What a bet actually costs, measured every day

Method

For every bookmaker's quote on every market, we sum the implied probabilities of all outcomes. A fair market sums to 1.00; anything above that is the margin. We record each quote as a separate observation, so a market quoted by 40 bookmakers contributes 40 measurements rather than one average.

Handicap markets are paired on the absolute value of the line, because the two sides carry opposite numbers. Head-to-head markets may have two or three outcomes; every other market type is measured two-way. Observations that resolve outside 0–30% are mispaired or stale quotes and are excluded.

The index reports aggregates only. It publishes no odds, no prices and no bookmaker identities, so it can be reused freely without redistributing licensed market data.

Use the data

The full daily series is available as CSV: betbetter-margin-index.csv — released under CC BY 4.0.

The research behind it

The method here is the one used in three published working papers, each with its own DOI: The Price of a Bet (margins across bookmakers, competitions and props), Well Calibrated, Unevenly Priced (market calibration and the favourite–longshot tilt), and The Price of Complexity (why quoted margins near 5% become a realized hold near 10%).

Companion studies: bookmaker margins · price vs hold · market calibration