Horse racing odds are determined using a formula that takes into account the total amount of money that has been bet on a race, the amount that has been bet on each horse and the percentage of the money that the track or offtrack betting site keeps. Before any bets have been placed, the odds are based on the oddsmaker's estimate of the percentage of the total betting pool that will be bet on each horse; these odds are often referred to as the morning line. As bets are placed and the calculations are made, the odds can change if the oddsmaker's estimates were not accurate. Unlike many other forms of betting, the payouts on horse racing odds usually are based on the final odds for each horse, rather than the odds that a bettor received when placing the bet. For example, someone might bet on a horse when the odds are 41, but if that horse's odds are 21 when the race begins, the bettor will win only half as much money as he or she would have if the odds had not changed.
The type of betting that is done on most horse races is called parimutuel betting. The name comes from the French words for "mutual stake" and reflects the fact that all of the bettors on a race are competing against each other for the same pool of money. In parimutuel betting, the odds are determined by the bets that have been placed. The business that accepted the bets keeps a portion of the money — often called a "take" or "rake" and usually 10 to 20% — so that it can pay for expenses and taxes and still make a profit. The rest of the money is then paid to the winning bettors.
Calculating horse racing odds is rather simple. An example can be shown by a fivehorse race on which a track has taken a total of $1,000 US Dollars (USD) in bets to win — $300 USD on one horse, $250 USD on another, $200 USD on the third, $150 USD on the fourth and $100 USD on the last. If the track's take is 15%, all of the bettors are competing for the remaining $850 USD. The odds for each horse are calculated by subtracting the amount bet on that horse from the available payout — $850 USD, in this case — and dividing that number by the amount bet on that horse. The result is then rounded downward, if necessary.
In this example, the horse on which the most money was bet — the "favorite" — would have odds of 95. Starting with the $850 USD payout pool and subtracting the $300 USD that was bet on the horse leaves $550 USD in potential winnings for the people who bet on that horse. Dividing $550 USD by $300 USD is 1.83, which is rounded down to 1.8. Horse racing odds are expressed in whole numbers, so 1.8 is converted to 95 odds. This means that a bettor would win $9 USD for every $5 USD that he or she bets. The bettor also would receive back the amount or his or her original bet, so the total payout for a winning $5 USD bet, in this case, would be $14 USD.
The same math is used to calculate the odds for the other horses. The second favorite would have 125 odds — $850 USD minus $250 USD is $600 USD, divided by $250 USD is 2.4. The last three horses would have odds of 134, 143 and 152, respectively, based on the same calculations.
No matter which horse wins, the track will pay out roughly the same amount. Rounding the odds downward when necessary might result in slightly less money being paid out, depending on which horse wins. In this example, only $840 USD will be paid out if the favorite wins, and $850 USD will be paid out if any other horse wins. If the 152 long shot in this example ended up winning the race, the track would return the $100 USD that was bet on that horse to those bettors and would give them a total of $750 USD in winnings — $15 USD for every $2 USD that they bet.
In horse racing, many types of bets can be placed in addition to bets on a horse to win. For example, in North America, people can also bet on horses to "place" or "show." A place bet wins when the horse finishes first or second, and a show bet wins when it finishes first, second or third. In addition, bets can be made on various combinations of horses and their order of finish. These are called exotic bets.
Like bets to win, the odds for other types of bets are calculated based on the same factors — the total amount bet, the take and the amount bet on each horse. When the payout for a specific type of bet is divided among more than one horse's bettors, such as the first three horses' bettors for show bets, the payout pool must be divided by that number. So, for example, the payout pool for show bets, also called the show pool, must be divided by three before the odds for each horse are calculated. Calculating the odds for other exotic bets can be quite complicated and involve combining various odds.
anon266060 Post 12 
If I bet $100 on a horse to show with 10:1 odds, how much will I win if he comes in third? 
anon230140 Post 11 
can we put our bet on a single horse only for a single race too? there is no quota for it?

anon154120 Post 10 
How are the place odds built and are they a function of the "to win" odds? 
anon98156 Post 9 
Actually, odds are rounded down to the nearest dime on the dollar, a nasty little concept known as breakage. In your example, your 1.491 horse pays off at 75, not 32. Think this sucks? Wait until that horse you bet in the show pool should have paid $2.78 to show and you get $2.60. That's 23 percent of your rightful payoff up in smoke. In New York they only round down to the nearest nickel on the dollar. Better, but not good enough. We have the technology to eliminate breakage, but the tracks just don't want to screw around with pennies and nickels or leave that easy breakage money on the table. 
anon63979 Post 8 
"Yes, it is legal to bet on all the horses, but you will lose money, guaranteed. " Not true. If you bet $2 on every horse in a 9 horse race and the horse that happens to win goes off at 20:1, you win money. It's still a dumb strategy, but the statement was incorrect. 
anon39930 Post 7 
Yes, it is legal to bet on all the horses, but you will lose money, guaranteed. Doing it that way, you are taking into account the pool money deducted from the pool (for the winning horse) multiple times, dropping the odds over and over again.
$1000 at 51 odds will pay you back $6,000, because you get your $1,000 back for winning. 
anon33443 Post 5 
What's the payoff if i bet 100.00 at 5 to 1 odds? 
anon19129 Post 4 
Is it legal to bet on all competing horses in a single race? 
anon16285 Post 3 
The $375 goes back to the bettors who bet on the winning horse. If you bet $2 at 3:2, then you get $3 for winning (based on total of $560 to be split among winners) plus your original $2 (your share of $375 bet on that horse) for a total of $5. The $375 really isn't deducted from the payout as it may appear from first glance. It is necessary to use the $375 to calculate out to divvy up the $560. Example of the above. Assume the track only sells $2 tickets. Then in this case 187.5 tickets were sold. With a pool of $935, divide by number of tickets sold to get payout: 935/187.5 = $5. 
anon8852 Post 2 
No one gets the $375 because that's what is bet on the horse for which you are calculating the odds. It is the value that 'our horse' has in the total prize pool. The prize pool is still $935 (after track take) but the odds are not 935/375 or $2.50 because you would then be including our horse's total bets in its own odds. In other words, it is a 3 to 2 chance to come in a winner or a 1.5:1 chance to win because it has the majority of the total prize pool (minus its own bets) bet on it. The greater the contributing share the lower its odds because of the higher chance
it will win (all of this is determined by how much is bet on it, so it could be a crap horse but still be punter's favorite to win due to the amount bet on it and the relatively small amounts bet on other horses).

irishdanIII Post 1 
What happens to the $375 deducted? Who gets it? Why is it deducted if it is part of the pool? 