The probability implied by a price: 1 / price × 100. A price of 4.0 implies a 25% chance. If a runner were truly 25% likely to win, 4.0 is the break-even price — no edge either way.
Overround
The sum of all implied probabilities minus 100%. A fair market sums to exactly 100%. Betfair markets typically sum to 101–107% — the excess is the bookmaker's margin, shared between Betfair (commission) and the layers. Higher overround means worse value for backers.
Market Efficiency
100 / total implied × 100. A 103% implied total = 97.1% efficiency. Betfair exchange markets are the most efficient in the world (97–99.5%) compared to bookmakers (85–93%). Efficiency measures how much of your money returns to the market vs. the house.
Normalised Probability
The implied probability rescaled so all runners sum to exactly 100%. This gives the market's best estimate of each runner's true win chance, stripping out the overround. The grey inner bar shows normalised vs the coloured outer bar for implied.
Fair Price
The break-even price if the normalised probability were correct: 100 / normalised%. If the market is efficient, this is the price where you have zero edge. Prices shorter than fair favour layers; prices longer than fair favour backers.
Commission
Betfair charges commission on net winnings, reducing effective back prices. At 5% commission, a price of 4.0 nets 1 + (4-1) × 0.95 = 3.85 effective odds. The commission tool shows how this shifts overall market efficiency and the effective implied total.