Odds against is a term used to denote a price larger than that of even money (evens, 1/1). If a punter bets at odds against, they will receive more money than they wagered if their bet is successful (with the exception of a rule 4 in horse racing).
When a price is larger than that of evens, for example 9/1, the first number in the fraction is what a bookmaker is liable for. The second part of the fraction is the amount the customer is liable for. In this case, if the customer won and the 9/1 shot was landed, the customer would win the £90 lay by the bookmaker, plus the original £10 back. If the selection loses, the bookmaker receives the customer’s £10 stake in addition to keeping the £90 they lay.
When deciding to back an outcome of an event, it’s important to try and estimate the percentage chance your selection has of winning. In what’s known as a “fair book”, a 2/1 shot would have a 33% chance of winning and a 4/1 shot would have a 20% chance of winning. However, bookmakers do need to make money (which is known as an over-round). A bookmaker’s over-round would likely make a genuine 2/1 chance somewhere in the region of a 6/4 shot and a 4/1 chance a 7/2 shot. That said, bookmakers don’t always price markets up as accurately as some might think and if one can identify percentage chances, value can be identified long-term. Persistently backing 2/1 shots at 6/4 will put a punter in the red while backing 2/1 shots at 5/2 will give a punter every chance of making a decent return long-term.
When customers are trading on a betting exchange, odds are commonly seen in decimal format. When using decimals, the stake is always included in the price. Therefore, if a bet is struck at 8.0 on the betting exchange, the customer is effectively receiving odds of 7/1 and if the bet is struck at 2.0 on the exchange, that would represent even money (1/1).
The Betfair exchange will actually tell the customer what the fractional price is when hovering over the price with the cursor. It’s thought to make it easier for customers when betting at odds on; not everyone knows what a 4/11 shot is but when translated to 1.36 on the exchange a customer can see that for every £10 wagered, £3.60 is returned.