Kidney stones form when certain minerals in your urine become so concentrated that they crystallize and stick together into solid masses. This process, called supersaturation, is the core mechanism behind every type of kidney stone. The specific reasons your urine reaches that tipping point vary widely, from what you eat and drink to underlying medical conditions and genetics.
How Stones Actually Form in the Kidney
Your kidneys filter waste products from your blood and dissolve them in urine for removal. Under normal conditions, the water in your urine keeps these waste minerals diluted enough that they flow out as liquid. But when the concentration of minerals like calcium, oxalate, phosphate, or uric acid rises too high relative to the water available, the urine becomes supersaturated. At that point, tiny crystals begin to form, a process called nucleation.
These crystals can then grow and clump together over weeks or months, eventually forming a stone large enough to cause problems. Research supports the idea that variation in supersaturation alone can regulate whether stones form. Anything that increases the concentration of stone-forming minerals or decreases the volume of urine tips the balance toward crystallization.
The Most Common Cause: Calcium and Oxalate
About 80% of kidney stones are made of calcium oxalate, sometimes mixed with calcium phosphate. These stones form when calcium and oxalate levels in the urine are both elevated. High urinary calcium (hypercalciuria) is one of the strongest risk factors, and it often runs in families. About half of people who develop calcium stones have hypercalciuria.
Oxalate is a natural compound found in many everyday foods: spinach, beets, rhubarb, Swiss chard, sweet potatoes, nuts, chocolate, wheat bran, and tea are all high-oxalate sources. When oxalate from these foods is absorbed in your gut and filtered through your kidneys, it can combine with calcium in the urine to form crystals.
Here’s a counterintuitive detail that matters for prevention: eating calcium-rich foods (like dairy) at the same meal as high-oxalate foods actually reduces your stone risk. The calcium binds to oxalate in your digestive tract before it ever reaches your kidneys, so less oxalate ends up in your urine. Cutting calcium from your diet, which many people assume would help, tends to make things worse by leaving more free oxalate available for absorption.
Dehydration and Low Fluid Intake
Not drinking enough water is one of the most straightforward paths to a kidney stone. Less water means more concentrated urine, which means a higher chance of supersaturation. This is why stones are more common in hot climates, during summer months, and in people who sweat heavily without replacing fluids.
Urologists recommend that people who have had a kidney stone drink enough water to produce 2.5 to 3 liters of urine per day. For most people, that translates to roughly 3 liters (about 100 ounces) of fluid daily, though the exact amount depends on your activity level, climate, and body size. If your urine is pale yellow or nearly clear, you’re generally in a good range.
Diet Patterns That Raise Risk
Beyond oxalate, several dietary habits contribute to stone formation:
- High sodium intake. Excess salt increases the amount of calcium your kidneys excrete into urine. Processed foods, restaurant meals, and salty snacks are the biggest contributors for most people.
- High animal protein. Diets heavy in red meat, poultry, and fish raise uric acid levels and lower urinary citrate, a natural substance that helps prevent crystals from forming. This can promote both calcium stones and uric acid stones.
- Sugar and fructose. Sugary drinks and foods high in fructose have been linked to changes in urine composition that may promote stone formation, though the exact mechanisms are still being studied. Research has found that higher intakes of fructose and sucrose are associated with shifts in urine pH, which influences how easily different minerals crystallize.
Medical Conditions That Cause Stones
Sometimes kidney stones are a symptom of an underlying health problem rather than a diet or hydration issue.
Hyperparathyroidism
Your parathyroid glands regulate calcium levels in your blood. Normally, they release parathyroid hormone (PTH) when blood calcium drops too low, pulling stored calcium from your bones. Once levels normalize, PTH production stops. In hyperparathyroidism, the glands ignore that stop signal and keep releasing PTH, flooding the blood with excess calcium. Your kidneys then filter that extra calcium into the urine, creating the perfect conditions for calcium stones. Recurrent kidney stones are sometimes the first clue that someone has this condition.
Urinary Tract Infections
Certain bacteria that cause urinary tract infections produce an enzyme called urease, which breaks down urea in urine into ammonia at a rate roughly 10,000 times faster than it would break down on its own. This chemical reaction raises urine pH dramatically, making it alkaline. In that environment, ammonium ions combine with magnesium and phosphate already present in urine to form struvite stones. These infection stones can grow rapidly and become very large, sometimes filling the entire interior of the kidney (called staghorn stones). Proteus species are the most common culprits, though Klebsiella, Pseudomonas, and even MRSA can produce urease and trigger struvite formation.
Other Contributing Conditions
Obesity, type 2 diabetes, gout, inflammatory bowel disease, and chronic diarrhea all increase stone risk through various mechanisms. Bowel conditions, for instance, can increase oxalate absorption, while gout and insulin resistance raise uric acid levels in the urine.
Genetics and Family History
Kidney stones have a clear hereditary component. If a parent or sibling has had stones, your own risk is significantly higher than the general population’s. The most common genetic link is familial hypercalciuria, where the tendency to excrete too much calcium in urine is passed through families. Rarer genetic conditions also cause stones directly. Cystinuria, for example, causes the kidneys to excrete abnormally high amounts of the amino acid cystine, which crystallizes into cystine stones. These typically begin forming in childhood or early adulthood.
Why Stones Keep Coming Back
One of the most important things to understand about kidney stones is that they tend to recur. The recurrence rate after a first stone is as high as 50% within five years and 80 to 90% within ten years. The underlying conditions that caused the first stone, whether metabolic, dietary, or genetic, are usually still present after the stone passes or is treated. This is why urologists often recommend a metabolic evaluation after a first stone, testing your urine over 24 hours to identify which specific minerals are elevated and what’s driving the supersaturation.
The Four Main Stone Types
Not all kidney stones are the same, and knowing the type matters because it determines the cause and prevention strategy:
- Calcium oxalate and calcium phosphate stones are by far the most common, driven by high urinary calcium, high oxalate, low fluid intake, or a combination.
- Uric acid stones form when urine is persistently acidic (low pH) and uric acid levels are high. They’re more common in people with gout, diabetes, or high-protein diets.
- Struvite stones result from urinary tract infections with urease-producing bacteria and can grow very large very quickly.
- Cystine stones are caused by the genetic condition cystinuria and are relatively rare.
If you pass a stone or have one removed, keeping it for lab analysis is one of the most useful things you can do. The composition tells your doctor exactly what type you’re dealing with, which shapes every recommendation that follows.