Calculate the correct cephalexin dose for your cat by weight, always confirm with your veterinarian first
| Weight (kg) | Weight (lb) | 15 mg/kg | 20 mg/kg | 25 mg/kg | 30 mg/kg | 35 mg/kg |
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Faiq Ur Rahman is a web designer, digital product developer, and founder of Toolraxy, a growing platform of web-based calculators and utility tools. He specializes in building structured, user-friendly tools focused on health, finance, productivity, and everyday problem-solving.
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Cephalexin dosing in cats isn’t one number. It depends on your cat’s weight, the infection being treated, the dose rate your vet chose, and which product form you actually have in the house. This calculator takes all four and returns the milligrams per dose, the daily total, and how much of a 250 mg capsule, 500 mg capsule, 75 mg chewable tablet, or 50 mg/ml suspension that works out to. Always confirm the dose with your veterinarian before giving your cat anything.
Cephalexin is a first-generation cephalosporin antibiotic. Vets prescribe it for skin infections, wounds, abscesses, urinary tract infections, and some respiratory infections. The standard veterinary range is 15 to 35 mg/kg by mouth every 6 to 12 hours, and the most common starting point is 15 mg/kg twice daily for five days. That range is wide on purpose. A mild wound and a deep pyoderma don’t get the same dose, and a cat with reduced kidney function may need less than a healthy one. The calculator puts the range into a single number for your cat, so you know what a prescribed dose rate actually looks like in milligrams and in real capsule fractions.
Pick your weight unit, then select the product form your vet prescribed. The product dropdown tells the calculator which capsules or liquid to break the dose into.
Enter your cat’s current weight. Weigh at home on a kitchen scale or a pet scale if you can.
Choose the dose rate. 15 mg/kg is the standard label dose; higher rates are for more serious infections.
Set the frequency your vet specified: every 12 hours, 8 hours, 6 hours, or once daily.
Select the condition being treated to load the typical treatment duration.
Check the dose per administration in the large panel, then look at the product breakdown for capsule, tablet, and millilitre amounts.
Read the verdict banner to see whether the chosen dose rate sits in the standard, moderate, or maximum band.
Confirm the number with your veterinarian before the first dose. The calculator is a double-check, not a prescription.
The calculator multiplies your cat’s weight in kilograms by the chosen dose rate in mg/kg. That gives the milligrams per dose. Frequency is separate and only affects the daily total. Product breakdown divides the dose per administration by each product’s strength.
Formula: Dose per administration (mg) = body weight (kg) × dose rate (mg/kg)
Formula: Daily total (mg) = dose per administration × (24 ÷ frequency in hours)
Formula: Product amount = dose per administration ÷ product strength
Weight converts at 0.45359237 kg per pound. If you enter pounds, the tool converts to kilograms before anything else runs. The dose rate dropdown holds five values: 15, 20, 25, 30, and 35 mg/kg. Frequency holds four: every 12 hours (BID), 8 hours (TID), 6 hours (QID), and once daily (SID, which is off-label for cephalexin in cats).
| Product | Strength | Typical use |
|---|---|---|
| Cephalexin capsule | 250 mg | Standard small-cat dosing |
| Cephalexin capsule | 500 mg | Larger cats or higher dose rates |
| Chewable tablet (Rilexine) | 75 mg | Palatable, scored, easy to split |
| Oral suspension | 50 mg/ml | Kittens, small cats, fractional doses |
Duration is preset by condition: 5 days for skin, wound and abscess, 14 days for urinary tract and respiratory infections, 28 days for deep pyoderma, or a custom number you enter. Blank or zero weight returns an invalid input message. The verdict banner changes with dose rate: 15 and 20 mg/kg read as standard, 25 mg/kg as moderate-severe, 30 mg/kg as severe, and 35 mg/kg as maximum.
Your cat weighs 3.8 kg and has a draining abscess from a fight. The vet prescribes 15 mg/kg twice daily for 5 days.
Dose per administration: 3.8 × 15 = 57 mg
Daily total: 57 × 2 = 114 mg per day
250 mg capsule: 57 ÷ 250 = 0.228 of a capsule
75 mg chewable tablet: 57 ÷ 75 = 0.76 of a tablet
50 mg/ml suspension: 57 ÷ 50 = 1.14 ml per dose
At 57 mg per dose, the 250 mg capsule is too large to split accurately. The chewable tablet gives you roughly three-quarters of a 75 mg tablet, which is easier with a scored tablet and a pill cutter. The suspension gives 1.14 ml, which is a clean measurement with a dosing syringe. This is exactly the situation where the product form matters as much as the dose. Same cat, same dose, three different practical answers.
