Can You Pass a 2mm Kidney Stone? Symptoms & What to Do

A 2 mm kidney stone passes on its own in the vast majority of cases. In a study tracking nearly 400 ureteral stones, stones in the 0–2 mm range had a spontaneous passage rate of 98%.¹ That is about as close to a guaranteed outcome as you will find in medicine. Still, “it will probably pass” does not tell you how much it will hurt, how long you will wait, or when waiting stops being safe.

How Long It Takes to Pass

For stones 2 mm or smaller, the average time to passage is about eight days. In a study of 75 patients whose stones were tracked until they either passed or required a procedure, stones in this size range averaged 8.2 days, and only about 5% of patients needed any intervention. For 95% of these small stones to clear, the window stretched to roughly 31 days.² So while most people are done in a week or so, a small number wait a few weeks before the stone finally drops into the bladder.

Where the stone sits in your ureter also affects how quickly it moves. Stones closer to the bladder (the distal ureter) pass at higher rates than stones stuck near the kidney. One study found passage rates of about 48% for stones high in the ureter versus 79% for stones right at the junction with the bladder.³ Those numbers pool all stone sizes together, so a 2 mm stone in the upper ureter still has excellent odds compared to a larger one in the same spot. But location explains why two people with the same size stone can have very different timelines.

What the Pain Feels Like

Kidney stone pain, sometimes called renal colic, is often described as one of the most intense pains people experience. It typically starts in the flank, the area between your lower ribs and hip on one side, and can radiate down into the groin or inner thigh as the stone moves. The pain comes in waves rather than staying constant, and it is not really the stone itself scraping along the ureter that causes most of the agony. The pain is primarily driven by the ureter clamping down in spasm around the stone and by the pressure that builds when urine backs up behind the obstruction.

Other common symptoms include blood in the urine, nausea, and vomiting.⁴ You might also feel a persistent urge to urinate or a burning sensation when you do, especially once the stone reaches the lowest part of the ureter near the bladder. A 2 mm stone can absolutely produce all of these symptoms. Size affects how likely a stone is to pass, not how much pain it causes along the way, because even a tiny stone can trigger significant ureteral spasm.

Managing Pain at Home

Since a 2 mm stone is almost certain to pass without a procedure, pain control is the primary medical task. Over-the-counter anti-inflammatory drugs like ibuprofen or naproxen are the first-line choice, and the evidence strongly supports them over opioid painkillers for this purpose. A Cochrane review found that patients given NSAIDs reported lower pain scores in the majority of trials, needed less rescue medication, and experienced far less vomiting than those given opioids.⁵ A later meta-analysis confirmed NSAIDs offered a modest edge in pain reduction at 30 minutes and required fewer rescue treatments.⁶ NSAIDs work well here because much of the pain is inflammatory, and they directly target the swelling and spasm in the ureter wall.

Heat can also help. A randomized trial testing a heat-patch applied to the flank found it provided meaningful pain relief as a non-drug option.⁷ A heating pad or hot water bottle against the painful side is a simple complement to medication, especially during the wave-like spasms. Staying well hydrated is standard advice during an active episode. The logic is straightforward: more urine flow means more pressure pushing the stone along and less opportunity for the stone to sit in one place.

Do You Need Tamsulosin for a 2 mm Stone?

Your doctor might mention tamsulosin, an alpha-blocker originally prescribed for enlarged prostate, which relaxes the smooth muscle in the ureter and can help stones pass faster. A large meta-analysis of 46 studies found that tamsulosin improved overall stone passage rates and shortened the time to passage.⁸ But when the analysis drilled down to stones smaller than 6 mm, the benefit largely disappeared: passage rates were nearly the same whether patients took tamsulosin or not. The drug’s advantage was significant only for stones 6 mm and larger.

