Seresto Small Dog vs Large Dog: Which Size Do You Need?
Posted by Flea Collar Direct on Nov 5th 2026
Seresto makes two dog collars, and the only thing that decides between them is your dog’s weight. Seresto Small Dog is for dogs up to 18 lbs. (8 kg); Seresto Large Dog is for dogs over 18 lbs.
Side-by-side comparison
| Seresto Small Dog | Seresto Large Dog | |
|---|---|---|
| Pet weight | Up to 18 lbs. (8 kg) | Over 18 lbs. (8 kg) |
| Minimum age | 7 weeks | 7 weeks |
| Collar length | 15 in. | 27.5 in. |
| Net contents | 1 collar, 0.44 oz. (12.5 g) | 1 collar, 1.6 oz. (45 g) |
| Weight band on tin | Green: “For Dogs up to 18 lbs.” | Red: “For Dogs above 18 lbs.” |
| Protection | 8 months | 8 months |
| Protects against | Fleas, flea larvae, ticks, lice | Fleas, flea larvae, ticks, lice |
| EPA Reg. No. | 11556-155 | 11556-155 |
Both collars contain the same active ingredients at the same percentages (imidacloprid 10.0%, flumethrin 4.5%). The Large Dog collar is longer and heavier, so the dose matches a bigger dog. That is why weight, not breed, decides the size.
How to choose
- Weigh your dog. Use a vet visit, or weigh yourself holding your dog and subtract your own weight.
- 18 lbs. or under: choose Seresto Small Dog.
- Over 18 lbs.: choose Seresto Large Dog.
- Fit it correctly: two fingers should fit under the collar; trim extra length to 1 inch past the loop. See how to put on a Seresto collar.
What about a growing puppy?
Puppies can wear Seresto from 7 weeks of age. Choose the size for your puppy’s weight today, leave some extra length when you trim the collar, and check the fit regularly. If your puppy grows past 18 lbs., move to the Large Dog collar. More in Seresto for puppies and kittens.
| Collar | For | Pet weight | Minimum age | Collar length |
|---|---|---|---|---|
| Seresto Small Dog | Dogs & puppies | Up to 18 lbs. (8 kg) | 7 weeks | 15 in. |
| Seresto Large Dog | Dogs & puppies | Over 18 lbs. (8 kg) | 7 weeks | 27.5 in. |
| Seresto Cat | Cats & kittens | Any | 10 weeks | See label |
Frequently asked questions
What weight is Seresto Small Dog for?
Dogs and puppies 7 weeks and older that weigh up to 18 lbs. (8 kg).
What weight is Seresto Large Dog for?
Dogs and puppies 7 weeks and older that weigh over 18 lbs. (8 kg).
Can I cut a Large Dog collar down for a small dog?
No. The sizes contain different amounts of active ingredient for different weights. Choose the collar labeled for your dog’s weight.
How long is each Seresto dog collar?
Seresto Small Dog is 15 inches and Seresto Large Dog is 27.5 inches before trimming.
Information from the EPA-registered Seresto product labels. This is general product information, not veterinary advice; always read the full label and ask your veterinarian about your pet’s health. Flea Collar Direct is an independent retailer, not affiliated with Elanco. Seresto is a trademark of Elanco or its affiliates.