Kidney stones are hard deposits of minerals and salts that form inside your kidneys when certain substances in your urine become too concentrated. About 1 in 10 people will develop a kidney stone at some point, with men (11%) slightly more likely than women (9%). Most stones are small enough to pass on their own, but larger ones can block the flow of urine and cause intense pain that sends people to the emergency room.
How Kidney Stones Form
Your kidneys filter waste from your blood and send it out through urine. Normally, urine contains enough water to keep waste minerals dissolved. When the concentration of certain minerals rises too high, the urine becomes “supersaturated,” meaning it can no longer hold those minerals in liquid form. That’s the starting point for a stone, but high mineral concentration alone isn’t always enough to produce one.
Stone formation follows a chain of events. First, mineral salts clump together into tiny clusters called nuclei. These clusters usually form on an existing surface inside the kidney rather than appearing out of thin air. Once a nucleus reaches a certain size, it becomes stable and starts attracting more mineral deposits, growing larger. Then individual crystals begin sticking to each other, a step called aggregation that many researchers consider the most critical part of the process. If these growing clusters get trapped in the kidney’s narrow drainage passages instead of washing out with urine, they continue to build into what eventually becomes a stone.
Types of Kidney Stones
Not all kidney stones are made of the same material, and knowing the type matters because it shapes prevention strategies.
- Calcium oxalate stones account for 40 to 60% of all cases and form when calcium binds with oxalate, a compound found in many foods.
- Uric acid stones make up 5 to 10% of cases. They develop when urine is too acidic, which is more common in people with diabetes or gout.
- Calcium phosphate stones represent 2 to 4% of cases and tend to form in more alkaline urine.
- Cystine stones are the rarest at 1 to 3%, caused by a genetic condition that causes the kidneys to leak too much of a specific amino acid.
What Kidney Stone Pain Feels Like
A stone sitting quietly in the kidney often causes no symptoms at all. Pain begins when a stone drops into the ureter, the narrow tube connecting the kidney to the bladder. The hallmark symptom is a sudden, severe pain in the side and back, just below the ribs. This pain typically radiates forward and downward toward the groin, and in men it can extend to the testicle.
The pain often comes in waves as the ureter contracts trying to push the stone along. As the stone moves closer to the bladder, you may notice a shift in symptoms: more frequent urination, urgency, burning during urination, or difficulty starting a stream. Nausea and vomiting are common during the worst episodes. Blood in the urine, which can appear pink, red, or brown, is another frequent sign.
Risk Factors
Dehydration is the single biggest modifiable risk factor. When you don’t drink enough fluid, your urine becomes more concentrated, giving minerals a better chance to crystallize. Beyond fluid intake, several metabolic and dietary factors raise your risk.
A diet high in sodium increases calcium in the urine. Research has shown a clear link between higher body mass index and greater urinary calcium excretion, likely driven by higher animal protein and sodium consumption. People with type 2 diabetes tend to produce more acidic urine and excrete more oxalate, both of which favor stone formation. A family history of stones roughly doubles your risk, and once you’ve had one stone, the chance of forming another within five years is significant.
How Kidney Stones Are Diagnosed
A non-contrast CT scan is the gold standard, with sensitivity above 95% for detecting stones of any size or composition. It shows the stone’s exact location, its size in millimeters, and whether it’s blocking urine flow. Ultrasound is a radiation-free alternative sometimes used as a first step, especially during pregnancy, but it catches only about 77% of stones and is less precise for measuring size and pinpointing location. If you’ve had multiple CT scans for recurring stones, your doctor may switch to ultrasound for routine follow-ups to reduce cumulative radiation exposure.
Passing a Stone on Your Own
Whether a stone will pass naturally depends almost entirely on its width. Research tracking nearly 400 ureteral stones found stark differences by size:
- 3 mm or smaller: 98% pass on their own
- 4 mm: about 81% pass
- 5 mm: about 65% pass
- 6 mm: roughly 33% pass
- 6.5 mm or larger: only about 9% pass
For stones with a reasonable chance of passing, treatment focuses on pain control and hydration while you wait. A medication that relaxes the smooth muscle in the ureter can help. A large meta-analysis found that patients taking this type of medication passed stones at a rate of about 81% compared to 71% without it, and the stones came out roughly 3.5 days sooner on average. The medication also helps reduce pain by dampening nerve signals from the ureter.
When Surgery Is Needed
Stones that are too large to pass, cause persistent blockage, or trigger infection need procedural treatment. The approach depends on where the stone is and how big it is.
For kidney stones under 1 cm, especially those in the lower part of the kidney, several options are equally reasonable: shock wave therapy (which uses focused sound waves to break the stone into fragments from outside the body), a scope passed up through the urinary tract, or a minimally invasive approach through the back. The best results with shock wave therapy come when stones are 10 mm or smaller, not too dense on imaging, and located within 10 cm of the skin surface.
For stones larger than 1 cm in the lower part of the kidney, a procedure through the back consistently achieves higher stone-free rates than either shock waves or a scope. For any kidney stone over 2 cm, the American Urological Association recommends this approach as first-line treatment. The same goes for ureteral stones over 2 cm, or smaller stones that have already failed other treatments.
Ureteral stones 10 mm or smaller can typically be treated with either shock waves or a scope, with the choice depending on the stone’s exact position and the patient’s preferences.
Preventing Recurrence
Fluid intake is the most effective and simplest prevention measure. A meta-analysis from the National Kidney Foundation found that people who produced 2 to 2.5 liters of urine daily were 50% less likely to develop stones. Reaching that urine volume takes roughly 8 to 10 glasses of water per day, though you’ll need more in hot weather or with heavy exercise.
For calcium stone formers, dietary guidelines may seem counterintuitive. Restricting calcium actually increases stone risk because dietary calcium binds oxalate in the gut and prevents it from reaching the kidneys. The recommended intake is 1,000 to 1,200 mg of calcium per day from food sources, not supplements. Sodium intake should stay at or below 2,300 mg per day, since excess sodium forces the kidneys to excrete more calcium. Reducing animal protein helps lower uric acid and calcium in urine while making urine less acidic.
If you’ve passed a stone, saving it for analysis is one of the most useful things you can do. Knowing the exact composition lets you and your doctor target the specific dietary and metabolic factors that led to it forming in the first place.