Kidney stones are hard mineral deposits that form inside your kidneys when certain substances in your urine become too concentrated. About 1 in 10 Americans will develop one at some point, with roughly 11% of men and 9% of women affected. Most small stones pass on their own, but larger ones can cause intense pain and may require medical treatment to remove.
How Kidney Stones Form
Your kidneys filter waste from your blood and send it out through urine. When the concentration of certain minerals rises above what the liquid can hold in solution, tiny crystals begin to form. This process, called supersaturation, is the starting point for every kidney stone.
Once those first crystals appear, they can grow in two ways: individual crystals clump together into larger clusters, or new crystals form on the surface of existing ones. For a stone to reach a size that causes problems, these clusters need to anchor somewhere inside the kidney or its drainage system. A crystal floating freely through the urinary tract would normally wash out before it grew large enough to matter. But when crystals attach to the inner lining of the kidney, they have time to accumulate layer by layer into a solid stone that can range from a grain of sand to a golf ball.
The Four Main Types
- Calcium stones are by far the most common. They’re made of calcium combined with oxalate, phosphate, or both. Low citrate levels in your urine, excess calcium, and high oxalate intake all raise the risk.
- Uric acid stones make up about 8 to 10% of all stones. They form when urine is too acidic (below a pH of 5.5), often in people who are dehydrated or eat large amounts of animal protein.
- Struvite stones account for 7 to 8% of cases and are caused by urinary tract infections. Certain bacteria produce ammonia that makes urine alkaline, triggering the formation of magnesium ammonium phosphate crystals. These stones can grow quickly and become quite large.
- Cystine stones are the rarest type, caused by an inherited condition that leads to too much of the amino acid cystine in the urine. People with this condition tend to form stones repeatedly, often starting in childhood.
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 moves into the ureter, the narrow tube connecting your kidney to your bladder. This triggers waves of severe, cramping pain in your side or lower abdomen that can shift to your groin. In men, the pain sometimes radiates to the testicles. These waves of pain, called renal colic, come and go as the ureter contracts to push the stone along.
Other common symptoms include blood in the urine (which may look pink, red, or brown), nausea or vomiting, feeling feverish or shivery, and signs of a urinary tract infection. The pain is often described as one of the most intense experiences people go through, sometimes compared to childbirth.
What Raises Your Risk
Not drinking enough water is the single biggest modifiable risk factor. When your urine is concentrated, minerals are more likely to crystallize. Beyond hydration, three dietary habits play an outsized role.
High sodium intake forces your kidneys to excrete more calcium, which feeds calcium stone formation. Adults should aim for less than 2,300 mg of sodium per day, roughly one teaspoon of table salt. This guideline is especially important if you’ve already had a calcium stone, even if you’re taking medication to prevent recurrence.
Eating large amounts of animal protein (beef, pork, chicken, eggs, fish, and shellfish) makes urine more acidic, which promotes both uric acid and calcium stones. If you’re prone to stones, reducing portion sizes of these foods can help. For calcium oxalate stones specifically, it helps to limit high-oxalate foods like spinach, rhubarb, nuts, peanuts, and wheat bran.
Will It Pass on Its Own?
The size of the stone is the strongest predictor of whether it will pass without intervention. Research tracking stones over 20 weeks found a clear pattern: stones under 3 mm pass on their own about 98% of the time. At 4 mm, the rate drops to around 81%. At 5 mm, roughly 65% still pass. But once a stone reaches 6 mm, only about a third will pass naturally, and stones 6.5 mm or larger have just a 9% chance of making it through on their own.
Where the stone is lodged also matters. Stones closer to the bladder have a shorter distance to travel and pass more easily than those stuck near the kidney. While waiting for a stone to pass, your doctor will typically recommend drinking plenty of water and may prescribe pain medication to manage the waves of discomfort.
How Kidney Stones Are Diagnosed
A CT scan without contrast dye is the gold standard for diagnosing kidney stones, picking up about 95% of them regardless of composition or location. It also reveals the stone’s size and exact position, which helps determine the best treatment approach.
Ultrasound is a radiation-free alternative often used as a first step, especially for pregnant women and children. It’s very good at confirming a stone is present when it finds one, but it misses stones more often than CT, detecting only about 45% of stones in the ureter. For many emergency room visits, though, studies have shown that starting with ultrasound leads to similar outcomes compared to jumping straight to CT.
Treatment Options for Larger Stones
When a stone is too large to pass or is causing complications, three main procedures can remove it.
Shock wave lithotripsy uses focused sound waves from outside the body to shatter the stone into tiny fragments that you then pass naturally over the following days or weeks. It’s done as an outpatient procedure under anesthesia, meaning you go home the same day. It works best on stones that aren’t too large or too hard.
Ureteroscopy involves threading a thin, flexible scope up through the urethra, into the bladder, and up into the ureter or kidney. Once the doctor locates the stone, they can either grab it with a small basket or break it apart with a laser. This is also typically a same-day procedure with anesthesia.
Percutaneous nephrolithotomy is reserved for the largest or most stubborn stones. The surgeon makes a small incision in your back and inserts a scope directly into the kidney to locate and remove the stone, sometimes using a laser to break it into manageable pieces first. This procedure requires a hospital stay of several days.
What Happens if a Stone Goes Untreated
A stone that blocks the ureter can cause urine to back up into the kidney, creating pressure and swelling known as hydronephrosis. Short-term obstruction is uncomfortable but usually reversible. Prolonged blockage, however, can damage kidney tissue and reduce function.
The most dangerous scenario is when a blockage coincides with a urinary tract infection. Infected urine trapped behind an obstruction can lead to a kidney infection that escalates rapidly into sepsis, a life-threatening condition. Research shows that having a ureteral stone with hydronephrosis more than doubles the risk of acute kidney injury in patients with a urinary tract infection. If you have stone symptoms along with fever, chills, or cloudy, foul-smelling urine, that combination requires urgent medical attention.
Preventing Recurrence
Kidney stones are a recurring problem. Without changes to diet or lifestyle, many people who’ve had one stone will form another. The single most effective prevention strategy is drinking enough fluid to produce at least 2.5 liters of urine per day, which keeps minerals diluted below the threshold where crystals form.
Citrate, found naturally in lemons, limes, and oranges, is one of the body’s built-in defenses against stones. It works in three ways: it binds to calcium in the urine so there’s less free calcium available to form crystals, it directly blocks crystal growth and clumping, and it can reduce how much calcium your body absorbs from food in the first place. People with low urinary citrate levels are at significantly higher risk for calcium stones. Increasing citrus fruit intake or using citrate supplements (as recommended by a physician) can help restore that protective effect.
Keeping sodium under 2,300 mg daily, moderating animal protein, and staying well-hydrated form the core prevention strategy for most stone types. If you’ve had a stone analyzed, knowing its composition allows you to tailor your diet more precisely, whether that means reducing oxalate-rich foods for calcium oxalate stones or addressing urine acidity for uric acid stones.