Now raise the dose rate to 30 mg/kg for a deep infection. Dose per administration jumps to 114 mg. That’s 1.52 of a 75 mg tablet, or 2.28 ml of the 50 mg/ml suspension. The 250 mg capsule is still too large. The suspension becomes the most accurate option at that dose for a cat this size.
The standard range is 15 to 35 mg/kg by mouth every 6 to 12 hours. The most common regimen, and the one that matches the label on veterinary products like Rilexine and Therios, is 15 mg/kg twice daily. Severe infections may go up to 30 or 35 mg/kg, sometimes at shorter intervals.
The dose your vet picks depends on the infection and the cat. A superficial wound gets 15 mg/kg twice daily for five days. A urinary tract infection usually gets the same rate for 14 days. A deep pyoderma may need 25 to 30 mg/kg for 28 days or longer. The range exists because cephalexin is effective across it, and vets adjust within it based on how the infection responds.
Multiply the cat’s weight in kilograms by the prescribed mg/kg rate. A 4.5 kg cat at 15 mg/kg needs 67.5 mg per dose. At 25 mg/kg, the same cat needs 112.5 mg per dose. If you’re working in pounds, convert to kilograms first: divide the weight in pounds by 2.2046.
Frequency doesn’t change the per-dose amount. It changes the daily total. A 67.5 mg dose given twice daily is 135 mg per day. The same dose given three times daily is 202.5 mg per day. That’s why the calculator shows both numbers.
A 250 mg capsule contains 250 mg of cephalexin. The question is how much of it your cat needs. A 4 kg cat at 15 mg/kg needs 60 mg, which is 0.24 of a capsule. That’s less than a quarter, and quartering a capsule is unreliable. The contents spill, the split is uneven, and you can’t guarantee the dose.
This is the most common practical problem with cephalexin in cats. The capsules are designed for human-sized doses, and most cats need fractions. If your vet prescribes a dose below about 125 mg, ask whether the chewable tablet or oral suspension would be easier. Both measure more accurately at small doses.
That depends on the suspension concentration. A 50 mg/ml suspension delivers 50 mg of cephalexin per millilitre. A 60 mg dose needs 1.2 ml. A 100 mg dose needs 2 ml. If the suspension is a different strength, the volume changes accordingly.
Use the dosing syringe that comes with the suspension rather than a kitchen teaspoon. A teaspoon holds roughly 5 ml, but the actual volume varies by spoon and by how full you fill it. For a 1.2 ml dose, a syringe is the only reliable option.
The veterinary standard is 15 to 35 mg/kg per dose. The lower end, 15 mg/kg, is the label dose for soft tissue infections. The middle, 20 to 25 mg/kg, covers moderate infections. The top end, 30 to 35 mg/kg, is reserved for severe or deep infections and may be given more frequently.
Some references list 10 mg/kg as a minimum for very mild cases, and some compounded products are dosed at 30 mg/kg once daily. Those are variations on the same range, not separate standards. Follow the number your vet wrote on the prescription.
Five days for a simple wound or abscess. Fourteen days for a urinary tract or respiratory infection. Twenty-eight days or longer for deep pyoderma. Your vet may adjust based on how quickly the infection clears.
The rule is to finish the full course even if the cat looks better in two days. Stopping early is the fastest way to leave surviving bacteria behind and encourage resistance. If your cat won’t take the medication or keeps spitting it out, call the vet rather than skipping doses.
The most common side effects are vomiting and diarrhea, and they’re often mild and short-lived. Loss of appetite, drooling, and lethargy also show up. These signs are more likely at higher dose rates, and they usually ease if you give the medication with a small meal.
More serious reactions are rare but possible. Hives, facial swelling, and difficulty breathing suggest an allergic reaction and need immediate veterinary attention. Fever, pale gums, and unusual tiredness can also signal a problem. Call your vet if any of those appear.
The drug is the same molecule, and vets routinely prescribe human-labelled cephalexin because it’s often cheaper than veterinary products. The problem is dosing, not the drug. Human capsules come in 250 mg and 500 mg strengths, and most cats need a fraction of one.
Splitting a capsule accurately is hard. You also need a prescription, because giving antibiotics without veterinary direction risks the wrong drug, the wrong dose, and antibiotic resistance. Talk to your vet first. If they’re happy with human cephalexin, ask them to write the dose in milligrams so you can check the math.
Cats with a known allergy to cephalosporins, penicillins, or other beta-lactam antibiotics should not get cephalexin. Cats with severe kidney disease may need a reduced dose, because the drug is cleared through the kidneys. Kittens under 9 weeks and cats weighing under 2 kg are outside the labelled use for some veterinary products.
The most serious warning is for small pets other than cats and dogs. Cephalexin can be fatal in rabbits, guinea pigs, hamsters, and gerbils. If you have a multi-pet household, keep the medication well away from them.
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