What factors govern the field size in horse races?

Obviously, the field size of any horse race depends, first and foremost, on the number of horses declared to run. Depending on the nature and timing of the race in question, declarations to run must be made by 10.00am one or two days before the race. However, for safety reasons, the number of runners in any race is limited according to the size and configuration of the racecourse on which the race is run; the Derby at Epsom, for example, has a safety limit of 20, while the Wokingham Stakes at Ascot has a safety limit of 30 and the Grand National has a safety limit of 40.

At the declaration to run stage, if the number of declarations exceeds the safety limit, horses are eliminated to reduce the field size. Horses are eliminated in a specified order, starting with the lowest weighted, in handicap races, or the lowest rated, in Listed or Pattern races, with a ballot if necessary, to restrict the number of runners to the maximum field size. If the number of declarations exceeds the safety limit and is 18 or more, another strategy is to divide the race into two, or possibly more, divisions.

Another consideration is the stabling capacity of the racecourse, which cannot be exceeded if every horse is to have its own stable on arrival. If the declarations for all races at a fixture, including divisions, exceeds the so-called ‘field size limit’, horses are eliminated, race-by-race, as previously described, until those that remain can adequately be accommodated at the racecourse.