Stones smaller than about 5 millimeters pass on their own most of the time, while stones larger than roughly 7 millimeters usually do not. That 5-to-7 mm range is the gray zone where size alone cannot tell you the outcome. But the answer is messier than a single cutoff number, because where the stone is sitting, what shape it takes, and even how it was measured all shift the odds in ways that matter for your treatment decisions.
Passage Rates by Size
The most widely cited data on spontaneous passage comes from CT-based studies that tracked what happened to ureteral stones of various sizes. One study using unenhanced CT scans found passage rates of about 87% for stones around 1 mm, 76% for stones 2 to 4 mm, 60% for stones 5 to 7 mm, roughly half for stones 7 to 9 mm, and only about a quarter for stones larger than 9 mm.1PubMed. Relationship of spontaneous passage of ureteral calculi to stone size and location as revealed by unenhanced helical CT Those numbers paint a clear trend: the bigger the stone, the worse your chances. But notice that even in the 7-to-9 mm group, nearly half still passed without surgery. The cutoff is not a cliff edge so much as a sliding scale.
A large meta-analysis that pooled data across many studies found that stones between 5 and 7 mm had about one-fifth the odds of passing compared to smaller stones, and stones above 7 mm had about one-tenth the odds.2PubMed Central. Nomogram predicting the probability of spontaneous stone passage in patients presenting with acute ureteric colic That is why most urologists use 5 mm as the practical line: stones under that size deserve a trial of watchful waiting, while stones above it warrant a more serious conversation about intervention. But 5 mm is a starting point for that conversation, not an automatic referral to the operating room.
Why Location Matters as Much as Size
The ureter is not a uniform tube. It has natural narrow points, and a stone’s location along this path changes its odds of getting through. One study found that stones sitting near the bladder entrance, called the ureterovesical junction, had over five times the odds of passing compared to stones higher up.3PubMed Central. Prediction for spontaneous passage of ureteral stones with indwelling ureteral stent: PASS score Stones in the middle-to-lower ureter also had better odds than those near the kidney. A stone that has already traveled most of the way down has less distance to cover and has cleared the tightest anatomical bottlenecks.
This is why two people with the same 6 mm stone can have completely different outcomes. If yours is already in the lower ureter near the bladder, your doctor may be comfortable giving it more time. If it is jammed up high near the kidney, the same stone is more likely to need help. Both size and location predict whether a stone will pass on its own, and research confirms these two factors work together.4PubMed Central. Size matters: The width and location of a ureteral stone accurately predict the chance of spontaneous passage
Volume Tells a Different Story Than Diameter
Most of the size numbers you hear, including the 5 mm threshold, are based on the stone’s largest diameter on a CT scan. But stones are not spheres. A flat, disc-shaped 8 mm stone occupies far less volume than a chunky, round 8 mm stone, and volume turns out to matter. One study found that stones with a volume under about 53 cubic millimeters passed far more reliably than larger-volume stones. Among stones bigger than 7 mm in their longest dimension, those with a low enough volume still had a 75% passage rate, while larger-volume stones in the same diameter range mostly required intervention.5PubMed. CT-Based Determination of Ureteral Stone Volume: A Predictor of Spontaneous Passage
This finding suggests that the diameter-only approach misses part of the picture. A stone that looks borderline on a single measurement might actually be quite passable if it is thin in one dimension. Some centers now consider stone volume when making treatment decisions, though diameter remains the standard metric most doctors reference in practice.
Bigger Stones Do Not Always Hurt More
If you have ever writhed through renal colic, you might assume a larger stone means worse pain. That is not what the evidence shows. One emergency medicine study found no correlation between stone size and pain scores.6PubMed. The relationship between the severity of pain and stone size, hydronephrosis and laboratory parameters in renal colic attack A tiny stone can produce agonizing pain if it lodges in a tight spot, while a larger stone sitting in a wider part of the ureter might cause surprisingly mild discomfort.
What does predict pain severity is the degree of hydronephrosis, which is the backup of urine behind the stone that causes the kidney to swell. Larger stones tend to produce more hydronephrosis, but the relationship is not perfectly linear.7PubMed. Can ureteral stones cause pain without causing hydronephrosis? A study that looked specifically at what predicts severe renal colic found that a high degree of kidney swelling and the patient’s anxiety level were the strongest predictors of pain intensity.8PubMed Central. The severity of renal colic pain: Can it be predicted? The practical takeaway is that you should not use pain level to guess whether your stone is small enough to pass. Pain tells you about the obstruction and inflammation happening right now, not about the stone’s dimensions.
