Enter your cat’s weight to see the commonly published weight-based estimate range — milligrams per dose and liquid milliliters — with the prescription and safety checks that matter for antibiotics. Runs entirely in your browser; nothing you type ever leaves this page.
Weight-based estimate from the published 10–15 mg/lb range (liquid = 125 mg/5 mL or 250 mg/5 mL suspensions). The output is always a range.
Weigh your cat first — a kitchen scale works for small cats.
twice daily (every ~12 hours), as prescribed
Estimate only — not veterinary advice. Antibiotics require a veterinarian’s prescription and diagnosis. Never give antibiotics without veterinary guidance. Your vet chooses the exact point in (or outside of) this range based on the infection type, severity, culture results, and your cat’s health.
Quick reference from the published 10–15 mg/lb range (twice daily as typically prescribed). Educational estimates — confirm with your vet.
| Weight | Per-dose range | Liquid 125 mg/5 mL | Liquid 250 mg/5 mL | Daily total (BID) |
|---|---|---|---|---|
| 3 lb (1.4 kg) | 30–45 mg | 1.2–1.8 mL | 0.6–0.9 mL | 60–90 mg |
| 4 lb (1.8 kg) | 40–60 mg | 1.6–2.4 mL | 0.8–1.2 mL | 80–120 mg |
| 5 lb (2.3 kg) | 50–75 mg | 2–3 mL | 1–1.5 mL | 100–150 mg |
| 6 lb (2.7 kg) | 60–90 mg | 2.4–3.6 mL | 1.2–1.8 mL | 120–180 mg |
| 8 lb (3.6 kg) | 80–120 mg | 3.2–4.8 mL | 1.6–2.4 mL | 160–240 mg |
| 10 lb (4.5 kg) | 100–150 mg | 4–6 mL | 2–3 mL | 200–300 mg |
| 12 lb (5.4 kg) | 120–180 mg | 4.8–7.2 mL | 2.4–3.6 mL | 240–360 mg |
| 15 lb (6.8 kg) | 150–225 mg | 6–9 mL | 3–4.5 mL | 300–450 mg |
| 18 lb (8.2 kg) | 180–270 mg | 7.2–10.8 mL | 3.6–5.4 mL | 360–540 mg |
| 20 lb (9.1 kg) | 200–300 mg | 8–12 mL | 4–6 mL | 400–600 mg |
This page covers cephalexin (a cephalosporin antibiotic) — a completely different medicine from Benadryl (diphenhydramine, an antihistamine). nrtechstudio.com has a separate cat Benadryl dosage estimator; make sure you are on the page for the medicine your vet actually prescribed.
Stop the medication and get to your vet or an emergency clinic now if you see any of these after a dose:
ASPCA Animal Poison Control Center: 1-888-426-4435 (a consultation fee may apply).
Side effects usually appear within hours. Persistent or severe GI upset warrants a call to your vet — the dose or drug may need changing.
No. Cephalexin is a prescription-only antibiotic. It requires a veterinarian’s diagnosis to confirm the infection is bacterial and that cephalexin is the right drug, plus a prescription with the correct dose, frequency, and course length for your cat. Giving antibiotics without veterinary guidance risks the wrong drug, the wrong dose, and antibiotic resistance. This calculator only shows what the commonly published weight range looks like as an educational estimate.
Using the commonly published 10–15 mg per pound range, a 10 lb cat’s per-dose estimate range is 100–150 mg, given twice daily (every ~12 hours) as typically prescribed — a daily total estimate of 200–300 mg. That is a range, not a recommendation: your veterinarian picks the exact amount based on the infection type, severity, and your cat’s health. Never dose from this page without your vet’s instructions.
Twice daily (every ~12 hours) is the most common schedule; some infections are prescribed every 8 hours. Typical courses run about 7–14 days for uncomplicated skin infections and up to 14–28 days for deeper infections, per university veterinary guidelines. Your veterinarian sets the schedule and duration on the prescription label — follow it exactly.
No. Stopping antibiotics early is one of the most common mistakes owners make. The surviving bacteria can multiply again, the infection can return worse, and surviving bacteria may develop resistance. Finish the entire course your veterinarian prescribed, even if your cat looks fully recovered. If you are worried about side effects, call your vet instead of stopping on your own.
