This chart uses 149,000 horse race starts from UK
and Irish racing in 2020, sourced from Kaggle.
Each row is one horse in one race. The market favourite
is identified by the isFav flag, derived from Betfair
Starting Price odds. Implied win probability is
calculated as 1 / BSP odds, it represents what the betting market
believes the favourite’s chance of winning is.
The overall favourite win rate in this dataset is 32.3%, far better than a random baseline, but nowhere near certain. However, this single number hides a dramatic variation depending on how many horses are in the race.
Hover over each bar to see full details.
1. Win rate drops steeply with field size. In races with 2–5 runners, the favorite wins around 46% of the time. In races with 17 or more runners, that drops to around 24%. The decline is consistent with no exceptions across all five bands.
2. The market knows this, but only partially. The red dotted line also declines with field size, meaning the market does assign lower implied probabilities to favorites in bigger fields. However, in large fields the bar sits noticeably below the red line, suggesting the market still slightly overestimates the favorite’s chances even after accounting for field size.
3. The overall 32.3% average is misleading on its own. Because most races fall in the 9–12 runner range, the aggregate is pulled toward that middle group. A favorite in a 2–5 horse race is a fundamentally different proposition to one in a 17+ horse race, and should never be treated the same way.
Data: Kaggle Horse Racing Dataset