Do betting odds tell the truth?
18,238 settled two-outcome markets, five sports, November 2024 – July 2026 ·
Published 3 August 2026 · Companion dataset free under
CC BY 4.0
Betting odds are predictions with money on them. Strip out the bookmaker's margin and every
price becomes a probability — a claim about how often something should happen. We tested
those claims against what actually happened, across every settled two-outcome market in our
archive: both sides of 18,238 markets, scored against real results.
36,476
outcome observations (both sides of every market)
±2 pts
how closely outcome rates track vig-free probabilities, in every decile
+2 pts
how much longshots overperform their vig-free price
−2 pts
how much favourites underperform theirs
Finding one: the market is a very good forecaster
Across every probability band, real outcome frequencies land within about two percentage
points of the market's vig-free probabilities. A thing the market prices as a 65% chance
happens about 63% of the time; a 25% chance happens about 27% of the time. As a pure
forecasting machine, the betting market is impressively calibrated — which is exactly
why beating it is hard.
| Vig-free probability band | Observations | Mean implied | Actually happened | Gap |
|---|
| 10–20% | 2,144 | 17.5% | 18.8% | +1.4 |
| 20–30% | 5,263 | 25.2% | 26.7% | +1.4 |
| 30–40% | 4,909 | 34.7% | 37.0% | +2.2 |
| 40–50% | 5,515 | 45.7% | 46.4% | +0.7 |
| 50–60% | 6,077 | 53.8% | 53.2% | −0.6 |
| 60–70% | 4,945 | 65.2% | 63.0% | −2.2 |
| 70–80% | 5,251 | 74.7% | 73.3% | −1.4 |
| 80–90% | 2,157 | 82.5% | 80.9% | −1.6 |
Finding two: the margin lives on the longshots
The residual pattern is systematic, not noise: outcomes priced below 50% happen
slightly more often than their vig-free probability suggests, and favourites slightly less.
Read together with our margin measurements, the
interpretation is straightforward: bookmakers do not spread their margin evenly
— they load it onto the longshot side of the market. In raw prices this is the
classic favourite–longshot bias, documented since the 1940s; measured here on modern,
multi-sport fixed-odds markets with the vig removed.
The tilt is roughly twice as large in player-prop markets (±5 points in the 30–40%
band) as in match-level markets — the same markets that carry the
highest margins.
What this means for a bettor, in plain English
- The market's probabilities are mostly honest once you remove the vig.
The dishonesty is in where the vig sits: longshot prices carry more of it.
- A long-odds multi is mispriced twice — every leg carries
longshot-loaded margin, and the legs multiply.
- "The bookies always know" is roughly true — to within about two
points. Which is why disciplined price comparison matters more than hunches.
Method
Every settled two-outcome market in our grading archive was included from both
sides: for each outcome at vig-free probability p, its complement enters
at 1−p with the opposite result. The dataset therefore nets to exactly
50/50 by construction and contains no information about anyone's selections — only
about the market. Probabilities were de-vigged across each market's outcomes; de-vig method
choice affects tail allocation, a caveat the dataset documents. No odds, prices, bookmaker
identities, teams or players appear in the data.
The full dataset (36,476 rows) is free under CC BY 4.0 — links to the archived copies
are in the citation block below as they come online.
Citing this study
Bet Better (2026). Do Betting Odds Tell the Truth? Market Calibration Across 18,238
Settled Markets. https://betbetter.world/studies/market-calibration
Companion research: the Bookmaker Margin Study ·
Australian gambling statistics ·
the free model API.
18+. Nothing here is betting advice. If the
market is calibrated to within two points and carries a
4.78% margin, the average bet loses by construction
— that is the honest arithmetic of gambling. If it is causing you harm:
Gambling Help Online or
1800 858 858.