Horse Dewormer Dosage Calculator From Weight

Dewormer dose volume calculator

Fill in the fields above and press Calculate.

Why the weight is the whole calculation

Every equine dewormer is dosed by body weight, and the weight on the syringe dial is an estimate of the animals it was designed for — not your horse. A "medium" setting on a dial is often 900–1,100 lb, which under-doses a 1,300 lb warmblood and over-doses a 500 lb pony.

Get a real number first with the horse weight calculator, then read the dose volume here and round up to the next syringe mark.

Label weights used by this calculator

Product class	Weight treated by one full syringe
Ivermectin 1.87% paste	up to 1,250 lb (567 kg)
Pyrantel pamoate paste	up to 1,200 lb (545 kg)
Fenbendazole 10% paste	up to 1,100 lb (499 kg)

Products and formulations differ between countries and brands. Always read the label of the tube in front of you; the calculator above reproduces the common labelled weights so you can see the arithmetic, not to replace the label.

Safety note. Under-dosing promotes resistant worms, which is a herd-level problem. Over-dosing risks toxicity, particularly with older drugs and in foals. Verify the weight, follow the label, and take veterinary advice on which drug class to use and when.

A worked example, and why it matters

A 500 lb pony on ivermectin paste labelled at 1,250 lb per syringe:

500 ÷ 1,250 = 0.4 of a syringe Rounded up to the next quarter: 0.5 syringe ≈ 2.5 dial marks on a 5-mark syringe

Left on a default "medium" dial setting the pony would receive more than twice the required dose. The same rounding logic protects an oversized horse in the other direction: 1,300 lb needs 1.05 syringes, so 1¼ syringes correctly rounded up — but the arithmetic only works if the weight is real.

Targeted deworming in practice

Calendar-based dosing of whole herds every eight weeks is the practice that produced today’s resistance. Current guidance leans towards:

Faecal egg counts to identify which horses are actually shedding eggs. Around 20–30% of horses carry most of the worm burden.
Treating the shedders with an accurately calculated dose, rather than everyone.
Rotating drug classes annually, not within a season, and checking efficacy with a follow-up count.
Pasture management — harrowing, resting paddocks and not overstocking does as much work as any chemical.

Your vet can set a testing and treatment plan for your specific yard. This page only does the arithmetic.

Frequently asked questions

How much dewormer should I give my horse?

Dose is determined by body weight and stated on the product label. Calculate the horse’s weight, work out the fraction of a syringe the label implies, then round up to the next mark.

What happens if I under-dose a dewormer?

Under-dosing kills only the most susceptible worms, leaving the resistant survivors to breed. That is the main driver of dewormer resistance, and it is why rounding up is standard practice.

Can I use one syringe for two horses?

No. Sharing a syringe risks transferring parasites and makes accurate dosing impossible. Use a fresh syringe per horse.

Do I need to deworm every horse every eight weeks?

Modern guidance says no. Use faecal egg counts to treat only the horses shedding significant numbers of eggs, and follow your vet’s programme for the yard.

Related tools and guides

Sources and method

Ontario Ministry of Agriculture, Food and Rural Affairs — Estimating body weight for horses.
NSW Department of Primary Industries — Primefact 928: Estimating a horse’s condition and weight.
Carroll, C.L. & Huntington, P.J. (1988) — the girth-and-length body weight formula used on weight tapes.

Page last reviewed 2026-10-16. Results are estimates from published formulas and are not veterinary advice.