This makes intuitive sense for a 2 mm stone. When a stone is already small enough to pass almost every time, there is not much room for a drug to improve on a 98% success rate. Some clinicians still prescribe it on the theory that it might reduce pain or speed things up by a day or two, but the evidence for that specific benefit in very small stones is thin.⁹ If your doctor offers tamsulosin for a 2 mm stone, it is unlikely to hurt, but it is reasonable to ask whether the likely benefit justifies the side effects, which can include dizziness and low blood pressure.

Why Your Stone Might Not Actually Be 2 mm

This is a detail that catches people off guard: the size on your imaging report can be wrong, especially if the stone was measured by ultrasound. Ultrasound consistently overestimates the size of small stones. One study found that ultrasound measured stones in the 0–10 mm range as significantly larger than they actually were when compared to CT measurements.¹⁰ Another study put the average ultrasound measurement at 8.7 mm versus 5.5 mm on CT for the same stones, with the overestimation more pronounced for smaller stones and in patients with a higher body mass index.¹¹

This matters because treatment decisions hinge on size. A stone that ultrasound calls 5 mm might really be 3 mm on CT, which would change the expected passage rate dramatically. CT is the gold standard for sizing, so if you were told you have a 2 mm stone based on a CT scan, that number is reliable. If the measurement came from ultrasound, the stone could be even smaller than reported, which is actually reassuring. The reverse scenario, where a stone seems small on ultrasound but is actually larger on CT, is less common for kidney stones because the tendency is to overestimate, not underestimate.

Red Flags That Change Everything

Even with a tiny stone, certain warning signs mean you need urgent medical attention rather than watchful waiting at home. The biggest concern is an infected obstructed kidney: a stone blocking urine flow while bacteria are present upstream. This combination can lead to sepsis quickly. Signs include fever, chills, or a feeling that something is seriously wrong on top of the usual stone pain.

Other reasons to head to the emergency room include pain that you cannot control with oral medication, inability to keep fluids down because of persistent vomiting, or only having one functioning kidney. In one study of over 800 emergency department patients suspected of having kidney stones, about 6.5% turned out to have a different and acutely important diagnosis found on CT.¹² That is not a trivial number, and it reinforces why severe or unusual symptoms deserve imaging even when you think you know what is going on.

Not Every Flank Pain Is a Kidney Stone

The classic presentation of one-sided flank pain with blood in the urine points strongly toward a stone, but several other conditions can mimic it convincingly. Research looking at CT scans performed for suspected renal colic found that up to about one in three revealed findings unrelated to stone disease. The most common alternative diagnoses included gynecologic conditions like ovarian cysts, non-stone urinary problems such as kidney infections, and gastrointestinal issues like appendicitis and diverticulitis.¹³

A separate study found that among patients whose CT scans were done for flank or back pain, a ureteral stone explained the symptoms in about 48% of cases, while about 9% had a non-stone diagnosis, and in roughly 43% no definitive cause was identified on imaging at all.¹⁴ The takeaway is not that you should panic about every episode of flank pain, but that self-diagnosing based on a previous stone experience can miss something important. If you have never had imaging confirming the stone, the symptoms alone do not guarantee the diagnosis.

Catching the Stone on Its Way Out

Your doctor will likely ask you to strain your urine through a fine mesh or filter so you can collect the stone when it passes. This step is easy to skip, especially once the pain fades and you assume the worst is over, but it is one of the most useful things you can do for your long-term care. Analyzing the stone’s composition tells your doctor what type of stone you form, which directly shapes the prevention plan.

Most kidney stones are calcium-based, mainly calcium oxalate, but stones can also be made of uric acid, struvite (linked to infections), or cystine (linked to a genetic condition). The treatment and dietary changes differ substantially by type. Infrared spectroscopy is the standard method used to identify the makeup of a collected stone.¹⁵ A 2 mm stone is small and easy to miss in the toilet, so consistent straining is important. Passing it painlessly is common, as many people realize the stone has passed only because the pain suddenly stops or because they find a tiny grain in the strainer.

