A kidney stone is a hard deposit of minerals and salts that forms inside your kidney when certain substances in your urine become too concentrated. Stones range from a grain of sand to a golf ball in size, and most people don’t know they have one until it starts moving through the urinary tract, where it can cause severe pain. About half of all people who pass a kidney stone will form another one within five years, and the rate climbs to 80 to 90 percent at ten years.
How Kidney Stones Form
Your kidneys filter waste from your blood and send it out in urine. During that process, they pull back most of the water, which concentrates stone-forming substances like calcium, oxalate, phosphate, and uric acid. When the concentration of these substances exceeds what the liquid can hold in solution, a state called supersaturation, crystals can begin to form.
Urine is actually more resistant to crystal formation than you might expect. Natural proteins and other molecules slow the process down considerably, which is why most people don’t form stones even though their urine is regularly supersaturated to some degree. But when the balance tips, whether from too little water, too much of a particular mineral, or a shortage of those protective molecules, crystals can stick together and grow into a stone over weeks or months.
The Four Main Types
Most kidney stones fall into one of four categories, and knowing which type you have matters because prevention strategies differ for each.
- Calcium stones are the most common. They’re usually made of calcium oxalate, though calcium phosphate stones also occur. Diet, hydration, and metabolism all influence their formation.
- Uric acid stones form when urine is too acidic. They’re more common in people with gout, diabetes, or chronic diarrhea.
- Struvite stones develop after urinary tract infections and can grow large surprisingly fast, sometimes filling a significant portion of the kidney.
- Cystine stones are the rarest type, caused by an inherited condition called cystinuria that causes the kidneys to excrete too much of a specific amino acid.
What a Kidney Stone Feels Like
Small stones sitting quietly in the kidney often cause no symptoms at all. The pain starts when a stone drops into the ureter, the narrow tube connecting the kidney to the bladder, and partially or fully blocks urine flow. This produces what’s called renal colic: an intense, cramping pain in your side or lower back, between your ribs and hip. It often radiates to the groin, lower abdomen, or inner thigh.
The pain tends to come in waves. You might have a dull, constant ache punctuated by sharp flare-ups that last 20 to 60 minutes as the ureter spasms around the stone. Many people describe it as the worst pain they’ve ever experienced. Nausea and vomiting are common because the kidneys and gut share nerve pathways. You may also notice blood in your urine (it can look pink, red, or brown), pain during urination, cloudy or foul-smelling urine, or an urgent need to urinate more often than usual. Fever and chills can appear if an infection develops alongside the blockage.
Risk Factors
Not drinking enough water is the single most straightforward risk factor. When urine volume drops, stone-forming minerals become more concentrated. But several medical and lifestyle factors stack the odds further.
Obesity, high blood pressure, elevated blood sugar, unhealthy cholesterol levels, and a large waist circumference are the five traits of metabolic syndrome, and people who have all five are three times more likely to develop kidney stones than people with none of them. Even having four of the five doubles the risk. This connection matters because a kidney stone can sometimes be the first visible sign of conditions like diabetes or high blood pressure that haven’t been diagnosed yet.
Other risk factors include a family or personal history of stones, diets high in sodium or animal protein, frequent urinary tract infections (especially for struvite stones), certain digestive conditions that affect how your body absorbs calcium and water, and medications that alter urine composition.
How Kidney Stones Are Diagnosed
If your symptoms suggest a stone, imaging is the fastest way to confirm it. A non-contrast CT scan is the preferred first-line test, with a sensitivity of 98 percent and specificity of 97 percent for detecting stones in the ureter. It takes only a few minutes and doesn’t require any dye injection. Ultrasound is sometimes used as a radiation-free alternative, particularly in pregnant patients and children, though it catches only about 61 percent of stones. Standard X-rays miss nearly half of all stones because some types don’t show up well on film.
Beyond imaging, a urine sample can reveal blood, infection, or crystals, and blood work can flag elevated calcium, uric acid, or signs of kidney function changes. If you do pass a stone, saving it for lab analysis helps identify which type you formed, which directly shapes your prevention plan.
Passing a Stone on Your Own
Size is the biggest factor in whether a stone will pass without a procedure. Research using CT measurements found the following spontaneous passage rates: stones 1 millimeter or smaller pass about 87 percent of the time, stones 2 to 4 millimeters pass 76 percent of the time, stones 5 to 7 millimeters pass about 60 percent, and stones larger than 9 millimeters pass only 25 percent of the time.
For small stones likely to pass on their own, the typical approach involves drinking plenty of water to keep urine flowing, using pain relief as needed, and waiting. Most passable stones clear within a few days to a few weeks. Your doctor may prescribe a medication that relaxes the ureter to help the stone move along. Straining your urine through a filter to catch the stone is worth the effort because lab analysis guides long-term prevention.
When Stones Need a Procedure
Stones that are too large to pass, that cause unrelenting pain, or that lead to infection or kidney damage typically require intervention. The most common approach for stones under about 2 centimeters is shock wave lithotripsy, which uses focused sound waves from outside the body to break the stone into smaller fragments you can then pass naturally. For larger or harder-to-reach stones, a ureteroscopy involves passing a thin scope through the bladder and up the ureter to laser the stone into tiny pieces. Very large stones, particularly those filling the kidney’s central drainage area, may require a small incision in the back for a procedure called percutaneous nephrolithotomy.
Recovery varies by procedure. Shock wave lithotripsy is typically outpatient with a day or two of soreness. Ureteroscopy may involve a temporary stent in the ureter for a week or so, which can cause some discomfort and frequent urination. The more invasive approach requires a short hospital stay and a longer recovery period.
Preventing the Next Stone
Because recurrence rates are so high, prevention after a first stone is critical. The single most effective step is also the simplest: drink enough water to produce at least six to eight glasses worth of fluid daily. The goal is pale, dilute urine throughout the day.
For calcium oxalate stones, the most common type, reducing high-oxalate foods can help. The top offenders include spinach, rhubarb, wheat bran, nuts, and peanuts. Counterintuitively, you should not cut back on dietary calcium. Calcium in food binds to oxalate in the gut and prevents it from reaching the kidneys. Cutting calcium actually increases oxalate absorption and raises stone risk.
Limiting sodium also matters because high salt intake forces the kidneys to excrete more calcium. Reducing animal protein helps lower uric acid levels and makes urine less acidic, which benefits people prone to uric acid stones in particular. For some people, a metabolic evaluation (24-hour urine collection and blood tests) can pinpoint the specific chemical imbalance driving stone formation, allowing for targeted dietary or medication adjustments.