NB-PT Casino Notes
Odds Basics
Odds are the first thing anyone sees on a betting screen and the thing newcomers most often misread. They describe two things at once: how much a winning bet pays, and how likely the bookmaker thinks the outcome is, with its own margin added. This page explains the three common formats and how to read them. It is written for adults aged 18 and over and is not a recommendation to bet.
One price, three notations
| Format | Example | How to read it | Return on a 10 stake |
|---|---|---|---|
| Decimal | 2.50 | Total return per 1 staked, stake included | 25.00 |
| Fractional | 3/2 | Profit of 3 for every 2 staked | 25.00 |
| American (plus) | +150 | Profit on a stake of 100 | 25.00 |
| American (minus) | −200 | Stake needed to make a profit of 100 | 15.00 |
The first three rows are the same price shown in different ways. The last row is a short-priced favourite, where you risk more than you stand to make.
From a price to a probability
Every price can be converted into the probability it implies. That is the most useful skill on this page, because it reveals the margin.
- Decimal: divide 1 by the price. 2.50 implies 40%.
- Fractional: divide the second number by the sum of both. 3/2 gives 2 ÷ 5, also 40%.
- American plus: divide 100 by the price plus 100. +150 gives 100 ÷ 250, again 40%.
- American minus: divide the number by itself plus 100. −200 gives 200 ÷ 300, about 66.7%.
Finding the margin
Add up the implied probabilities of every possible outcome in a market. A market with no margin at all would land on exactly 100%. Bookmakers set prices so the total comes out higher, and the excess is their margin, also called the vig, juice or overround. Two sides both priced at 1.91 imply about 52.4% each, roughly 104.7% together, so the margin is close to 4.7%.
That margin is why a bettor must be right more often than the prices suggest just to break even. The same idea applies inside casino games, where it is called the house edge, and inside slots, where it is the gap between the return-to-player figure and 100%.
Lines and prices are different things
In handicap and totals markets, a second number sits beside the price: the line. In a market like Team A +3.5 at −110, the line is +3.5 and the price is −110. The line decides whether the bet wins; the price decides how much it pays. Our entry on what +3.5 means in betting works through an example, and what a push is explains what happens when a result lands exactly on a whole-number line.
What odds cannot tell you
A price is an estimate with a fee attached, not a forecast you can rely on. Favourites lose and outsiders win; that is what makes sport worth watching. Odds can help you understand what a bet costs, but they cannot make any bet a good idea. Check the rules where you live before betting, and keep any stake to money set aside for entertainment.
Pinned to the inside cover
Truths about odds that never change
General points that hold for any sportsbook and any price format.
Decimal, fractional and American odds can all express exactly the same price.
Any price can be turned into an implied probability with one division.
When the implied probabilities of a market add up to more than 100%, the surplus is the bookmaker's margin.
A line decides whether a bet wins; the price decides how much it pays.
Combining selections on one ticket multiplies the margin along with the payout.
Short prices on favourites still lose whenever the favourite does.
Translate a price
Choose the notation you have in front of you, type the price, and read it in the other two along with the probability it implies.
– implied probability
| Decimal | |
|---|---|
| Fractional | |
| American | |
| Return on a 10 stake |
Add the other side of a two-way market
Type the second price to see the margin.
A reading aid, not a recommendation to bet. A price is an estimate with a fee attached; it cannot tell you what will happen. 18+ only, and check the rules where you live.