Kidney stones are hard deposits of minerals and salts that form inside your kidneys when your urine becomes too concentrated. They range from a grain of sand to a golf ball in size, and passing one through your urinary tract can cause some of the most intense pain people experience. About 13% of men and 7% of women will deal with a kidney stone at some point in their lives.
How Kidney Stones Form
Your kidneys filter waste from your blood and send it out through urine. Normally, the waste products stay dissolved. But when urine becomes overly concentrated, those dissolved minerals can start clumping together into tiny crystals. This happens when your body excretes too much of a stone-forming substance, when your urine volume drops too low, or when urine pH shifts in a direction that favors crystallization.
The process begins with something called supersaturation. Think of stirring sugar into water: at some point, the water can’t hold any more sugar and crystals start forming. The same principle applies to minerals in your urine. Once the concentration crosses a threshold, microscopic crystals appear. Those crystals then grow by pulling more minerals out of the surrounding fluid, or they clump together with other crystals. This clumping, called aggregation, is what turns invisible particles into a stone large enough to cause problems. The whole process can take weeks to months.
Sometimes a tiny bit of cellular debris or an existing crystal acts as a seed, triggering crystallization the same way dangling a string in a supersaturated sugar solution produces rock candy.
The Four Main Types
Not all kidney stones are made of the same material, and the type you form matters for both treatment and prevention.
- Calcium oxalate and calcium phosphate stones account for 75% to 85% of all kidney stones. Calcium oxalate alone makes up 70% to 75%. These form when excess calcium or oxalate ends up in the urine, often from diet, dehydration, or conditions that cause the body to absorb or excrete too much calcium.
- Uric acid stones make up 8% to 10% of cases. They develop when urine is consistently too acidic (pH below 5.5) and are linked to diets high in red meat, fish, and organ meats. People with gout are at higher risk.
- Struvite stones represent 7% to 8% of stones and are caused by urinary tract infections. Certain bacteria produce an enzyme that breaks down urea into ammonia, raising urine pH and creating conditions for rapid crystal growth. These stones can grow large quickly, sometimes filling the entire interior of the kidney in a branching shape called a staghorn stone.
- Cystine stones are rare, accounting for just 1% to 2% of cases. They result from a genetic condition that causes the kidneys to leak too much of an amino acid called cystine into the urine.
What a Kidney Stone Feels Like
A stone sitting quietly in your kidney may cause no symptoms at all. The trouble starts when it shifts and enters a ureter, the narrow tube connecting each kidney to the bladder. That’s when most people experience renal colic: sudden, severe pain that comes in waves.
The pain typically starts in the side or back, just below the ribs, and can radiate to the lower abdomen and groin as the stone moves. Many people describe it as pain so intense they can’t sit still or find a comfortable position. Nausea and vomiting are common. You may also notice blood in your urine, ranging from pink to red to brown. Some people develop fever and chills, which can signal an infection that needs urgent attention. As the stone travels through the urinary tract, the location and intensity of the pain shift, which can be disorienting but is actually a sign the stone is progressing toward the bladder.
How Kidney Stones Are Diagnosed
A CT scan without contrast dye is the most accurate way to diagnose a kidney stone. Both the American College of Radiology and the American Urological Association recommend it as the first-line test for adults with symptoms. A CT scan reveals the stone’s size, location, and density, all of which help determine the best treatment approach. The density reading can even hint at what the stone is made of: uric acid stones typically measure 200 to 400 on the density scale, while calcium oxalate stones register much higher at 600 to 1,200.
Ultrasound is preferred for pregnant women and children under 14 because it avoids radiation exposure. It’s also a reasonable first step for adults when the suspicion of a stone is strong and the person has a normal body weight, since ultrasound works better in leaner patients. European guidelines recommend starting with ultrasound and using CT only when results are unclear.
Which Stones Pass on Their Own
Size is the biggest predictor of whether a stone will pass without a procedure. A study tracking nearly 400 stones over 20 weeks found that stones 3 millimeters or smaller passed on their own 98% of the time. At 4 millimeters, the rate dropped to 81%. At 5 millimeters, 65%. At 6 millimeters, only 33% passed spontaneously, and stones 6.5 millimeters or larger had just a 9% chance.
For stones under 10 millimeters, watchful waiting with pain management and plenty of fluids is a reasonable first step. Your doctor may prescribe medication to relax the walls of the ureter and help the stone pass more easily. Passing a stone can take anywhere from a few days to several weeks. If the pain becomes unmanageable, or the stone doesn’t budge, a procedure is the next step.
Procedures for Larger Stones
Three main approaches exist for stones that won’t pass on their own, and the choice depends largely on the stone’s size and location.
For stones smaller than 10 millimeters, shock wave therapy is often effective. You lie on a table while a machine sends focused sound waves through your body to break the stone into smaller fragments that can pass naturally. It’s noninvasive and usually done as an outpatient procedure, though it works less reliably on larger or very dense stones.
For stones between 1 and 2 centimeters, a thin scope can be threaded up through the bladder and ureter to reach the stone directly. A laser at the tip breaks the stone into fragments small enough to remove or pass. This approach avoids any incisions and has become a go-to option for mid-sized stones.
For stones larger than 1.5 to 2 centimeters, a more direct approach works best. A small incision is made in the back, and a tube is passed directly into the kidney to break up and extract the stone. This is the most effective technique for large stones, with clearance rates of 93% to 100% depending on size, but it involves a short hospital stay and a longer recovery.
Preventing Kidney Stones
About half of people who form a kidney stone will form another within five to ten years, so prevention matters. The single most effective strategy is also the simplest: drink enough fluid to produce about 2.5 liters (roughly 80 ounces) of urine per day. That’s more than most people produce naturally, and it usually means drinking water throughout the day, not just at meals. Clear or light yellow urine is a good sign you’re on track.
For the most common type, calcium oxalate stones, dietary adjustments make a real difference. Counterintuitively, you should not cut calcium from your diet. Getting about 1,200 milligrams of calcium daily, ideally from food rather than supplements, actually helps prevent stones. Calcium binds to oxalate in the gut before it reaches the kidneys, reducing the amount of oxalate in your urine. Cutting calcium has the opposite effect: more oxalate reaches the kidneys and stone risk goes up.
Keeping sodium below 2,000 milligrams per day also helps, because excess salt causes the kidneys to excrete more calcium. Reducing high-oxalate foods like spinach, rhubarb, beets, and nuts can lower oxalate levels in urine, though most people don’t need to eliminate these foods entirely unless their oxalate levels test high. For uric acid stones, reducing red meat, shellfish, and organ meats helps keep urine less acidic. Struvite stones require treating the underlying infection, since the bacteria are the root cause. Cystine stones are managed through aggressive hydration and medications that make cystine more soluble in urine.