The three odds formats, side by side
Decimal odds (2.50) show your total return for every 1 unit staked, including your stake. A 10 stake at 2.50 returns 25.
Fractional odds (3/2) show profit relative to stake. 3/2 means 'win 3 for every 2 staked' — the same as 2.50 decimal.
American / Vegas odds use a plus or minus sign. +150 means 'stake 100 to win 150 profit'. -200 means 'stake 200 to win 100 profit'. Favourites carry the minus, underdogs carry the plus.
They are three different ways of saying the same thing. 2.50 decimal = 3/2 fractional = +150 American. Pick the format you find easiest and convert the rest to it.
The one calculation that matters: implied probability
For decimal odds, divide 1 by the odds to get the chance the bookmaker is giving that outcome. 1 ÷ 2.50 = 0.40, so the odds imply a 40% chance. 1 ÷ 7.00 = 0.14, so about 14%.
For a positive American line like +150: 100 ÷ (150 + 100) = 40%. For a negative line like -200: 200 ÷ (200 + 100) = 66.7%.
This one number lets you compare the bookmaker's opinion with your own. If you think a team has a 55% chance to win but the odds imply only 40%, you may have found a smart bet. That gap is the whole idea behind value betting.
Reading a Vegas sportsbook line at a glance
A US sportsbook line usually shows three columns per game: moneyline (straight winner), point spread (a handicap so the favourite must win by a margin) and total (over/under on the combined score).
A -7.5 spread with -110 next to it means the favourite must win by 8+ points, and you stake 110 to win 100. The total ('O 47.5, -110') means Over 47.5 combined points pays the same 110-to-100 price.
In UK and European sportsbooks the same information is shown as decimal odds — the maths is identical, just easier to read at speed.
Why the percentages add up to more than 100%
Add up the implied percentages for home win, draw and away win in a football market and you will usually get something like 105%. That extra 5% is the bookmaker's built-in margin — their cut for taking the bet.
Lower-margin markets (major leagues, big fixtures) give you better long-term value. Exotic markets (correct score, HT/FT, jackpots) carry much bigger margins, which is exactly why they pay bigger odds when they land.
A 30-second cheat sheet you can memorise
1.20 ≈ 83% chance. Very likely, tiny reward. 1.50 ≈ 67%. Strong favourite. 2.00 ≈ 50%. A coin-flip. 2.50 ≈ 40%. 3.00 ≈ 33%. 5.00 ≈ 20%. 7.00 ≈ 14%. 10.00 ≈ 10%. 20.00 ≈ 5%.
Learn those seven numbers and you can eyeball any decimal price for the rest of your betting life.