A calculus of the kidney, more commonly called a kidney stone, is a hard deposit that forms inside your kidney when minerals in your urine become too concentrated and crystallize into a solid mass. These stones can range from a grain of sand to a golf ball in size, and they affect roughly 1 in 10 people at some point in life. Most cause no symptoms while sitting in the kidney, but when a stone moves into the narrow tube connecting your kidney to your bladder (the ureter), it can trigger some of the most intense pain the human body experiences.
How Kidney Stones Form
Your urine naturally contains dissolved minerals like calcium, oxalate, phosphate, and uric acid. Normally, your body produces chemical inhibitors that prevent these minerals from clumping together. But when urine becomes too concentrated, either because you’re not drinking enough fluid or because you’re excreting unusually high amounts of certain minerals, those inhibitors can’t keep up. The urine reaches a state called supersaturation, where dissolved minerals begin to solidify.
The process starts small. In the most common type of stone, tiny calcium phosphate crystals first form deep in the kidney tissue, near structures called the loops of Henle. These deposits gradually extend outward through the surrounding tissue until they reach the inner surface of the kidney. Once this solid material breaks through into the urinary space, which is already supersaturated with calcium oxalate, it acts as a seed. Crystals latch onto it and grow layer by layer, eventually forming a stone large enough to cause problems.
The Four Main Types
- Calcium stones make up 65 to 70% of all kidney stones. Most are calcium oxalate, though some contain calcium phosphate. They form when your body excretes too much calcium or oxalate in the urine.
- Struvite stones account for about 15% of cases and are sometimes called infection stones. They develop in response to urinary tract infections, when bacteria produce ammonia that changes urine chemistry. These stones can grow quickly and become quite large.
- Uric acid stones represent roughly 10% of stones. They form when urine is persistently acidic, which is common in people with gout, diabetes, or diets very high in animal protein.
- Cystine stones are rare, causing about 2% of cases. They result from a genetic disorder that causes the kidneys to excrete too much of an amino acid called cystine.
What a Kidney Stone Feels Like
A stone sitting quietly in the kidney may cause no symptoms at all. Pain begins when a stone drops into the ureter and blocks the flow of urine. This blockage creates pressure that stretches the kidney and ureter, producing what’s known as renal colic.
Renal colic is an intense, cramping pain that typically starts in your flank, the area between your lower ribs and hip on one side. It can radiate to your back, groin, or lower abdomen. The pain often comes in waves lasting 20 to 60 minutes, with the worst intensity hitting one to two hours after onset. Between waves you might feel a dull, persistent ache. Many people also experience nausea, vomiting, blood in the urine, or a frequent, urgent need to urinate, especially as the stone moves closer to the bladder.
Diagnosis
A non-contrast CT scan is the preferred first-line imaging test for kidney stones. It picks up stones with a sensitivity of 98% and specificity of 97%, far better than ultrasound or X-ray. The scan also reveals the stone’s size, location, and whether it’s causing obstruction. Ultrasound is sometimes used as an initial step for pregnant women or children to avoid radiation exposure, but CT remains the gold standard for accuracy.
Beyond imaging, your doctor will typically order blood and urine tests to check kidney function and look for signs of infection. If you’ve had more than one stone, a 24-hour urine collection can identify the specific metabolic imbalance driving stone formation, whether that’s too much calcium in the urine, too much oxalate, too much uric acid, or too little citrate (a natural stone inhibitor).
Will the Stone Pass on Its Own?
Size is the biggest factor in whether a stone will pass without intervention. Stones 1 mm or smaller pass spontaneously about 87% of the time. For stones 2 to 4 mm, the passage rate is 76%. At 5 to 7 mm, it drops to 60%. Stones between 7 and 9 mm pass on their own less than half the time (48%), and stones larger than 9 mm have only a 25% chance of passing naturally.
When a stone is small enough to pass, the process usually involves drinking plenty of water, managing pain, and waiting. Your doctor may prescribe a medication that relaxes the ureter to help the stone move along. Passing a stone can take days to a few weeks depending on its size and location.
When Procedures Are Needed
Stones that are too large to pass, that cause persistent pain, or that block urine flow and threaten kidney function require active treatment. The three main approaches are:
Shock wave lithotripsy (SWL) uses focused sound waves from outside the body to break a stone into smaller fragments that can then pass through the urinary tract. It works best for stones 1 cm or smaller and is a noninvasive outpatient procedure.
Ureteroscopy (URS) involves passing a thin, flexible scope through the bladder and up the ureter to reach the stone directly. The surgeon can then break it apart with a laser and remove the fragments. For stones in the lower part of the kidney between 1 and 2 cm, ureteroscopy has a median success rate of 81%, compared with only 58% for shock wave lithotripsy. That difference makes ureteroscopy the preferred option for medium-sized lower-pole stones.
Percutaneous nephrolithotomy (PCNL) is reserved for large stones, typically over 2 cm, or complex branching stones. The surgeon makes a small incision in your back and passes instruments directly into the kidney to break up and remove the stone. It requires general anesthesia and a short hospital stay but is highly effective for large stones that other methods can’t handle.
Recurrence and Prevention
Kidney stones have a frustrating tendency to come back. About 11% of people form a second stone within 2 years of their first. By 5 years, the recurrence rate climbs to 20%. At 10 years it’s 31%, and by 15 years, nearly 4 in 10 people will have had another stone. That makes prevention just as important as treatment.
The single most effective preventive measure is drinking enough fluid to produce at least 2.5 liters of urine per day, which keeps minerals diluted. Reducing sodium intake also helps because excess sodium causes your kidneys to excrete more calcium. One counterintuitive but well-established point: getting adequate dietary calcium (around 1,200 mg daily from food, not supplements) actually lowers your risk of calcium oxalate stones. Calcium in your gut binds to oxalate from food and prevents it from being absorbed into the bloodstream and filtered into the urine. Skipping calcium-rich foods can paradoxically increase the oxalate concentration in your urine and raise stone risk.
For people with recurrent stones and confirmed metabolic abnormalities, medications can target the specific problem. A type of diuretic helps reduce the amount of calcium your kidneys release into the urine by increasing calcium reabsorption. Potassium citrate raises urine citrate levels and makes urine less acidic, both of which inhibit stone formation. For uric acid stones, medications that lower uric acid production may be used, particularly in people who also have gout. Each of these is typically tried only after dietary changes alone haven’t been enough to prevent recurrence.