Give the missed dose as soon as you remember, unless it is almost time for the next scheduled dose — in that case, skip the missed one and continue normally. Never give two doses at once to make up for a missed dose. If you are unsure, call your veterinarian for instructions.
The most common side effects are gastrointestinal: vomiting, diarrhea, decreased appetite, and drooling or lethargy. Giving the dose with a small amount of food can reduce stomach upset. Contact your veterinarian promptly if vomiting or diarrhea is severe or persistent, or if you see facial swelling, hives, difficulty breathing, or bloody stool — those can signal an allergic reaction and need urgent care.
Not without explicit veterinary approval — and often not at all. Cephalexin is a cephalosporin, and cross-reactivity with penicillin allergy is a known risk: a penicillin-allergic cat can have a serious allergic reaction, including anaphylaxis, to cephalexin. Always tell your veterinarian about any past drug allergy before any antibiotic is prescribed.
Excessive vomiting, diarrhea, abdominal pain, unusual lethargy, or in severe cases seizures and kidney problems have been reported with overdose. If you suspect an overdose — for example, a double dose or a pet eating the medication — contact your veterinarian or an emergency animal clinic immediately. The ASPCA Animal Poison Control Center (1-888-426-4435, a consultation fee may apply) can also advise.
Owners often notice improvement within 24–48 hours, though some infections take several days to respond. If there is no improvement at all after the period your vet indicated — or if the condition worsens at any point — call your veterinarian; the bacteria may not be susceptible to cephalexin and the treatment plan may need to change.
Cephalexin can generally be given with or without food; with food is easier on the stomach. Shake liquid suspensions well before each use, and refrigerate the reconstituted suspension — it is typically only good for about 7–14 days after mixing (check the pharmacy label). Use the oral syringe your pharmacy provides for accurate measuring. Never split or share a prescription between pets.
Both are beta-lactam antibiotics, but cephalexin is a first-generation cephalosporin while amoxicillin is a penicillin. They cover somewhat different bacteria, have different dosing schedules, and are chosen for different infections — they are not interchangeable. Your veterinarian picks the antibiotic based on the likely bacteria, culture results, and your cat’s history.
No. This page is about cephalexin, a prescription antibiotic for bacterial infections. Benadryl (diphenhydramine) is an over-the-counter antihistamine for allergies — a completely different drug with different dosing and different risks. nrtechstudio.com also has a separate cat Benadryl dosage estimator; make sure you are reading the page for the medicine your cat was actually prescribed.
Only under direct veterinary supervision. Young kittens, pregnant or nursing cats, and cats with kidney or liver disease need extra caution: the dose may need adjustment, and in some cases a different antibiotic is safer. Do not use leftover cephalexin or guess a smaller dose for these cats — the margin for error is much smaller.
Veterinarians most commonly prescribe it for skin and soft-tissue infections (wounds, abscesses), and sometimes for urinary or respiratory infections when the bacteria are susceptible. Cephalexin does not work against viruses, fungi, or parasites, and it is not effective against some bacteria such as Pseudomonas — which is why a veterinary diagnosis and sometimes culture testing come before the prescription.
The estimate multiplies your cat’s weight by the commonly published cephalexin range of 10–15 mg per pound per dose, twice daily (every ~12 hours) as typically prescribed. The exact metric equivalent is 22.0–33.1 mg/kg (published sources often round this to 22–30 mg/kg). Liquid volumes assume standard suspensions of 125 mg/5 mL (25 mg/mL) and 250 mg/5 mL (50 mg/mL). Daily totals assume twice-daily dosing.
Published references consulted (ranges summarized, not copied): the cephalexin-for-cats dosing guide at iditect.com (10–15 mg/lb every 8–12 hours), the OSU Veterinary Medical Center antimicrobial guidelines for companion animals (15–30 mg/kg PO q12h for feline soft-tissue infections), and GoodRx’s veterinary cephalexin overview (15–30 mg/kg every 12 hours for cats). Honest scope: this is arithmetic on a published range, not a diagnosis and not a prescription. Infection type, severity, culture results, age, kidney and liver function, and other medications all move the real prescription — only your veterinarian can weigh those.
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