Kidney stones in cats form when minerals in the urine become too concentrated and crystallize into solid masses. The two most common types, calcium oxalate and struvite, account for over 90% of all urinary stones in cats, and each forms under different conditions. Understanding what drives stone formation can help you reduce your cat’s risk.
The Two Main Types of Kidney Stones
In a large study of nearly 4,000 stone submissions from cats between 2005 and 2018, calcium oxalate stones made up 46.2% and struvite stones made up 47.1%. These two types are almost equally common, but they form for very different reasons and require different approaches to management.
Less common types include urate stones (9.2%), apatite stones (7.4%), and a handful of rare mineral compositions that together account for a small fraction of cases. Among cats with urinary stones, kidney stones specifically (as opposed to bladder stones) are quite common. In one study, 66% of cats with urinary stones had stones located in the kidneys.
How Calcium Oxalate Stones Form
Calcium oxalate stones form when urine becomes oversaturated with calcium and oxalate, the two building blocks of this stone type. This typically happens in acidic urine with a pH below 6.8. When urine pH drops below 6.5, calcium oxalate saturation increases significantly. When it rises above 7.2, saturation drops.
Acidic urine causes problems in several ways at once. It pulls calcium out of bone, increases the amount of calcium filtered through the kidneys, and reduces the kidney’s ability to reabsorb that calcium before it reaches the urine. At the same time, acidic conditions lower the concentration of citrate, a natural substance that normally binds to calcium and prevents it from crystallizing. Acidic urine also impairs the function of certain proteins that act as crystal inhibitors, blocking calcium and oxalate from combining into stones.
Healthy urine contains several natural defenses against stone formation, including citrate, magnesium, and specialized proteins that interfere with crystal growth. When these inhibitors are reduced or overwhelmed, crystals can form, clump together, and grow into stones.
What Causes Struvite Stones
Struvite stones form under the opposite conditions: alkaline urine rather than acidic. Contributing factors include urine that stays too alkaline for too long and low urine volume. One important distinction in cats is that struvite stones are almost always sterile, meaning they form without a bacterial infection. This is different from dogs, where struvite stones are commonly linked to urinary tract infections. Because feline struvite stones aren’t infection-driven, they can often be dissolved with dietary changes alone.
Dehydration and Low Activity
Concentrated urine is one of the most consistent risk factors for kidney stones in cats. When a cat doesn’t drink enough water, the minerals in their urine become more concentrated, making crystallization more likely. Dilute urine reduces the concentration of stone-forming substances and leads to more frequent urination, which flushes those substances out before they can crystallize.
Cats are notoriously poor drinkers. They evolved as desert animals and don’t always have a strong thirst drive, which means many indoor cats live in a state of mild, chronic dehydration. Researchers at Cornell University’s Feline Health Center note that kidney stones tend to occur frequently in cats that are not very active, don’t take in enough fluids, and don’t urinate enough. A sedentary indoor lifestyle compounds the problem. Less movement generally means less drinking and less frequent trips to the litter box.
Stress may also play a role. A sedentary lifestyle and environmental stress have been linked to recurrence of urinary problems in cats, and providing more physical activity and enrichment for indoor cats has been shown to help resolve urinary symptoms.
Diet’s Role in Stone Formation
What your cat eats directly affects urine chemistry. Diets that acidify the urine can push conditions toward calcium oxalate stone formation, while diets that produce more alkaline urine can favor struvite stones. This is one reason why commercial “urinary health” diets are carefully formulated to target a specific urine pH range.
For cats prone to calcium oxalate stones, restricting both calcium and oxalate in the diet seems like an obvious solution, but it’s not that simple. Reducing only one of those minerals can actually increase the intestinal absorption and urinary excretion of the other, potentially making the problem worse. Foods high in oxalate or its precursors should be avoided, but calcium shouldn’t be cut too aggressively either.
Magnesium, phosphorus, and citrate act as natural inhibitors of calcium oxalate stone formation and should not be restricted in the diet. Excessively restricting phosphorus is particularly risky because low blood phosphorus triggers a chain of hormonal changes that ultimately increases calcium absorption from the intestines, adding more calcium to the urine.
Wet food is an important part of prevention. Because canned food is roughly 75% water, it significantly increases your cat’s total fluid intake compared to a dry-food-only diet. Cornell’s veterinary experts recommend that at least 50% of a cat’s diet be wet food to help maintain adequate hydration.
Breeds at Higher Risk
Genetics play a documented role in kidney stone formation. Persian, Himalayan, Ragdoll, and Burmese cats are all predisposed to calcium oxalate stones. There is evidence of heritability for this condition, and a genetic disorder called primary hyperoxaluria (where the body produces excessive oxalate) has been identified in some cats. If your cat is one of these breeds, proactive attention to hydration and diet is especially important.
Medical Conditions That Increase Risk
Several underlying health issues can tip urine chemistry toward stone formation. Metabolic acidosis, a condition where the blood becomes too acidic, is a significant driver. It promotes bone breakdown, which releases calcium into the bloodstream. That extra calcium is then filtered through the kidneys and excreted in the urine, creating the oversaturated conditions that calcium oxalate stones need to form.
High blood calcium levels from any cause increase stone risk for the same reason: more calcium in the blood means more calcium in the urine. Cats can develop elevated calcium for a variety of reasons, including certain cancers, kidney disease, and a condition called idiopathic hypercalcemia where calcium rises without an identifiable cause.
Signs Your Cat May Have Kidney Stones
Kidney stones can be tricky to detect because many cats show no obvious symptoms, especially when stones are small and stationary inside the kidney. When signs do appear, they can include blood in the urine, straining to urinate, urinating in unusual places, frequent small urinations, and visible discomfort or vocalization during urination. Some cats become lethargic or lose their appetite. If a stone moves from the kidney into the ureter (the narrow tube connecting the kidney to the bladder), it can cause sudden, severe pain and potentially block urine flow, which is a medical emergency.
Ultrasound is the most versatile tool for detecting kidney stones because it can identify both types that show up on X-rays and types that don’t. Standard X-rays can miss certain stone compositions entirely.
Reducing Your Cat’s Risk
The most practical steps focus on increasing water intake and encouraging frequent urination. Make sure fresh water is always available, and consider a pet water fountain since many cats prefer moving water. Feed wet food as a significant portion of the diet. Keep litter boxes clean, because a dirty box discourages cats from urinating as often as they should, giving minerals more time to concentrate and crystallize.
Encourage your cat to stay active. Interactive toys, climbing structures, and regular play sessions help combat the sedentary lifestyle that contributes to urinary problems. For cats with a history of stones or breeds at higher risk, your veterinarian can recommend specific therapeutic diets designed to maintain urine chemistry in a range that discourages stone formation.