Before and after a race, jockey and kit are weighed, to ensure that the horse carries the correct weight, ensuring that the race is both safe and fair. This is specified by the racing secretary at the event, and can be found in the race day programme or form guide.
Weighing in before and after a race is crucial, for several reasons.
It ensures the race is fair
Ensuring that the horses carry the same weight makes it a level playing field. This makes sure that you are testing the skill of the jockey and the racing ability of the horse, with everything else taken out of the equation.
It ensures the horse is safe
Carrying too much weight in a race puts the horse in danger, risking serious harm in the demanding conditions of the race. Weighing in ensures that everyone around the horse knows what to expect.
It ensures there is no doubt about the result
Once riders are weighed in and weighted out, the result can be ratified by the authorities, as there is proof that the race was a fair and open contest.

How weigh in and weigh out is performed
To perform a weigh in and weigh out, the jockey must be weighed with all the kit that the horse is to carry in the race – saddle included. The jockey then hands the kit to the trainer or trainer’s assistant to saddle up the horse, ready to run.
After the race, the jockey must weigh in again with all the kit, confirming that the horse carried the correct weight. Once all the jockeys have been weighed after a race, you will hear the commentator announce “Weighed in. Weighed in.” This confirms that the result is valid.
What to do when the weights are wrong
If the jockey and the gear are under the combined total weight, special lead weights in a saddle cloth are added, to ensure that each horse is carrying the precise weight they need to carry for the race.
If the rider is too heavy, they may be replaced by another rider, or accepted to carry “overweight”, which is permitted by the rules but must be announced as such before the race begins. However, the margin for being overweight is just four pounds, which is why jockeys have such an intense training regime!

An important scientific detail – riders can lose or gain weight during a race, depending on weather conditions, dehydration and other factors. So how is this accounted for?
The rules from the British Horseracing Authority (BHA) state the following –
Should a rider weigh in more than one pound under the weight at which he weighed out, the Clerk of the Scales will lodge an objection with the Stewards, who will automatically hold an enquiry. The horse involved will be disqualified and the rider penalised.
If a rider weighs in two pounds or more over the weight that he weighed out, the Clerk of the Scales will report the rider to the Stewards and may be suspended. The one pound under and two pound over tolerances reflect the weight loss or gain a rider may incur, depending on weather conditions.
In this way, the authority accounts for the natural variation that happens during a race.

Is that all there is to it?
Well, no. It gets a little more involved than that.
Usually all horses will carry the same amount of weight for fairness, but sometimes newer, less experienced horses get to carry a slightly lighter load than the competition in order to help even the playing field.
Usually the amount of weight a horse is carrying ranges from 110 to 130 lbs. As discussed, if a Jockey weighs more than the maximum weight with his riding equipment it is announced as an overweight at the track, and either the horse races at a disadvantage with too much weight or a backup jockey races instead.
But there are also handicaps, which account for natural variations.
In handicapping, the most common thing people say is 5lbs = 1 second. In other words, a horse can run an average-length race 0.2 seconds faster for each pound you take off its back.
In the past, racing secretaries were notorious for using weight as the main condition in making races even, whereas today races are much more specific to skill level. For example, the famous horse Man O’War could carry up to 138 lbs as a 3 year old while still blowing away the competition, who were much more lightly weighted. There is also much discussion about how weight is distributed, and how it affects the running of the horse.

Essentially, weight is used to handicap a horse’s chance of winning. The more successful the horse, the more weight it carries and the less successful it should be in future races. At the other end of the scale, lighter horses are allowed to carry less weight and should thus be able to compete.
From a simple question, to many layers of complexity – it’s no wonder racing fans become transfixed! Weight is just one of the things to take into account when trying to pick a winner, which makes horse racing one of the most fascinating sports to watch.