Pet Medication Dose Calculator

What's the key thing to understand about Always Consult Your Vet?

This calculator provides reference doses only. Many medications interact with other drugs, are contraindicated in certain conditions, or require different doses for puppies/kittens. Always confirm the dose and safety with your vet or a phone triage service before administration.

What should I know about Cats Are Not Small Dogs?

Cats lack certain liver enzymes and cannot metabolise many drugs that are safe for dogs. Aspirin, ibuprofen, paracetamol (acetaminophen), and many human medications are toxic to cats. Never give a cat any medication intended for dogs without explicit veterinary guidance.

What's the difference between Prescription and OTC?

Most effective pet medications are prescription-only in the UK and EU. Over-the-counter flea and worm treatments vary significantly in efficacy. Prescription spot-on treatments from the vet are typically more effective than supermarket alternatives.

What do I need to know about Preventive Veterinary Care?

Regular preventive veterinary care reduces long-term costs and improves pet health outcomes. Annual vaccinations (dogs and cats) protect against serious diseases. Regular flea, tick, and worm treatments prevent infestations and parasite-borne diseases. Annual dental checks — dental disease affects over 80% of dogs and cats over 3 years old and causes chronic pain that is often undetected. Microchipping is compulsory for dogs in England and recommended for cats. Pet insurance taken out when pets are young and healthy is significantly cheaper than insuring an older pet or one with a pre-existing condition, since most policies exclude conditions that existed before the policy started. Taking out cover early, even for a healthy young pet, is usually the most cost-effective long-term approach.

What should I know about mg/kg Dosing and Why Weight Accuracy Matters?

Most veterinary medications are dosed according to the animal's body weight, typically expressed as milligrams of drug per kilogram of bodyweight (mg/kg), which is exactly what the dose_mg calculation in this tool is built around. This weight-based approach exists because a drug's effect and safety margin both scale with how much of an animal's body mass the medication has to reach and act on. It also means that the smaller the animal, the more a given weight measurement error matters proportionally: a 200g scale error is a rounding error for a 40kg dog but could represent a meaningfully large percentage error for a 2kg cat or a small-breed puppy, potentially pushing an actual dose outside the standard_range this calculator reports as safe and effective. Using the pet's most recent, accurately measured weight — ideally from a veterinary scale rather than an estimate — is one of the simplest and most important things an owner can do to make any mg/kg-based dosing calculation meaningful.

What should I know about Species-Specific Drug Toxicity?

One of the most important warnings this calculator carries is that medications cannot simply be scaled by weight from one species to another, because different species metabolize drugs through genuinely different biochemical pathways. The clearest example is that cats lack sufficient activity of a liver enzyme called glucuronyl transferase, which many other mammals (including dogs and humans) use to safely break down and eliminate a range of common drugs. This is a core reason why paracetamol (acetaminophen), a medication many people consider mild and over-the-counter for themselves, is highly toxic and potentially fatal to cats even in very small amounts, with no safe mg/kg dose at all. Several common non-steroidal anti-inflammatory drugs (NSAIDs) also carry much narrower safety margins and different approved uses in cats compared to dogs. This is exactly why species is a required input alongside weight in this calculator, and why the warnings output exists as a dedicated field — a mg/kg dose that would be entirely reasonable in one species can be genuinely dangerous in another, and this tool is intended as a reference starting point alongside veterinary guidance, never a substitute for a vet's or veterinary pharmacist's specific dosing instructions for your pet's situation.

What should I know about Why Human Medications Are Often Dangerous for Pets?

Many over-the-counter human medications that seem harmless are genuinely dangerous for pets, even at doses that look proportionally small. Paracetamol (acetaminophen), common in human pain relievers, is highly toxic to cats specifically because they lack a liver enzyme needed to process it safely, and even a single standard adult tablet can be fatal to a cat. Ibuprofen and other NSAIDs common in human pain relief can cause serious gastrointestinal ulceration and kidney damage in both dogs and cats at doses far lower than a human equivalent would suggest. Xylitol, a sugar substitute found in sugar-free gum and some peanut butters, causes a rapid, life-threatening insulin surge in dogs. Because pet metabolism differs from human metabolism in ways that aren't predictable just by scaling for body weight, no human medication should be given to a pet without specific veterinary guidance confirming both the drug and the dose.

Worked Example: Converting Body Weight for Accurate Dosing Calculations

Because most veterinary medication doses are calculated in milligrams per kilogram of body weight (mg/kg), and many owners in the UK still think in pounds or stones for a pet's weight, an inaccurate unit conversion is a surprisingly common source of dosing errors — always double-check which unit a prescribed dose or vet instruction is using.

Worked example: a cat weighed at the vet as 10 lb needs to be converted to kilograms before any mg/kg-based dose can be calculated: 10 ÷ 2.205 ≈ 4.5kg. If a vet has prescribed a dose at, say, X mg/kg (the specific figure always set by the prescribing vet for the specific medication and condition, never by a generic calculator), the total dose for this cat would be calculated as X multiplied by 4.5. The critical point this calculator exists to illustrate is unit accuracy and weight accuracy, not to suggest or confirm a specific drug dose — medication dosing, frequency, and formulation (including whether a human medication is safe at any dose) must always come directly from a vet or pharmacist who has examined the individual animal.

Why Can the Same Drug Need a Very Different Dose in Cats vs Dogs?

Beyond the general metabolic differences referenced above, cats and dogs can process the same drug at genuinely different rates due to differences in liver enzyme pathways — cats, for example, have a reduced capacity for a specific liver detoxification pathway (glucuronidation) compared to dogs, which is a core reason certain drugs that are dosed safely in dogs at a given mg/kg rate can build up to toxic levels in cats at an equivalent weight-based dose. This is a genuine species-level pharmacological difference, not just a matter of cats being smaller, which is why a dose calculated correctly for a dog of a given weight should never be assumed to simply scale down safely for a cat of equivalent weight. It is also why reputable veterinary dosing references list separate dosing guidance per species for the same drug, rather than one weight-based formula applied universally.

Why Do Vets Sometimes Prescribe 'Off-Label' Doses?

Many medications used in veterinary practice, including some very commonly prescribed ones, are technically licensed for human use or for a different animal species, and are prescribed by vets for pets based on established clinical experience and veterinary pharmacology knowledge rather than a formal pet-specific licence covering every possible use. This is a normal, accepted part of veterinary practice under the prescribing cascade used in UK veterinary medicine, not a sign of anything irregular, and it's precisely why the specific dose, frequency and suitability for an individual pet needs to come from a vet who understands both the general pharmacology and the specific animal in front of them, rather than from an owner extrapolating a human dose or a dose intended for a different species or condition.

Weight Conversion Quick Reference for Pets

Since dosing instructions and product labels sometimes use kilograms and sometimes pounds, a fast, accurate conversion matters before any weight figure is passed to a vet or used with a product label.

A small error here compounds directly into any weight-based calculation, so it's worth weighing pets on a scale that displays the same unit used on the product packaging where possible, or double-checking a converted figure against this kind of reference table rather than relying on a rounded mental estimate — particularly for small animals, where even a half-kilogram discrepancy is a large percentage of total body weight.