The Importance of Weight in Horse Racing


Weight is the basis around which both handicapping and traditional form study is based. The well-established principle is that 3lbs = 1 length, or put another way if a horse loses to a rival by a length, if the rival is raised by 3lbs in the handicap and the horses subsequently meet again, that the result should be a dead heat. But is this actually true and can you gain an edge by better understanding the effect of weight on a racehorses performance.

In my own form study I actually totally ignore weight carried, undoubtedly there must be some effect of weight but in my opinion its impact is to minimal to materially affect the result. The reason horses stop winning when they go up in the handicap is in my opinion due to them having to go up in grade and race against better horses, not due to the weight they carry, A telling statistic is that in UK racing top weights win 27% of races, which is far more than they should statistically if weight had the effect it is meant to.

To put this into perspective. Horses are large animals, the average weight of a thoroughbred is around 550 kg's. Two kilograms(5 lbs) represents 0.36% of the horses body weight. Let's translate this illustration to human terms. Two 90kg athletes compete with athlete A narrowly victorious over athlete B over 200m. Same conditions 1 week later this time athlete A must carry 0.36% of his body weight strapped to his back as a penalty. This equates to 330 grams or about the size of a family block of chocolate.

Do you think that the 330 gram penalty will stop a powerful finely tuned athlete from repeating the win? Or will a 2kg weight turn around impede the winning chances of a 550 kg thoroughbred?

So that’s my view is there any analysis to back this up, or disprove my theory?

If we look at what percentage who are carrying more weight from their last race won compared with horses that carried less weight and won. Then this gives us the following stats:

Horses that carried more weight and won 11.5%

Horses that carried less weight and won 7.8%

This appears to show that extra weight carried is not a negative, however this data obviously includes lots of other factors which might be affecting the win %. So another, probably better way is to look at a horses performance as an individual, rather than if it won or lost.

A way of doing this is to look at the change in speed figure produced when weight was either added or taken away. Below are some analysis taken from US Dirt racing.

Test Results - Dirt Sprint - Carrying 5 lbs MORE weight since last start

Speed figure remained about the SAME from previous = 28%
Speed figure IMPROVED from the previous start = 34%
Speed figure DECLINED from the previous start = 38%

Test Results - Dirt Sprint - Carrying 5 lbs LESS weight since last start

Speed figure remained about the SAME from previous = 26%
Speed figure IMPROVED from the previous start = 34%
Speed figure DECLINED from the previous start = 37%

Test Results - Dirt middle distance race - Carrying 4 lbs MORE weight since last start

Speed figure remained about the SAME from previous = 30%
Speed figure IMPROVED from the previous start = 32%
Speed figure DECLINED from the previous start = 38%

Test Results - Dirt middle distance race - Carrying 4 lbs LESS weight since last start

Speed figure remained about the SAME from previous = 23%
Speed figure IMPROVED from the previous start = 37%
Speed figure DECLINED from the previous start = 40%

The improvement or decline of speed figures is virtually identical regardless of whether weight is being added or taken away. This shows to me that my initial thoughts that the effect of weight carried by a racehorse is exaggerated are correct, and I will continue to ignore weight when evaluating a horse race.