For most dogs in the 20 to 40 pound range, the Small or Medium size of an Ark Naturals dental chew is the sensible starting point. Be aware of one thing before you buy: Ark does not publish dog weight bands for its chew sizes on the product pages, so the size names are the only guidance available.
The honest answer about weight bands
Many dental chew brands print a chart on the bag mapping size to a pound range. Ark Naturals does not publish one on its online listings. We checked the live product data for the dental line, and the size options are named Mini, Small, Medium, Large and Extra Large with no accompanying weight ranges, no per chew dimensions, and no calorie counts.
That matters, and we are not going to invent numbers to fill the gap. If you see a blog confidently telling you that Ark's Medium is for dogs of 41 to 60 pounds, ask where the figure came from, because it is not on Ark's product pages.
What Ark does publish is pack weight and price per size, and those are real. Use them to understand what you are paying for, then use the size name plus your dog's chewing style to pick. If the pack in your hand carries a weight band printed on the physical label, that printed guidance beats anything online, including this article.
What Ark actually publishes, size by size
Here is the current published lineup for the dental range, with prices and pack weights as listed at the time of writing.
Two things jump out of that table. First, pack weight does not track size name in a straight line across products. The Protection+ Small pack is listed at 513 grams while the Brushless Toothpaste Small pack is 340 grams, so the Small in one line is a different quantity from the Small in another. Second, Extreme Clean currently exists only in Small, which limits your options if you have a large power chewer.
Picking for a 20 to 40 pound dog
A dog in this band is squarely in the middle of the range, which is the easiest place to be and also the place where two sizes will both plausibly work. Here is how to decide.
Start with Small if your dog is at the lower end, around 20 to 28 pounds, or is a light, methodical chewer who takes his time. Start with Medium if he is at the upper end, closer to 35 or 40 pounds, or if he is an enthusiastic chewer who tends to dispatch treats quickly.
Then run the practical test on the first chew. Watch one session start to finish. If the chew disappears in under a minute, or your dog gets his back teeth around it and crushes it in one or two bites, the size is too small and you should move up. If he gnaws at it for a long stretch and loses interest without making progress, the size may be too large or too firm for his enthusiasm, and a soft format may suit him better.
Take away the final piece once it is small enough to swallow whole. That rule holds at every size and is the single most important handling habit with any chew. It is also the reason supervision is not optional: the riskiest moment with a dental chew is the last ten seconds, not the first.
For a 20 to 40 pound dog specifically, Protection+ is a reasonable choice because it exists in Mini, Small and Medium, which brackets the range from both directions. You can see the full size and price list on the Protection+ Brushless Toothpaste page. Ark lists it as a proactive dental supplement, describes it as clinically proven to help reduce plaque and tartar and improve the oral microbiome, and notes that it is approved by the National Animal Supplement Council. Note that at the time of writing the Large option shows as unavailable, so check stock before planning around it.
If you would rather start gentler, the Soft Brushless and Soft Shield lines both offer Small, which will suit a 20 to 40 pound dog with an unremarkable mouth perfectly well. What neither offers is a Medium, so a dog at the top of the band moves straight from Small to Large in those two lines, and Large is a meaningful jump.
What the size name does not tell you
Size is only one of three variables, and the other two are easy to overlook.
Texture is the second. Ark publishes a texture label for each line: Medium for Brushless Toothpaste and Protection+, Soft for Soft Brushless and Soft Shield, Hard for Extreme Clean. A 30 pound dog with a sensitive mouth, worn teeth or a recent dental procedure wants the soft end regardless of his weight. A 30 pound terrier who destroys everything may want the firm end even though he is not a large dog.
Format is the third. Brushless Toothpaste and its soft sibling are chews. Protection+ and Soft Shield are positioned as daily oral supplements in chew form. Breath Bursts is a small, low calorie soft treat intended for breath rather than for prolonged chewing, so it is not a substitute for a dental chew at any weight.
It is also worth saying that chewing style is not predictable from breed. Two dogs of the same weight and the same breed can differ enormously in how hard and how long they chew, and the only way to find out is to watch one session with a new product. Budget one chew as a test rather than buying the largest pack on a guess.
Jaw shape matters too, though nobody publishes a chart for it. A flat faced 25 pound dog and a long muzzled 25 pound dog do not handle the same object the same way. Watch, do not assume.
Getting a definitive answer for your dog
The American Animal Hospital Association's guide to pet dental care is a good primer on what home care can and cannot achieve, and on how professional cleanings fit into the picture.
Beyond that, your veterinarian is the one person who can look at your specific dog's mouth and tell you which texture and size are appropriate. Bring the question to a routine visit rather than waiting for a problem. It takes thirty seconds to ask.
Raise it sooner if you notice bleeding gums, breath that has turned strongly unpleasant, a reluctance to chew on one side, dropped food, or a tooth that looks discolored or moves. Those are examination findings, not sizing questions. Periodontal disease is diagnosed by a veterinarian, typically with an oral exam under anesthesia and dental radiographs, and no chew treats it. What a chew can do is help manage the soft plaque that accumulates between professional cleanings.
For clarity, since this comes up constantly: no Ark Naturals product carries a Veterinary Oral Health Council seal, and none of Ark's listings claim one. Nothing in this sizing guide should be read as implying otherwise.
Frequently asked questions
Can I just buy the biggest size and break it up?
No, and this is a common mistake. Breaking a chew changes its geometry, which removes the resistance the dog is supposed to work against and creates sharp fragments he can swallow. Buy the size intended to be given whole.
Is Mini appropriate for a 20 pound dog?
Probably not. Mini is the smallest option in the range and a 20 pound dog is likely to dispatch it too fast for the chew to do useful work. It may suit a toy breed household or serve as a lighter option, but for a 20 pound dog Small is the better starting point.
Why does the Small pack weigh more in one product than another?
Because pack weight reflects how many chews are in the bag and how large each one is, and those differ by product line. Read pack weight as a value indicator, not as a proxy for the size of an individual chew.
Does my dog's age change what I should buy?
Age changes texture more than size, because adult weight tends to be stable while the mouth does not. Older dogs often have worn or missing teeth and do better in a soft format at the same size, so review the texture choice at every annual dental check and move gentler if your veterinarian finds wear or recession. Puppies should not start any dental chew without veterinary clearance first.
What if my dog sits right on 40 pounds?
Buy Medium and watch the first session. If he works it for several minutes and does not crush it immediately, you have the right size. If it vanishes, go up. Observation beats any chart, including one Ark has not published.
Related reading: Ark Naturals Protection+ Brushless Toothpaste: What Makes It Different From the Original?