The Tamsulosin Question
If you visit an emergency room with a kidney stone, there is a good chance you will leave with a prescription for tamsulosin, a medication originally designed for prostate problems that also relaxes smooth muscle in the ureter. Whether tamsulosin actually helps depends on the stone’s size. A systematic review and meta-analysis found that for stones larger than about 5 mm, tamsulosin increased the passage rate by roughly 22 percentage points compared to standard care alone, meaning about one in five patients who took it avoided a procedure they would have otherwise needed.9Annals of Emergency Medicine. Effect of Tamsulosin on Stone Passage for Ureteral Stones: A Systematic Review and Meta-analysis For stones under 5 mm, the drug made no measurable difference, probably because those stones were going to pass anyway.
A separate meta-analysis confirmed the general pattern: the overall expulsion rate with tamsulosin was about 81% versus 71% without it, but the benefit was concentrated in stones larger than 6 mm.10PubMed Central. Is tamsulosin effective for the passage of symptomatic ureteral stones A systematic review and meta-analysis So if you are dealing with a borderline stone in that 5-to-7 mm range, tamsulosin is the medication most likely to tip the balance in your favor. For smaller stones, it is unlikely to speed things up. For very large stones, it can help but probably will not be enough on its own.
How Imaging Can Mislead You About Size
All of the size cutoffs above assume you know the stone’s actual dimensions, and that is where things get tricky. If your stone was measured by ultrasound rather than CT, the number you were told may be inflated. Ultrasound consistently overestimates stone size, and the error is largest for smaller stones. One study found that for stones 5 mm or smaller on CT, ultrasound overestimated the size by an average of about 85%. For stones between 5 and 10 mm, the overestimate was closer to 27%.11PubMed. Ultrasonography Significantly Overestimates Stone Size When Compared to Low-dose, Noncontrast Computed Tomography
This matters for decision-making. Another study found that when CT suggested a stone was small enough to watch and wait, ultrasound would have led the doctor to recommend intervention about 14% of the time. Going the other direction, when CT showed a stone large enough to consider treatment, ultrasound would have understated the urgency and suggested observation in about 39% of cases. Overall, roughly one in five patients could receive the wrong management advice based on ultrasound measurements alone.12PubMed. Accuracy of ultrasonography for renal stone detection and size determination: is it good enough for management decisions? A more recent comparison found mean stone size was about 8.7 mm on ultrasound versus 5.5 mm on CT for the same stones, with heavier patients and smaller stones showing the biggest discrepancy.13PubMed Central. Limitations of ultrasound compared with computed tomography for kidney stone surveillance
CT is the gold standard for measuring kidney stones. If you are making a decision about whether to wait or pursue treatment, and the only measurement you have is from an ultrasound, it is worth asking whether a CT might change the picture, especially for stones measured at or near the 5 mm threshold.
When a Stone Needs to Come Out
Even if your stone is small enough to pass in theory, certain situations push doctors toward intervention. Persistent pain that does not respond to medication, signs of infection alongside an obstructing stone, a solitary kidney, or worsening kidney function all change the calculus. When conservative management and medication do not resolve symptoms, the classic next step is either placing a temporary stent to allow urine to flow past the stone or inserting a drainage tube through the back into the kidney.14PubMed Central. Emergency management of ureteral stones: Recent advances
For stones that need active removal, the two most common outpatient procedures are ureteroscopy, where a thin scope goes up through the urinary tract to grab or laser the stone, and shockwave lithotripsy, which uses focused sound waves from outside the body to break the stone into smaller fragments. For stones up to about 2 cm, ureteroscopy tends to achieve higher stone-free rates in the first several months, though the difference narrows by one year.15PubMed. Flexible Ureterorenoscopy Versus Shockwave Lithotripsy for Kidney Stones ≤2 cm: A Randomized Controlled Trial Shockwave lithotripsy works well for smaller, softer stones but its effectiveness drops off for stones larger than about 10 mm.16PubMed Central. Ureteroscopy vs Shockwave Lithotripsy to Remove Kidney Stones in Children and Adolescents
For the largest stones, particularly staghorn calculi that fill much of the kidney’s internal collecting system and can grow to several centimeters, percutaneous nephrolithotomy is the recommended first-line treatment.17PubMed Central. Percutaneous nephrolithotomy for staghorn calculi: Troubleshooting and managing complications 18PubMed Central. Management of staghorn renal stones This involves making a small incision in the back to access the kidney directly. These very large stones are well beyond the range of anything that could pass naturally and typically require planning with a specialist.
