Drench Dose Checker NZ

This turns the dose rate printed on your own drench label into millilitres per animal, a setting you can actually get out of a drench gun, and the total volume a mob will take. It holds no dose rates of its own and it will not tell you what to use or how much: the rate comes from the label in your hand, because the label is the legal instruction and it differs between products, species and formulations. What the page does add is the arithmetic people get wrong. It works to the heaviest animal in the mob rather than the average, which is the standard New Zealand guidance, and it shows you what dosing to the average would actually deliver as a share of the label rate, because that gap is the whole problem. It also rounds up rather than down to the graduation your gun can deliver, since rounding down puts you under the label rate on every single animal. Underdosing is widely identified as a driver of drench resistance, and two of the most common causes run in the same direction: eye estimates of liveweight tend to run low, and a gun that has not been checked can deliver less than its setting shows. Weighing a sample and checking the gun into a measuring cylinder are what catch both, and no calculator can do either for you.

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Read the label, and use your vet. This is a calculator, not animal health advice. It does not recommend a product, a dose rate, a drenching interval or a treatment, and the rate shown below is a worked example to demonstrate the arithmetic, not a recommendation. Replace it with the figure from your own label. The label carries the dose rate, the meat and milk withholding periods, the species and class it is registered for and the safety directions, none of which appear here. Animal remedies in New Zealand are regulated by the Ministry for Primary Industries under the ACVM Act, and worm management guidance comes from your veterinarian and the Beef and Lamb New Zealand Wormwise programme. If you are unsure, ask your vet before you drench.
mL
kg
kg
mL
L
4.5 mL
per animal, dosed to the heaviest in the mob
Before rounding4.5 mL
Total for the mob1,125 mL (1.13 L)
Animals a pack does1,111
Packs needed1

Rounded up to what the gun can deliver, because rounding down puts every animal under the label rate.

If you dosed to the average weight instead

The heaviest animal in the mob would get 3.8 mL, which is 84.4% of what the label rate asks for at its weight. That shortfall is the underdose, and it lands on every animal above the average, not just the biggest one.

How it works

The dose is the animal's weight divided by the weight step on your label, multiplied by the millilitres for that step. A label reading 1 millilitre per 10 kilograms gives a 45 kilogram animal 4.5 millilitres. That figure is then rounded up to the next graduation your gun can deliver, so a gun that moves in half millilitre steps gets set to the next half millilitre at or above the calculated dose. Rounding is always up. A gun cannot deliver 3.8 millilitres if its smallest step is half a millilitre, and the choice is between 3.5 and 4.0, only one of which meets the label rate. Total volume is the rounded dose multiplied by the number of animals, since dosing to the heaviest means every animal gets that dose. If your label gives a flat dose per animal rather than a rate against weight, choose that option and weight drops out of the arithmetic.

Why the heaviest and not the average

Because a dose set on the average is below the label rate for every animal above the average, which is half the mob. The standard New Zealand guidance is to weigh, dose to the heaviest, and draft the mob into weight ranges where the spread is wide enough that dosing the whole lot to the top would be wasteful. The comparison above puts a number on it for your own figures rather than leaving it as a principle.

Two things this cannot check for you

The first is the weight itself. Estimates by eye tend to run low, and a low weight gives a low dose, so a set of scales or a weigh crate does more for dosing accuracy than any calculator. The second is the gun. Set it to a known volume, squirt several doses into a measuring cylinder and divide, because a worn or poorly primed gun shows the right number on the barrel and delivers less than that into the animal. Both of those errors push in the same direction as an eye estimate, which is why they compound.

Worked example

A mob of 250 lambs where the heaviest is 45 kilograms and the average is 38, with a label rate of 1 millilitre per 10 kilograms and a gun that steps in half millilitres. The heaviest animal calculates to 4.5 millilitres, which the gun can deliver exactly, so every animal gets 4.5 millilitres and the mob takes 1,125 millilitres, or 1.13 litres. A 5 litre pack does 1,111 animals at that dose. Had the mob been dosed on the average instead, the calculated dose would have been 3.8 millilitres, and the heaviest animal would have received 84.4 percent of the label rate for its weight, a shortfall of 15.6 percent on the animal carrying the largest worm burden by bodyweight.

Related calculators

Data sources: this page ships no dose rates. Rates come from your own product label, which is the legal instruction under the ACVM Act administered by MPI. Guidance on weighing, dosing to the heaviest and drench resistance comes from the Beef and Lamb New Zealand Wormwise programme. Figures are checked twice monthly.

Frequently asked questions

Should I drench to the average weight or the heaviest animal?

The standard New Zealand guidance is to weigh the mob and dose to the heaviest animal in it, and to split the mob into weight ranges where the spread is wide. Dosing to the average means every animal above the average gets less than the label rate. Underdosing is widely identified as a driver of drench resistance, because a sub-lethal dose leaves the more resistant worms behind to breed. The Beef and Lamb New Zealand Wormwise programme is the New Zealand reference on this.

How do I work out the drench dose from the label?

Labels give a dose rate against liveweight, most often as millilitres per 10 kilograms for sheep drenches and per kilogram or per 50 kilograms for some cattle products. Divide the animal's weight by the weight step on the label and multiply by the millilitres for that step. Then round up to the next graduation your drench gun can actually deliver, because rounding down puts you under the label rate.

Why do people underdose stock without meaning to?

Because weight is estimated by eye rather than measured, and eye estimates of liveweight tend to run low. A drench gun that has not been checked adds to it, since a gun delivering less than its setting shows the right number and gives the wrong volume. Both errors run in the same direction, towards a dose below the label rate. Weighing a sample of the mob and squirting a few doses into a measuring cylinder are what catch them.