Kidney stones form when minerals and salts in your urine become so concentrated that they crystallize and stick together. This happens through a straightforward process: when there’s too much of a stone-forming substance (like calcium, oxalate, or uric acid) relative to the liquid in your urine, crystals begin to form, grow, and eventually become large enough to cause problems. The lifetime risk is over 12% for men and about 6% for women, making this one of the most common urinary conditions.
How Stones Actually Form
Your kidneys filter waste from your blood and mix it with water to create urine. Normally, the waste stays dissolved. But when your urine becomes “supersaturated,” meaning it contains more dissolved minerals than the liquid can hold, tiny crystals begin to nucleate. Think of it like adding too much sugar to a glass of water: eventually the sugar stops dissolving and settles out as solid particles.
These microscopic crystals can attach to the inner lining of your kidney, where they slowly accumulate more mineral layers over weeks or months. Your urine naturally contains substances that inhibit crystal formation and growth, like citrate and certain proteins. When these protective factors are low or when mineral concentrations are high, crystals grow unchecked. A stone can sit quietly in your kidney for a long time, only causing pain when it moves into the narrow tube connecting your kidney to your bladder.
The Four Main Types of Kidney Stones
Not all kidney stones are made of the same material, and what yours is made of reveals a lot about why it formed.
- Calcium stones account for 65 to 70% of all kidney stones. Most are calcium oxalate, though some are calcium phosphate or a mix of both. These form when calcium and oxalate levels in urine are elevated.
- Struvite stones make up about 15% and are sometimes called infection stones. They develop in response to chronic or repeated urinary tract infections, and they can grow large quickly.
- Uric acid stones account for roughly 10%. They form when urine is consistently too acidic, often linked to high intake of animal protein.
- Cystine stones are rare, causing about 2% of cases. They result from a genetic disorder called cystinuria, which affects about 1 in 10,000 people.
Dehydration Is the Biggest Driver
The single most important factor in stone formation is how much water you drink. When you’re dehydrated, your urine becomes more concentrated, which pushes mineral levels past the point where crystals can form. Research on stone formers shows this clearly: when participants increased their fluid intake enough to nearly double their urine volume, the concentration of calcium oxalate (the most common stone material) dropped by roughly half. The protective “buffer zone” before crystals could form expanded significantly.
For prevention, the NHS recommends drinking up to 3 liters of fluid per day, enough to keep your urine consistently clear or pale yellow. Dark urine is a sign that waste products are concentrated enough to encourage crystal formation.
How Diet Contributes to Stone Formation
Several dietary patterns raise your risk, and some of them are counterintuitive.
High sodium intake is one of the strongest dietary risk factors. Your kidneys process sodium and calcium through similar pathways, so the more salt you eat, the more calcium your kidneys dump into your urine. That extra urinary calcium provides raw material for calcium-based stones. Reducing salt intake directly lowers urinary calcium.
Oxalate is a compound found in foods like spinach, rhubarb, nuts, and chocolate. It has a strong chemical affinity for calcium, and when the two combine in urine, they form an insoluble salt: calcium oxalate, the most common compound found in kidney stones. Here’s the counterintuitive part: eating too little calcium can actually increase your stone risk. When you eat calcium-rich foods alongside oxalate-rich foods, the calcium binds to oxalate in your gut before it ever reaches your kidneys. Cut calcium from your diet and more oxalate gets absorbed into your bloodstream, eventually ending up in your urine where it can form stones.
High animal protein intake contributes in multiple ways. It increases uric acid production, lowers urine pH (making it more acidic), and reduces citrate, one of the natural substances that prevents crystal formation. For uric acid stones specifically, the connection is direct: purines in red meat, organ meats, and shellfish break down into uric acid, which at high levels and low pH crystallizes in the urinary tract.
Medical Conditions That Raise Your Risk
Several health conditions make stones more likely by changing how your body handles minerals or acid.
Hyperparathyroidism causes your parathyroid glands to release too much hormone, which pulls calcium from your bones and raises calcium levels in both your blood and urine. Inflammatory bowel disease and chronic diarrhea alter how your gut absorbs calcium and water, concentrating stone-forming substances in your urine. Gastric bypass surgery creates similar absorption changes. Diabetes and metabolic syndrome increase the risk of uric acid stones by making urine more acidic. Repeated urinary tract infections can trigger struvite stone formation, as certain bacteria produce chemicals that make urine more alkaline and promote crystal growth.
Genetics and Family History
If your parents or siblings have had kidney stones, your risk is higher. The most well-defined genetic cause is cystinuria, an inherited condition where a mutation disrupts the kidney’s ability to reabsorb an amino acid called cystine. The cystine builds up in urine and forms stones. Both parents must carry the gene mutation for a child to develop the condition, and they often show no symptoms themselves.
Beyond cystinuria, broader genetic tendencies affect how your body handles calcium, oxalate, and citrate. Some people are simply more efficient at absorbing oxalate from food or less efficient at producing the protective substances that keep crystals from forming. This helps explain why two people with similar diets and hydration habits can have very different outcomes.
Medications That Can Cause Stones
Certain medications increase stone risk through two different mechanisms. Some are physically insoluble and can crystallize in urine themselves. Protease inhibitors used to treat HIV and some sulfonamide antibiotics fall into this category. Others change your urine chemistry in ways that promote stone formation. Topiramate, an anti-seizure medication also prescribed for migraines, is one of the strongest offenders. It inhibits an enzyme that normally helps your kidneys manage acid levels, leading to more alkaline blood but more acidic, calcium-rich urine. A large-scale analysis of FDA adverse event reports found that topiramate carried roughly 19 times the expected rate of kidney stone reports compared to other medications. Certain acid reflux drugs and anti-rheumatic medications also appear on the list of commonly reported stone-associated drugs.
Why Stones Recur
Having one kidney stone substantially increases your chance of getting another. Without changes to diet and fluid intake, up to half of stone formers will develop a second stone within five to ten years. The underlying conditions that created the first stone, whether that’s chronically low fluid intake, high sodium consumption, or a metabolic issue, typically persist unless actively addressed. This is why identifying what your stone is made of matters: it tells you which specific risk factor to target, whether that’s drinking more water, cutting salt, moderating animal protein, or managing an underlying condition.