Learn About Kidney Stones and What Causes Them

What Kidney Stones Actually Are and Why They Form

Kidney stones are hard, crystalline deposits that develop inside your kidneys when certain minerals and salts in your urine become concentrated. Think of it like salt crystallizing at the bottom of a pot when water evaporates—except this happens inside your urinary system. These stones can range from the size of a grain of sand to larger than a marble, though most are somewhere in between.

Learn About Managing Your Amazon Account Privacy →

Your kidneys filter waste from your blood to create urine, which normally contains dissolved minerals like calcium, phosphate, and oxalate. Under normal circumstances, these substances pass through your system without incident. But when your urine becomes too concentrated—either because you're not drinking enough water or because you have higher-than-normal levels of stone-forming substances—these minerals can crystallize and stick together, forming a stone.

The process usually happens gradually over weeks or months, though sometimes it can occur faster. As the stone grows, it may stay in your kidney, move into your ureter (the tube connecting your kidney to your bladder), or travel further down your urinary tract. The location of the stone determines what symptoms you'll experience, if any.

Kidney stones are surprisingly common. According to the National Kidney Foundation, about one in ten people will experience a kidney stone at some point in their life. Men are three times more likely than women to develop them, and the risk increases with age, peaking between ages 40 and 60. However, younger people—even teenagers—can develop kidney stones too.

Practical takeaway: Understanding that kidney stones form from concentrated minerals in urine helps explain why hydration and diet matter so much in prevention. A stone doesn't form overnight or because of a single meal; it's the result of prolonged conditions in your urine.

The Main Causes and Risk Factors

Dehydration is the single biggest cause of kidney stone formation. When you don't drink enough water, your urine becomes more concentrated, making it easier for minerals to crystallize. This is why kidney stones are more common in hot, dry climates and during summer months. If you're someone who doesn't drink much water throughout the day, or if you exercise heavily without replacing lost fluids, you're at higher risk.

Free COVID-19 Test Location Guide →

Dietary factors play a major role. Too much sodium (salt) increases the amount of calcium in your urine, which raises your stone risk. High animal protein intake can increase uric acid and phosphate in your urine, both stone-forming substances. Foods high in oxalate—including spinach, beets, nuts, chocolate, and sweet potatoes—can contribute to stone formation in people who are susceptible. Conversely, not getting enough dietary calcium paradoxically increases your risk because your body absorbs more oxalate without calcium present to bind it.

Family history matters significantly. If your parents or siblings have had kidney stones, your risk is higher. This suggests both genetic factors and shared dietary or lifestyle patterns run in families. Some people are simply born with metabolic differences that make their bodies produce more stone-forming substances or less of the compounds that prevent stones.

Certain medical conditions increase kidney stone risk. Hypercalcemia (too much calcium in your blood) and hyperoxaluria (too much oxalate in your urine) are metabolic disorders that frequently cause stones. Gout, inflammatory bowel disease, and urinary tract infections can all contribute. People who have had one kidney stone have about a 50 percent chance of developing another within 10 years if they don't make preventive changes.

Medications can be a factor too. Diuretics (water pills), some antacids containing calcium, vitamin D supplements in high doses, and certain other medications can increase stone-forming substances in your urine.

Practical takeaway: While you can't change your genes or family history, you can control the three biggest modifiable causes: hydration, sodium intake, and dietary choices. Knowing your personal risk factors helps you focus on the prevention strategies most relevant to you.

Different Types of Kidney Stones and What Causes Each One

Not all kidney stones are created equal. Understanding the type of stone you have (or might develop) helps explain why it formed and what might prevent another one.

Free Mahjong Strategy Guide for Beginners →

Calcium oxalate stones are by far the most common, accounting for about 80 percent of all kidney stones. These form when oxalate and calcium levels in your urine are high. As mentioned, this can happen from diet (too much oxalate or too little dietary calcium), dehydration, or genetic factors that make your body absorb more oxalate.

Calcium phosphate stones make up about 15 percent of cases. These are more likely to form in alkaline urine (urine that's less acidic than normal). They're associated with conditions like renal tubular acidosis and primary hyperparathyroidism. Diet plays a role here too—excessive sodium and protein increase your risk.

Uric acid stones form when urine is too acidic and uric acid levels are high. These are more common in people with gout or those eating very high-protein diets, especially from animal sources. Uric acid stones are interesting because they're one of the few types that can sometimes be dissolved with medication, rather than always requiring removal.

Struvite stones (also called infection stones) develop as a result of urinary tract infections, particularly those caused by certain bacteria. These stones can grow rapidly and become quite large. They're more common in women because women have higher infection rates, but they're less common overall than calcium stones.

Cystine stones are rare, accounting for less than one percent of kidney stones. These form in people with a genetic condition called cystinuria, where the kidneys excrete too much of an amino acid called cystine into the urine. People with this condition often develop stones repeatedly throughout their lives.

Practical takeaway: The type of stone you develop reveals what's causing it. If your doctor identifies your stone type through imaging or lab analysis, that information guides both treatment and prevention strategies specific to your situation.

How Kidney Stones Develop and Progress

Kidney stone formation isn't a sudden event—it's a process that unfolds in stages. Understanding this timeline helps explain why early intervention and prevention matter.

Learn How to Mix and Pour Concrete →

The process begins with nucleation, where individual molecules of stone-forming minerals begin to cluster together. This happens in your kidney's collecting ducts, the tiny channels where urine is processed. At this stage, you won't feel anything; there are no symptoms. Your kidney might be doing this right now without you knowing it.

As these clusters continue to accumulate mineral particles, they grow into larger crystals. This growth phase can take weeks or months, depending on how concentrated your urine is. If you're well-hydrated and your urine is dilute, crystal growth slows significantly or stops. If you're dehydrated, the stone can grow much faster.

Once a stone reaches a certain size—usually around 5 millimeters across—it may start to cause problems. A small stone might remain in your kidney indefinitely, causing no symptoms. But if it moves into your ureter, that's when pain typically begins. The ureter is narrow, and even a small stone can cause severe pain as your body tries to move it along through peristalsis (muscular contractions).

Some stones pass naturally. If a stone is small enough (typically under 6 millimeters), there's a good chance it will make its way out of your body through your urine within a few weeks, though this is usually painful. Larger stones typically don't pass on their own and may require medical intervention.

Without treatment, a stone can block urine flow, which is dangerous. Urine backing up into your kidney can cause swelling and infection. This is why kidney stones aren't something to ignore—even if they're not currently causing symptoms, they need to be monitored.

Practical takeaway: The key window for prevention is before any stone starts forming or in the very early stages. Once a stone reaches a certain size, prevention alone won't help—medical treatment becomes necessary. This is why maintaining good hydration and dietary habits is important even if you've never had a stone.

Recognizing Symptoms and When Stones Become Dangerous

Many kidney stones cause no symptoms at all. These are called "silent stones" and are often discovered accidentally

Free Guide to Setting Up Your Zoom Meeting →