The three odds formats and how to convert between them
The same price can appear in three different notations depending on the market and audience. Decimal is the standard across most of the world, including Brazil: the number represents the total return per unit staked, including the stake itself. Fractional is traditional in the UK and shows profit relative to the amount staked. American is the US standard: a positive number shows the profit on a 100-unit stake, a negative number shows how much needs to be staked to profit 100 units.
| Decimal | Fractional | American | Implied probability |
|---|---|---|---|
| 1.50 | 1/2 | -200 | 66.67% |
| 2.00 | 1/1 (evens) | +100 | 50.00% |
| 3.50 | 5/2 | +250 | 28.57% |
| 1.20 | 1/5 | -500 | 83.33% |
Converting between decimal and American follows a simple rule: if the decimal odd is 2.00 or higher, the American price is positive and equals (decimal − 1) × 100 — for example, 3.50 becomes (3.50 − 1) × 100 = +250. If the decimal odd is below 2.00, the American price is negative and equals −100 ÷ (decimal − 1) — for example, 1.50 becomes −100 ÷ 0.50 = −200. In the Brazilian market, practically every platform displays odds in decimal format by default, with fractional and American showing up only as a player preference option — most of the Brazilian audience never learned to read fractions like 5/2 or American numbers like +250, so keeping decimal as the default cuts down on reading friction.
What implied probability is and how it becomes margin
The implied probability of a decimal odd is simply 1 divided by the odd. Odds of 2.00 embed an implied probability of 1 ÷ 2.00 = 50%. If the true probability of a two-outcome event were exactly 50% on each side, and the operator charged no margin at all, both odds would be 2.00 and 2.00 — the sum of implied probabilities would land at exactly 100%. In practice, this almost never happens: the operator publishes odds slightly below the "fair" value on both sides, and the sum of implied probabilities lands above 100%. That gap is the overround (also called vig, short for vigorish). The overround does not have to be split evenly between sides — the operator can raise the margin on the more heavily bet side (usually the favorite, where volume concentrates) and lower it on the less-bet side, a technique called "book balancing," used to manage risk exposure without changing the market's total overround.
How the overround shows up in practice
Take a two-outcome market (moneyline): if the operator prices both sides at 1.90, the implied probability of each side is 1 ÷ 1.90 = 52.63%. Adding both sides: 52.63% + 52.63% = 105.26%. The overround is 105.26% − 100% = 5.26% — that is the margin built into the market, regardless of which side wins.
In a three-outcome market, like football's 1X2 (home win, draw, away win), the same calculation applies across three probabilities. With odds of 2.10 (home), 3.40 (draw), and 3.60 (away): 1÷2.10 = 47.62%, 1÷3.40 = 29.41%, 1÷3.60 = 27.78%. The sum is 47.62% + 29.41% + 27.78% = 104.81% — an overround of 4.81%.
During the match, live odds get recalculated after every relevant event — a goal, a card, a corner — and the overround tends to tick up slightly relative to pre-match, because the odds engine has to react faster to new information and absorbs part of that extra risk with a slightly wider margin.
Odds priced too tight (very low overround) squeeze the operator's margin to the point where the sportsbook depends on luck to close the month in the black; odds priced too wide (high overround) let the player quickly notice they are betting into a bad price relative to competing books, and move on. Overrounds of 4% to 6% in Brazilian football tend to balance both pressures.