Weight is a useful starting point, but it is not the full picture. Two dogs of the same weight can have very different builds. The most reliable way to choose is to measure your dog's stride and compare it against each size's running surface.
Weight alone does not always predict how much running surface a dog needs. A taller, leaner dog can need more length than a heavier, more compact one. A stride measurement gives you a far more accurate picture before you choose a size.
How to measure
Lay your dog gently on its side. Extend the front and back legs on one side so they are reaching outward, comfortably, not forced straight. Measure from the tip of the front paw to the tip of the back paw. This extended length is your dog's stride measurement.
Why it matters
Each F1 size has a belt length built to give a dog of that stride enough room to extend fully at a run without crowding the front or back of the mill. Comparing your measurement against belt length is more reliable than weight alone, especially for mixed breeds.
This is a general guide. Weight ranges below will fit most dogs of that build, but if your dog is unusually long, short, tall, or compact for its weight, lean on the stride measurement instead.
Typically fits
Whippets, Cocker Spaniels, Springer Spaniels, Beagles, Border Terriers, French Bulldogs, Staffordshire Bull Terriers, smaller Australian Cattle Dogs, Shiba Inus, and Standard Schnauzers.
Shop F1 Medium →Typically fits
Labrador Retrievers, Golden Retrievers, Border Collies, Boxers, Australian Shepherds, larger Springer Spaniels, Vizslas, Weimaraners, American Pit Bull Terriers, and Standard Poodles.
Shop F1 Large →Typically fits
German Shepherds, Belgian Malinois, Dobermans, American Bullies, larger Siberian Huskies, Rhodesian Ridgebacks, Dutch Shepherds, smaller Cane Corsos, Akitas, and Alaskan Malamutes.
Shop F1 XL →Typically fits
Rottweilers, Mastiffs, larger Cane Corsos, Great Danes, Newfoundlands, Bullmastiffs, Greater Swiss Mountain Dogs, Saint Bernards, Tibetan Mastiffs, and Caucasian Shepherds.
Shop F1 XXL →If you have dogs of different sizes, size for your largest dog. Smaller dogs adapt easily to a larger belt, while a smaller mill will not comfortably fit a larger dog. One F1 sized for your biggest dog will work for the whole household.
Send us your dog's breed, weight, and stride measurement if you have it, and we'll tell you exactly which size fits. No pressure, no upsell, just a straight answer.
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