Bladder stones in cats form when minerals in the urine become too concentrated and clump together into solid masses. The two most common types, calcium oxalate and struvite, account for over 93% of all feline bladder stones combined, and each forms under different conditions inside the urinary tract.
The Two Main Types of Bladder Stones
A large study analyzing nearly 4,000 feline uroliths submitted to a veterinary laboratory between 2005 and 2018 found that 46.2% were calcium oxalate and 47.1% were struvite. These two stone types dominate feline urology, but they form through very different mechanisms and respond to different treatments. Interestingly, the proportion of struvite stones has been increasing over time while calcium oxalate has been declining, likely reflecting shifts in how commercial cat foods are formulated.
How Struvite Stones Form
Struvite stones are made of magnesium, ammonium, and phosphate. They tend to form when urine is neutral to alkaline (higher pH). Cats whose urine pH stays consistently elevated, in the range of 6.5 to 6.9 rather than a lower 6.0 to 6.2, may be roughly twice as likely to develop these stones.
In cats, struvite stones often form without any infection present. The exact mechanism behind this “sterile” struvite formation isn’t fully understood, but dietary factors that push urine toward alkaline conditions and increase mineral concentrations are the primary suspects. When a urinary tract infection is involved, certain bacteria produce an enzyme called urease that raises ammonia levels in the urine. This shifts the pH upward and changes how phosphorus behaves chemically, creating ideal conditions for struvite crystals to grow.
The good news with struvite is that therapeutic diets are 100% effective at dissolving these stones, typically within one to three weeks. This makes identifying the stone type critical, because it determines whether your cat needs surgery or can be treated with food alone.
How Calcium Oxalate Stones Form
Calcium oxalate stones favor the opposite urinary environment: acidic pH. They form when calcium, oxalate, or both become supersaturated in the urine. High blood calcium levels (hypercalcemia) are an important risk factor, because excess calcium gets filtered through the kidneys and ends up concentrated in the bladder.
Unlike struvite, calcium oxalate stones cannot be dissolved with a special diet. Once they form, they need to be physically removed. Prevention focuses on keeping urine dilute and maintaining a neutral pH range of 6.6 to 7.5, which discourages both stone types from forming.
Concentrated Urine Is the Common Thread
Regardless of stone type, the underlying problem is the same: urine that’s too concentrated with stone-forming minerals. Think of it like dissolving sugar in water. A glass of warm water can hold a lot of dissolved sugar, but as the water evaporates and the volume shrinks, sugar crystals start forming at the bottom. The same principle applies to minerals in your cat’s bladder.
Cats are desert-evolved animals that naturally produce very concentrated urine. They’re not big water drinkers, and cats fed exclusively dry food take in far less moisture than those eating wet food. This means the minerals in their urine are packed more tightly together, making crystal formation more likely. Increasing water consumption dilutes those minerals and is one of the single most effective strategies for preventing stones of any type. Veterinarians measure this concentration through a test called urine specific gravity: the lower the number, the more dilute and safer the urine.
Practical ways to increase your cat’s water intake include switching to wet or canned food, adding water to dry food, using a pet water fountain (many cats prefer running water), and placing multiple water bowls around the house.
Diet and Mineral Balance
What your cat eats directly influences urine pH and mineral content. Foods high in magnesium and phosphorus can contribute to struvite formation, while diets that acidify the urine too aggressively may tip conditions toward calcium oxalate. This is part of why the balance between stone types has shifted over the decades. When pet food manufacturers began acidifying cat food to prevent struvite (which was more common in earlier years), calcium oxalate rates climbed in response.
Therapeutic and preventive diets work by carefully controlling mineral levels and targeting a neutral urine pH that discourages both stone types. These formulations also tend to promote water intake through higher moisture content. If your cat has had stones before, your vet will likely recommend a specific diet based on which type was identified.
Signs Your Cat May Have Bladder Stones
The two hallmark signs are blood in the urine and straining to urinate. Blood appears because the stones physically rub against the bladder wall, irritating and damaging the tissue. Straining results from inflammation and swelling of the bladder lining or the urethra, and muscle spasms can make it worse. You might notice your cat visiting the litter box more frequently, producing only small amounts of urine, or crying while urinating.
In male cats especially, a stone or cluster of crystals can lodge in the narrow urethra and cause a complete blockage. This is a life-threatening emergency. If your male cat is straining repeatedly with no urine output, that warrants an immediate trip to a veterinary clinic.
Why Some Cats Are More Prone Than Others
Several factors beyond diet influence which cats develop stones. Middle-aged to older cats are more commonly affected by calcium oxalate stones, while struvite stones can appear at any age. Indoor cats, who tend to be more sedentary and may drink less water, face higher risk. Overweight cats also appear more susceptible, partly because excess weight is linked to metabolic changes that affect mineral handling.
Underlying health conditions play a role too. Any disease that raises blood calcium levels increases the risk of calcium oxalate stones. Chronic kidney disease, certain hormonal disorders, and even some medications can shift urine chemistry in ways that favor stone formation. Cats who’ve already had one episode of bladder stones are significantly more likely to develop them again, which is why long-term dietary management and regular urine monitoring are standard recommendations after a first occurrence.