Combined odds are the single price several selections make once they are rolled into one bet: the decimal prices multiply. Three illustrative legs at $2.00, $1.50 and $3.00 combine to 2.00 x 1.50 x 3.00 = $9.00, so a $10 stake returns $90. That is how a multi is quoted, and a parlay or an accumulator is the same bet under another name. A same game multi is not quoted that way.
That rule is one line of arithmetic. What it does to the bet is not: multiplying prices multiplies the implied chance down and the bookmaker's margin up, and the single quoted price shows neither.
The multiplication rule
Combined odds = leg 1 price x leg 2 price, and so on for every leg, in decimal. The prices below are illustrative, not a real market.
| Leg | Price | Running combined price |
|---|---|---|
| 1 | $2.00 | $2.00 |
| 2 | $1.50 | $3.00 |
| 3 | $3.00 | $9.00 |
Add a fourth leg at $1.40 and the price becomes 9.00 x 1.40 = $12.60. Every leg stretches the price, and every leg also has to come in: one loser and the bet pays nothing. The multi bet calculator does the multiplication and the return, and how betting odds work has the arithmetic a single price rests on.
The implied chance multiplies down the same way
Implied probability = 1 / decimal odds, so those three legs imply 1 / 2.00 = 50%, 1 / 1.50 = 66.7% and 1 / 3.00 = 33.3%.
Multiply the chances and you get 0.5 x 0.6667 x 0.3333 = 0.1111, or 11.1%, which is exactly what $9.00 implies: 1 / 9.00 = 11.1%. Combining created nothing. Three legs that each looked likely enough have made one bet the price puts at about one chance in nine.
That 11.1% is the price's own figure, and every price carries the bookmaker's margin, so it overstates the chance the market is pointing at. Taking the margin out of the same three legs gives 9.60%.
Note: multiplying assumes the legs are independent. That holds across three different games and does not hold inside one.
Why the margin compounds leg by leg
Say each of those markets carries an illustrative 5% margin, so its prices add up to 105% rather than 100%. Scaling the margin out in proportion makes each leg's fair price the offered price multiplied by 1.05.
| Leg | Offered | Fair |
|---|---|---|
| 1 | $2.00 | $2.10 |
| 2 | $1.50 | $1.575 |
| 3 | $3.00 | $3.15 |
The fair combined price is 2.10 x 1.575 x 3.15 = $10.418625, implying 1 / 10.418625 = 9.60% against the 11.1% the $9.00 quotes. So the combined book is 10.418625 / 9.00 = 1.157625, a margin of 15.76%, because the margins multiply too: 1.05 x 1.05 x 1.05 = 1.157625.
Compounded margin = (1 + the margin on a leg) to the power of the number of legs, minus 1. At an illustrative 5% a leg:
| Legs | Combined book | Margin |
|---|---|---|
| 3 | 1.157625 | 15.76% |
| 5 | 1.276282 | 27.63% |
| 8 | 1.477455 | 47.75% |
Nothing in the eight-leg row is unusual pricing. It is the same 5% market eight times over. Bookmaker margin shows how to measure one market before you add its leg, and multi bets explained works out what each leg added costs per dollar staked.
Risk: Betting involves risk. Knowing the margin tells you what a price costs on average, not what one bet will do. A leg priced above its fair estimate can still lose, one losing leg loses the whole bet, and there is no guarantee of profit. See responsible gambling for limits and support.
Legs from the same game are not independent
Multiplication only describes independent events, and legs drawn from one game move together, so the product of the legs is the wrong number for a same game multi. A leg on a player to score and a leg on that player's side to win land together more often than multiplying assumes, which makes the product too long. Two legs that get in each other's way land together less often, and the product is then too short.
No bookmaker multiplies a same game multi. Each prices the correlation with its own model, which is why the same four legs quote differently at different bookmakers and why the quote has to be read rather than calculated. How same game multi odds are calculated works that pricing through with numbers, correlated bets covers how to measure a link, and bonus bets on multis covers what a bonus bet is worth on a long combined price.
The check on any builder quote is to multiply the legs yourself and compare. A quote well under the product is the correlation priced in, extra margin, or both.
Where combined odds show up on B337
The Terminal is B337's live odds board: racing and sports prices from 40+ bookmakers side by side, with price history and EV overlays. It shows each bookmaker's price selection by selection, so comparing the legs is comparing the multi. Terminal Pro adds an SGM builder and betting by hand from the board into your own accounts.
Two honest limits. Prices are read on a repeating cycle rather than tick by tick, so a price on screen can trail the bookmaker's own; confirm it before betting. And a same game multi quote is the bookmaker's own number, so it will not equal the product of the legs.
Risk: Betting involves risk. Bookmakers can restrict or close accounts and void bets, prices move, and a positive expected value bet can still lose. A longer combined price is a smaller chance, not a better bet. There is no guarantee of profit. See responsible gambling for limits and support.