When a Stone Sits Quietly in the Kidney

Sometimes a 2 mm stone is found incidentally on an imaging scan done for a completely different reason. If the stone is still sitting in the kidney and not causing symptoms, the question becomes whether to treat it or just watch it. The evidence suggests that observation is reasonable, but these stones are not as dormant as they seem. In one study following patients with asymptomatic kidney stones, about 54% eventually developed a stone-related event such as pain or the stone moving into the ureter. Roughly half the group had symptoms within 19 months of diagnosis, and about a quarter eventually needed some form of intervention.¹⁶

Smaller stones and younger patients were more likely to pass them spontaneously, which is good news for someone with a 2 mm stone. But men and those with a prior stone history had higher rates of events.¹⁶ A longer-term study found that over extended follow-up, about 77% of patients with asymptomatic stones experienced some form of disease progression, with stone growth being more common in stones sitting in the lower pole of the kidney.¹⁷ The practical message: if you know you have a small, quiet stone, periodic imaging to check whether it has grown is a worthwhile precaution.

Kidney Stones During Pregnancy

Pregnancy adds layers of complexity. The usual first-choice imaging tool, CT, involves radiation that clinicians understandably try to avoid in pregnant patients, so ultrasound is the go-to method. But as discussed earlier, ultrasound is less accurate for small stones and tends to overestimate their size. On top of that, a normal pregnancy causes mild swelling of the kidneys (physiologic hydronephrosis) that can look a lot like the swelling caused by a stone. These factors make diagnosis harder, and misdiagnosis is surprisingly common. One study found that nearly a quarter of pregnant women diagnosed with kidney stones were actually diagnosed incorrectly.¹⁸

The same study found that only about 48% of pregnant women with confirmed stones passed them spontaneously, which is lower than the rates seen in the general population for similar stone sizes. When surgical intervention is needed during pregnancy, ureteroscopy is considered the safest option and can clear the stone definitively.¹⁹ Medication choices are also more limited, as certain drugs used for stone management are not considered safe during pregnancy. If you are pregnant and develop flank pain, the threshold for seeing a doctor should be lower than it otherwise would be.

Preventing the Next Stone

Once you have had one kidney stone, your risk of forming another is substantially higher. Estimates vary, but roughly half of first-time stone formers will have a recurrence within several years. Prevention hinges on two main levers: fluid intake and diet.

The fluid story is simple. Higher fluid intake increases urine output, dilutes the minerals that form stones, and is consistently associated with reduced stone formation in the research.²⁰ The general target clinicians recommend is enough fluid to produce about 2.5 liters of urine per day, which usually means drinking around 2.5 to 3 liters of fluid. Water is the safest choice. The same review found that water high in calcium appeared to increase the rate of calcium oxalate stone formation, and that cranberry juice also raised the risk, while grapefruit, apple, and orange juices were associated with lower supersaturation of calcium oxalate in urine.

On the diet side, the DASH-style eating pattern, which emphasizes fruits, vegetables, whole grains, and moderate dairy while limiting sodium and animal protein, has shown strong results. Research found a 40–50% reduced risk of stones in people whose diets aligned with this pattern, and an intervention study showed the DASH group had about half the stone recurrences at five years compared to controls.²¹ One counterintuitive finding that surprises many people: adequate dietary calcium actually reduces stone risk rather than increasing it. The calcium binds oxalate in the gut before it reaches the kidneys. Restricting dietary calcium, which feels logical, tends to backfire by allowing more oxalate to be absorbed and excreted in the urine.

Sodium is arguably the most underappreciated dietary factor. High salt intake increases the amount of calcium your kidneys excrete, which raises the concentration of stone-forming minerals in urine. Keeping daily sodium below roughly 2,300 mg is a practical target that benefits both stone prevention and blood pressure. If your stone was made of uric acid rather than calcium oxalate, limiting red meat and other purine-rich foods becomes the central dietary strategy, since purines break down into uric acid.