Note: All medications should be prescribed by your veterinarian with directions including dosage, intervals at which the medication is given, and the form of the medication being prescribed.
What you need to know to calculate dosages
- Know the weight of your horse: Veterinarians are very good at estimating weight, but you can estimate by using a weight tape. EquiMed provides an Equine Weight Calculator to help you calculate your horse's weight.
- Know the recommended dose rate of the drug or medication per pound or kilogram of body weight.
- Know the concentration of the drug.
The basic formula for calculating a dose will be the same no matter what the form of the drug is: liquid, tablets, capsules, powder, or paste. The dose rate and form of administration are usually printed on the label and/or the package insert that comes with the medication.
To calculate the amount of drug to give your horse, use the following formula:
Weight of Horse X Dose Rate = amount of drug to give divided by the
Concentration (mg/ml or mg/pill or paste)
In the United State, dose rates are often given in milligrams (mg) per pound (lb). In other countries, the metric system is frequently used and the dose rate will be expressed in milligrams (mg) per kilogram (kg).
A kilogram contains approximately 2.2 pounds so if a dose rate is expressed in mg/kg simply divide the dose by 2.2 to convert to mg/lb: For example, if dose rate is 100 mg/kg, divide by 2.2 = 45 mg/lb.
To prevent mistakes, consistently choose either a pound or kilogram formula.
When working with tablets, pills or capsules:
- Multiply your horse's weight times the dose rate.
- Divide by the number of milligrams in the tablet, pill or capsule. This will give you the number of tablets, pills or capsules to give your horse.
When working with a liquid:
- Multiply your horse's weight in pounds times the dose rate in milligrams per pound. This gives you the total number of milligrams for the dose for your horse. For example: 1,000 lb horse X 10 mg/lb = 10,000 mg.
- The concentration of the drug is 500 mg/ml. Now divide the 10,000 mg by the 500 mg in the concentration. This will give you the number of milliliters of the drug to give. In this case, you will be giving 20 milliliters for your 1,000 lb horse.
When working with paste products:
Most paste products come in syringes with the dose rate marked on the plunger. The dose is administered by moving the ring on the plunger to the desired mark showing the correct number of milliliters to deliver per pound and depressing the plunger. Lock the ring in place before administering the medication to prevent giving the horse too much of the drug. If you are calculating the correct dosage for your horse, simply use the basic calculation formulas given above and apply them to the dose rate marked on the plunger.
Note that pills and tablets should not be given to foals and small ponies because of their small esophagi. Horses with a history of choke should not be given large pills or boluses.
Equine dose calculator
EquiMed provides an Equine Dose Calculator to help you with the calculations. In all cases, it is best to have your veterinarian prescribe all medications and drugs for your horses. That way, you will have the correct medication with the correct dosage and intervals for giving the medication. In no case, should you administer a medication or drug prescribed for one horse to any other horse.