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. These stones can range from a grain of sand to a golf ball in size, and they often cause no symptoms at all until they move into the narrow tube connecting your kidney to your bladder. At that point, they can trigger some of the most intense pain the human body experiences.
Kidney stones are remarkably common. In 2021, an estimated 106 million new cases were recorded worldwide, with men developing them more than twice as often as women. The highest rates occur in people between ages 50 and 54, though stones can form at any age.
How Kidney Stones Form
Your kidneys filter waste from your blood and concentrate it into urine. When certain minerals build up faster than your urine can dilute them, they reach a tipping point called supersaturation. At that concentration, dissolved minerals start crystallizing into tiny solid particles. These crystals can stick together over weeks or months, gradually growing into a stone large enough to cause problems.
The most common trigger is excess calcium in the urine, which raises the concentration of both calcium oxalate and calcium phosphate. That elevated concentration increases the likelihood that crystals will form and continue growing. Dehydration accelerates the process by making urine more concentrated overall, giving minerals less room to stay dissolved.
The Four Main Types
Not all kidney stones are made of the same material, and the type you have affects both treatment and prevention.
- 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 are the stones most directly tied to diet and hydration.
- Struvite stones make up about 15% of cases. They form in response to urinary tract infections and can grow quickly, sometimes becoming quite large with few early warning signs.
- Uric acid stones represent roughly 10% of stones. They develop when urine is consistently too acidic, which is more common in people who eat large amounts of animal protein or who have gout.
- Cystine stones are rare, accounting for about 2% of cases. They result from a genetic disorder that causes the kidneys to excrete too much of a specific amino acid.
What Kidney Stone Pain Feels Like
A stone sitting quietly in your kidney may cause no symptoms for years. The trouble starts when it drops into the ureter, the narrow tube leading to your bladder. If the stone gets stuck there, it blocks urine flow, causing the kidney to swell and the ureter to spasm. The resulting pain, called renal colic, is often described as worse than childbirth or broken bones.
The hallmark is severe, sharp pain in the side and back just below the ribs. This pain typically radiates downward into the lower abdomen and groin as the stone moves. It comes in waves rather than staying constant, with intense peaks that may last 20 to 60 minutes before easing briefly. You may also notice pink, red, or brown urine (from minor bleeding as the stone scrapes tissue), a burning sensation while urinating, nausea, and a persistent urge to urinate even when your bladder isn’t full.
The location of your pain often shifts as the stone travels. Pain higher in the back usually means the stone is still near the kidney, while pain in the groin or inner thigh suggests it has moved closer to the bladder.
How Kidney Stones Are Diagnosed
A non-contrast CT scan is the gold standard for detecting kidney stones. It picks up stones with 98% sensitivity and 97% specificity, far better than any other imaging method. The scan takes only a few minutes, requires no dye injection, and can precisely measure the stone’s size and location, both of which determine what happens next. Low-dose versions of the scan reduce radiation exposure by about 70% while still maintaining accuracy above 90%, and these are now preferred for most patients.
Ultrasound is sometimes used as a first step, particularly for pregnant women or children, since it avoids radiation entirely. It’s less accurate for small stones or those lodged in the ureter, but it can reliably detect the kidney swelling that a blockage causes.
Will It Pass on Its Own?
Size is the single biggest factor in whether a kidney stone will pass without intervention. Stones 4 millimeters or smaller pass on their own about 78% of the time. At 5 to 7 millimeters, that rate drops to around 60%. Stones 8 millimeters and larger pass spontaneously only about 39% of the time, and in one large study, no stone bigger than 10 millimeters passed without treatment.
When a stone is small enough to pass, the process typically takes a few days to a few weeks. During that time, drinking plenty of fluids helps keep things moving, and pain management becomes the primary concern. Your doctor may prescribe a medication that relaxes the smooth muscle in your ureter to help the stone travel more easily, though the evidence supporting this approach is still debated.
When Stones Need Procedures
For stones too large or too stubborn to pass naturally, three main procedures exist. The right choice depends on the stone’s size, location, and composition.
Shock wave lithotripsy uses focused sound waves from outside the body to break the stone into smaller fragments you can then pass naturally. It works best on stones 1 centimeter or smaller and is the least invasive option, typically performed as an outpatient procedure. For larger stones (1 to 2 centimeters), its success rate drops to around 58%, making it a less reliable choice.
Ureteroscopy involves threading a thin, flexible scope through the bladder and up into the ureter to reach the stone directly. The stone is either pulled out whole or broken apart with a laser. For stones between 1 and 2 centimeters, ureteroscopy succeeds about 81% of the time, making it the preferred option in that size range. Recovery usually takes a few days, and a temporary stent is often placed to keep the ureter open while swelling goes down.
Percutaneous nephrolithotomy is reserved for very large stones or complex cases. A surgeon makes a small incision in your back and uses a scope inserted directly into the kidney to break up and remove the stone. It requires general anesthesia and a short hospital stay but is the most effective approach for large or unusually shaped stones.
The Calcium Paradox in Prevention
Since most kidney stones contain calcium, it seems logical to cut calcium from your diet. This is actually one of the worst things you can do. When you eat calcium-rich foods alongside meals, the calcium binds to oxalate in your digestive tract before it ever reaches your kidneys. That means less oxalate gets absorbed into your bloodstream and less ends up in your urine. Reducing dietary calcium has the opposite effect: more free oxalate reaches the kidneys, increasing stone risk.
The foods to watch are those high in oxalate, which include spinach, rhubarb, beets, nuts, beans, and bran cereals. You don’t necessarily need to eliminate these, but pairing them with calcium-rich foods at the same meal helps neutralize the oxalate before it becomes a problem.
Hydration and Other Dietary Changes
Fluid intake is the single most effective prevention strategy. The goal is to produce enough urine that waste products stay diluted, and that means drinking up to 3 liters of fluid per day. A practical way to gauge this: your urine should be pale yellow or clear. If it’s consistently dark, you’re not drinking enough.
Reducing sodium also matters, because excess salt increases the amount of calcium your kidneys excrete. High animal protein intake raises uric acid levels and makes urine more acidic, creating conditions favorable for both uric acid and calcium stones. A diet that emphasizes fruits, vegetables, and moderate protein while keeping sodium in check addresses multiple stone-forming pathways at once.
For people who have already passed a stone, these changes are especially important. Without dietary adjustments, the recurrence rate is high, with roughly half of stone formers developing another stone within five to ten years.