sign, the underdog the plus. When both sides show a minus, such as −110 and −110, you’re looking at a roughly 50/50 market with the sportsbook’s fee baked in. More on that below.

Odds conversion table

Here’s the same set of prices in all three formats, with the implied probability and what a 10 stake returns. If your price isn’t in the table, the odds converter does any number in either direction.

Decimal	Fractional	American	Implied probability	Return on a 10 stake	Profit
1.20	1/5	−500	83.3%	12.00	2.00
1.50	1/2	−200	66.7%	15.00	5.00
1.91	10/11	−110	52.4%	19.09	9.09
2.00	1/1 (evens)	+100	50.0%	20.00	10.00
2.50	6/4	+150	40.0%	25.00	15.00
3.00	2/1	+200	33.3%	30.00	20.00
4.00	3/1	+300	25.0%	40.00	30.00
5.00	4/1	+400	20.0%	50.00	40.00
11.00	10/1	+1000	9.1%	110.00	100.00

One small note on the third row: 10/11 and −110 are exactly 1.909, which sites round to 1.91. That’s why the return is 19.09 and not 19.10.

Implied probability: what the odds are really saying

Implied probability is the chance of an outcome that a price assumes. It’s the most useful number in betting, and it’s one division away:

implied probability = 1 ÷ decimal odds

• Odds of 2.50 → 1 ÷ 2.50 = 0.40, or 40%.
• Odds of 1.50 → 1 ÷ 1.50 = 0.667, or 66.7%.
• Odds of 4.00 → 1 ÷ 4.00 = 0.25, or 25%.

If you prefer to work straight from American odds: for minus odds, divide the number by itself plus 100 (−200 → 200 ÷ 300 = 66.7%). For plus odds, divide 100 by the number plus 100 (+150 → 100 ÷ 250 = 40%).

Why does this matter? Because it turns “Arsenal at 1.50, sounds fine” into a question you can actually argue with: “Do Arsenal win this more than 66.7% of the time?” If your honest answer is yes, the bet has value. If it’s no, the payout is too small for the risk you’re taking, however likely the win feels. That single comparison is the whole idea behind value betting.

It’s also a quick way to sanity-check tips from prediction apps and tipsters. A pick at 1.30 needs to win about 77% of the time just to break even. A high hit rate at tiny odds can still lose money.

The bookmaker margin: why the percentages add up to more than 100

Here’s the part the odds board doesn’t advertise. In a fair market, the implied probabilities of all outcomes add up to exactly 100%. In a bookmaker’s market, they add up to more. The extra is the margin, also called the overround, the vig or the juice. It’s the bookmaker’s fee, and it’s built into every price you see.

Worked example: a football match

Take a football (soccer) match priced like this in the match result market:

Outcome	Bookmaker odds	Implied probability	Fair probability (margin removed)	Fair odds
Home win	2.10	47.6%	45.4%	2.20
Draw	3.40	29.4%	28.1%	3.56
Away win	3.60	27.8%	26.5%	3.77
Total		104.8%	100.0%

The three implied probabilities add up to 104.8%. The 4.8% over 100 is the overround. To strip it out, divide each implied probability by the total (47.6 ÷ 104.8 = 45.4%). The fair odds column shows what the prices would be with no fee at all.

So on the home win, you’re paid 2.10 for something that “should” pay about 2.20. It doesn’t look like much. Over hundreds of bets, it’s the reason most bettors lose and bookmakers stay in business.

The −110 coin toss

The same thing happens on US point spreads and totals. Both sides are priced at −110 (1.91). Each implies 52.4%, so the pair adds up to 104.8%. A true coin toss would pay +100 (2.00) on both sides. At −110, you need to win 52.4% of your bets just to break even.

A few practical things follow from this:

• Margins differ between bookmakers and between markets. The same match can carry a noticeably lower margin at one book than at another. That’s why comparing prices across bookmakers and sportsbooks is worth the two minutes.
• Popular markets are usually cheaper. Match result on a big league tends to have a smaller margin than a niche prop bet on a lower division.
• Margins multiply in parlays. Every leg you add to a parlay (an accumulator, in UK terms) stacks another fee on top. I’ve done the maths in what is a parlay, and you can test your own slip in the parlay calculator.

How to use odds like someone who’s done this before

You don’t need a spreadsheet. You need a short routine.

1. Switch your betting site to decimal odds. One rule to remember, and implied probability is one division away.
2. Convert the price to a percentage before you bet. Ask yourself if the outcome really happens that often.
3. Check the margin on the market. Add up the implied probabilities. The closer to 100%, the less you’re paying.
4. Compare at least two prices. Getting 2.20 where another book offers 2.10 means about 9% more profit on the same winning bet (12.00 instead of 11.00 on a 10 stake).
5. Size your stake sensibly. Good prices don’t help if one bad weekend wipes you out. That’s what bankroll management is for.

If a term in this guide was new to you, the betting glossary has short definitions for all of them, and what is sports betting covers the wider picture.

One last point: understanding odds doesn’t make you a winner. It makes you an informed customer who knows what each bet costs. No bet is a lock, whatever the number next to it says.

Bet only what you can afford to lose, and if it stops being fun, the responsible gambling page lists tools and help lines.