Pet Medication Dosage Calculator

How a mg/kg dose becomes a dose for your pet

Veterinary doses are written in milligrams per kilogram of body weight, usually shortened to mg/kg. Your pet's weight is in pounds on the scale at home, so the first step is always a conversion. One kilogram is 2.2046 pounds, which means you divide pounds by 2.2046 to get kilograms. A 40 lb dog is 18.14 kg. A 10 lb cat is 4.54 kg.

From there the arithmetic is short. Multiply kilograms by the mg/kg rate and you have milligrams per dose. If the label gives a range, such as 10 to 15 mg/kg, run the multiplication twice to get the low and high ends. Then divide by the strength on the package, 250 mg per tablet or 50 mg per mL, and you have a number of tablets or a volume in milliliters.

Three things decide the result as much as the weight does. The first is whether the drug is approved for that species. The second is how often the dose repeats, because a daily total is not the same as a single dose. The third is the concentration of the product in your hand, since a 50 mg/mL liquid and a 100 mg/mL liquid look identical on the shelf and one is twice as strong.

Worked example: a 40 lb dog and a 10 lb cat

A 40 lb dog comes out at 18.14 kg. Metronidazole for a dog runs 10 to 15 mg/kg every 12 hours, so the dose is 181 mg at the low end and 272 mg at the high end. With 250 mg tablets on hand that is 0.73 to 1.09 of a tablet, which in practice means one tablet twice a day. The daily total runs 363 to 544 mg.

The same drug for a 10 lb cat is a flat 10 mg/kg, not a range. The cat weighs 4.54 kg, so the dose is 45 mg. A 50 mg/mL oral liquid gives 0.91 mL every 12 hours. Notice how far apart those two numbers are: the dog gets roughly four times the milligrams of the cat, because the dog weighs four times as much and sits at a higher rate.

Amoxicillin-clavulanate shows the same pattern at a different rate. At 12.5 to 25 mg/kg the 40 lb dog needs 227 to 454 mg every 12 hours, which is one 250 mg tablet at the low end or two at the high end. The 10 lb cat needs 57 to 113 mg, so a 62.5 mg tablet is the usual fit. Guessing between those two tablet sizes is how cats end up underdosed for a week.

Dose rates for common pet medicines

These are the ranges vets use most often for dogs and cats, taken from the Merck Veterinary Manual drug tables. They are starting points for a prescription, not a prescription themselves, because the vet adjusts for the diagnosis, kidney and liver function, and other medicines.

The two rows at the bottom are the ones that hurt pets. Carprofen is a dog NSAID and is not given to cats at all. Meloxicam has a much narrower margin in cats, and the cat label is a one-time dose after surgery, not a daily pain pill. A cat owner who reaches for the dog's arthritis bottle is one of the most common poisoning calls vets take.

Why a dog dose is not a cat dose

Cats are not small dogs, and the difference is enzymatic rather than sentimental. Cats have limited glucuronyl transferase, the liver pathway that clears many drugs by attaching glucuronic acid so the kidney can flush them out. When that pathway is slow, a drug that a dog clears in a few hours can circulate for a day or more in a cat, and the same mg/kg turns into a much larger exposure.

The size difference compounds it. A 325 mg tablet is a small dose for a 150 lb person, about 2 mg/kg. Give that tablet to a 10 lb cat and it is 72 mg/kg, roughly 36 times the human rate. This is why a pill that barely registers for a person can be an emergency for a cat, and why the species on the label is not a suggestion.

Flea products are the other trap. Many dog spot-on treatments carry permethrin at concentrations that are fine for a dog and cause tremors, drooling and seizures in a cat. A cat that grooms a treated dog, or sleeps in the same bed, can be exposed. Never split a large dog tube among cats, and never apply a dog product to a cat to save a trip to the store. The FDA guidance on medicating animals safely covers the label rules that follow from this.

Human pain relievers that can kill a pet

Every week, someone gives a dog a Tylenol for a limp or an Advil for arthritis. The margins are narrow enough that a normal human dose can be a poisoning, and the damage is often delayed by a day or two, which makes it easy to miss the connection.

