Estimating your horse's weight

This becomes important every time your horse is wormed, or when given most other types of medication. This is because the amount of the product needed is directly related to the weight of the horse. A bigger horse needs more medication. There are significant consequences of getting this wrong and unfortunately many horse owners are a long way out when they make a guess.

Below are two ways of coming up with the answer. The best way (apart from going to a weigh bridge) is to calculate it using two measurements taken from the horse. The next best is to use your horse's type and height to get the weight from the "standard" published weights in the table below.

Calculating the weight

This is usually considered slightly more accurate than using a weigh-tape. Essentially the calculation is as follows:

heartgirth x heartgirth x body length ÷ 330

This gives the weight of the horse in pounds (lbs) when measurements are provided in inches. You can use the form below to get our system to calculate the weight of your horse for you.

Take a measuring tape and measure all the way around the horse’s girth from the highest point of the wither going to just behind the elbows.

Measure from the point of shoulder in a straight line around to the point of buttock on one side.

Your horse weighs approximately:


Approximate weights (in kilograms)

If you are unable to use the calculation above then you can use the following table to get an idea of your horse's weight. Each weight is given as a range so you can pick a wight depending on the build of your own horse. All weights are shown in kg.

Pony Cob Thoroughbred
Sport Horse
10 170-200 - - - -
11 200-240 - - - -
12 230-260 - - - -
12.2 250-310 - - - -
13 250-340 - - - -
13.2 280-380 - - - -
14 320-380 360-450 - - -
14.2 350-400 380-480 - - -
15 - 470-530 400-470 450-500 -
15.2 - 500-580 440-500 470-520 -
16 - - 480-560 560-630 630-680
16.2 - - 520-590 590-650 650-720

So what happens if I get the weight wrong?

This will result in either under-dosing or over-dosing. An under-dose means that the treatment will not work properly. Worse than that, if a worm treatment has been given, then you will "teach" surviving worms to become resistant to that wormer in future. Those which survive will have been exposed to a low dose of the wormer and so drug resistance soon develops. All worm preparations suffer eventually from becoming ineffective due to the worms developing a resistance to that drug. This happens much more quickly if horses are regularly under-dosed.

Over-dosing some drugs can be dangerous to the horse, though wormers are very safe and this is unlikely to be a concern. However it is wasteful and should be avoided if possible. Some veterinary prescription medication can be dangerous if over-dosed, but it is down to your vet to get the dose right. Beware if the vet asks you what your horse weighs. If you have a good idea then say so. If not, it is best to get the tape out and work it out.

Please note that VioVet would like to help horse owners to medicate their animals correctly, but ultimately the responsibility for this lies with you and your own vet.. If you have any difficulty, please feel free to email us at [email protected] However VioVet cannot be held responsible for any problems of any nature arising from the use or misuse of the information we provide on this page and elsewhere on the website. By using the website you acknowledge your acceptance of this.