Horse Deworming Dose Calculator
Accurate anthelmintic dosing is critical to effective parasite control in horses and to slowing the development of anthelmintic resistance. Most equine dewormers are dosed in milligrams per kilogram of body weight. Under-dosing allows parasites to survive at sub-lethal drug exposures, accelerating resistance. This calculator computes the total milligram dose required based on the horse's body weight and the prescribed mg/kg dose rate for the chosen anthelmintic. Use an accurate weight, either from a scale or a weight tape, to ensure therapeutic dosing. Always consult your veterinarian for a targeted parasite control program based on fecal egg count monitoring.
Horse deworming dose formula
Total dose (mg) = Dose rate (mg/kg) x Body weight (kg)
Common dose rates from product labels: ivermectin 0.2 mg/kg, moxidectin 0.4 mg/kg, fenbendazole 5-10 mg/kg, pyrantel pamoate 6.6 mg/kg. Most commercial paste dewormers are formulated and marked by weight in pounds or kilograms; verify the product format before using a calculated mg total.
Equine parasite control principles
- Fecal egg count (FEC) testing classifies horses as low (<200 EPG), moderate (200-500 EPG), or high (>500 EPG) shedders, allowing targeted treatment.
- Most adult horses in managed settings are low shedders and may need only 1-2 treatments per year.
- Anthelmintic resistance in small strongyles (cyathostomins) to benzimidazoles and pyrimidines is widespread; resistance testing or FEC reduction tests are recommended.
- Ivermectin and moxidectin remain effective for most strongyle species but should not be overused to preserve efficacy.
- Foals and weanlings require more frequent treatment due to their greater susceptibility to ascarids.
Horse deworming calculator: frequently asked questions
Why is body weight important for deworming horses?
Most equine anthelmintics are dosed by body weight in mg/kg. Under-dosing is a leading cause of anthelmintic resistance because parasites are exposed to sub-lethal drug concentrations, allowing resistant populations to survive and reproduce. Accurate weight-based dosing ensures therapeutic drug levels throughout the animal.
What is the dose rate for common horse dewormers?
Common dose rates: ivermectin 0.2 mg/kg; moxidectin 0.4 mg/kg; fenbendazole 5-10 mg/kg (10 mg/kg for benzimidazole-susceptible small strongyles); oxibendazole 10 mg/kg; pyrantel pamoate 6.6 mg/kg for roundworms. Praziquantel is often included in combination products for tapeworms. Always verify the current product label.
How do I estimate my horse's weight?
A weight tape around the heart girth provides a reasonable estimate. The most accurate measurement is on a livestock scale. The body weight formula using heart girth and body length (Heart girth^2 x Body length / 300 for pounds from inches) can also be used. For deworming, always err on the side of an accurate or slightly over-estimated weight to avoid under-dosing.
What is strategic versus targeted selective treatment?
Strategic deworming treats all horses on a fixed schedule regardless of parasite burden. Targeted selective treatment (TST), now recommended by parasitologists, treats only horses with high fecal egg counts identified by fecal egg count testing. TST reduces anthelmintic use and slows resistance development.
Which dewormers work against bot flies?
Ivermectin and moxidectin are the only anthelmintics with efficacy against equine bot larvae (Gasterophilus spp.). Other classes (benzimidazoles, pyrimidines) do not treat bots. Treatment is typically given after first frost to kill all larvae ingested for the season.
Official sources
- USDA Animal and Plant Health Inspection Service: APHIS Homepage.
- FDA Center for Veterinary Medicine: Animal and Veterinary.
Reviewed by the CalculatorHub team, edited by James Graham, 15 June 2026. See our methodology.