Acetaminophen is the worst of the four for cats. It turns hemoglobin into methemoglobin, which cannot carry oxygen, so the gums go brown or blue and the cat breathes hard. Ibuprofen and naproxen hit the stomach lining first, then the kidneys. In dogs the first sign is often vomiting or black stool, and by then the ulcer is already there.

There is no home dose of any of these for a cat, and no home dose of naproxen for a dog. If a pet has already swallowed one, the amount and the time matter more than anything you can calculate here. The Merck Veterinary Manual chapter on analgesic toxicosis lists the thresholds vets use when they decide whether to induce vomiting or start treatment.

Tablets, liquids and the math on the label

Tablet math is division, and the answer is rarely a whole number. A 181 mg dose against 250 mg tablets is 0.73 of a tablet, and against 100 mg tablets it is 1.81. Vets pick the tablet size that lands closest to a whole or half tablet, because a quarter tablet is hard to cut and easy to lose. Ask for a different strength rather than shaving a tablet with a kitchen knife.

Liquid math needs the concentration, which is printed as mg/mL. A 50 mg/mL suspension and a 100 mg/mL suspension need volumes that differ by a factor of two, so check the bottle every refill, since pharmacies switch manufacturers. Shake a suspension before every dose, because the drug settles and the last dose in the bottle can be much stronger than the first.

Measure with an oral syringe, not a kitchen spoon. A household teaspoon holds anywhere from 4 to 6 mL, and on a 0.9 mL cat dose that error is the difference between a therapeutic level and a miss. Draw the plunger to the mark, squirt into the cheek pouch rather than the back of the throat, and hold the head level so the pet swallows instead of inhaling.

Do not split extended release or enteric coated tablets, and do not crush them into food unless the vet says so. A crushed extended release tablet releases a full day of drug at once. If the pet will not take a pill, ask about a liquid, a transdermal gel or a chewable, all of which a compounding pharmacy can make at the strength you need.

Weighing your pet at home

Weight is the input that decides everything else, so it is worth getting right. A 1 lb error on a 10 lb cat is a 10% dose error, and for a drug with a narrow range like meloxicam that is enough to matter. A 1 lb error on a 60 lb dog is under 2%, which is noise.

Three ways to weigh, in order of accuracy. A baby scale gives the best number for cats and small dogs, since it is built for a wriggling 10 to 25 lb body. A bathroom scale works for larger dogs if you weigh yourself, then weigh yourself holding the dog, and subtract. A vet's scale is the reference, and most clinics will let you walk in and use it for free.

Weigh at the same time of day, before a meal, and write it down. A cat that drops from 11 lb to 9.5 lb over two months has lost 14% of its body weight, which is a medical finding, not a rounding error. The same log tells you whether a dose needs to change as a puppy grows, since a 20 lb puppy at 12 weeks may be 45 lb at 6 months, and the milligrams scale with it.

When to call poison control instead of calculating

This page converts a dose your vet already chose. It is the wrong tool the moment a pet has eaten something it should not have, because the question changes from how much to give to how much was taken and how fast it needs treatment. The ASPCA Animal Poison Control Center takes calls at 888-426-4435, and the ASPCA poison control page lists what to have ready. A consultation fee may apply.

Call right away for vomiting that will not stop, tremors, drooling, wobbling, a racing heart, brown or blue gums, collapse, or a pet that ate a pill bottle. Do not wait for symptoms, and do not induce vomiting unless a vet or the poison line tells you to. Hydrogen peroxide, the usual home remedy, burns the esophagus and is dangerous in cats.

Have the package in hand when you call. The strength, the number of tablets missing and the time of ingestion are what the toxicologist needs, and a photo of the label is faster than reading it out. If the pet is already showing signs, drive to the nearest emergency clinic and call on the way.

This is a reference estimate, not veterinary advice. Dose rates differ by diagnosis, and only your vet can set the amount and the interval for your pet.