The Danger of Silent Stones
One of the more troubling findings in kidney stone research is that the stones that hurt the most are not necessarily the ones that do the most damage. Some ureteral stones sit quietly without causing noticeable pain while steadily impairing kidney function. A study comparing asymptomatic and symptomatic stones found that patients with silent stones had reduced kidney function that did not improve even after the stone was removed, suggesting the damage had already become permanent.19Scientific Reports. A comparison between asymptomatic and symptomatic ureteral stones A separate study confirmed that patients with silent ureteral stones presented with significant loss of function on the affected side, and while hydronephrosis improved after treatment, the underlying kidney damage persisted.20PubMed. Irreversible Renal Function Impairment Due to Silent Ureteral Stones
The combination of obstruction and infection is especially dangerous. In patients with urinary tract infections, having a ureteral stone with hydronephrosis more than doubled the risk of acute kidney injury.21PubMed Central. Ureteral stone with hydronephrosis and urolithiasis alone are risk factors for acute kidney injury in patients with urinary tract infection The size question, then, is not just about whether you can tolerate the pain long enough for a stone to pass. A stone that is technically passable by size could still be doing real harm if it is partially blocking urine flow for weeks.
Different Thresholds for Children
If your child has a kidney stone, the size rules are not the same as for adults. Children’s ureters are smaller, and the data suggest lower thresholds for spontaneous passage. One pediatric study found a cutoff of about 6.7 mm as the best predictor of whether a stone would pass in children, with stones smaller than that size having a high likelihood of spontaneous passage.22PubMed. What predicts spontaneous passage of ≤1 cm ureteral stones in children? But a more recent study using the stone’s transverse (side-to-side) dimension rather than its longest axis arrived at a much more conservative cutoff: 3.5 mm.23PubMed. Predictive Factors for Ureteral Stone Passage in Children
The difference between those two numbers reflects partly what is being measured and partly the reality that pediatric stone passage data is less settled than adult data. Overall passage rates in children tend to hover around 30 to 40%, which is lower than you might expect.24PubMed. Pediatric urinary stone disease–does age matter? Pediatric urologists tend to be more cautious about watchful waiting, especially with larger stones, because children are harder to monitor and less able to articulate worsening symptoms.
Pregnancy Changes the Odds
Pregnancy is one situation where stones pass more easily than you would expect. The hormonal changes of pregnancy, particularly elevated progesterone, cause the ureters to dilate and relax. One study found that 81% of pregnant women passed their stones spontaneously, compared to only 47% of nonpregnant women.25PubMed. Stone formation and pregnancy: pathophysiological insights gained from morphoconstitutional stone analysis That is a dramatic difference. The tradeoff is that managing stones during pregnancy involves additional constraints: CT scans are avoided due to radiation, many medications are off-limits, and the overlap between normal pregnancy symptoms and stone symptoms can delay diagnosis. Ultrasound becomes the primary imaging tool despite its size-measurement limitations. Most pregnant patients are managed conservatively with hydration and pain control, and the high spontaneous passage rate works in their favor.
The Roller Coaster Experiment
No discussion of kidney stone passage would be complete without the study that put a silicone kidney model on a roller coaster. Researchers at Michigan State University created a model kidney with real stones embedded in it and rode a moderate-intensity roller coaster repeatedly, varying where they sat. The overall dislodgement rate across all trials was about 69%. Sitting in the rear of the coaster produced a passage rate of only about 17%, while sitting in the front produced a 100% rate in a small number of trials. Stones in the upper and middle parts of the kidney model moved more reliably than those in the lower pole.26PubMed Central. Evaluation of Renal Calculi Passage While Riding a Roller Coaster This was a bench-top model and not a clinical trial, so the results should be taken more as proof of concept than medical advice. Still, the researchers received an Ig Nobel Prize for the work, and some urologists have half-seriously suggested that moderate physical jostling after a diagnosis of small kidney stones is not an unreasonable thing to try while you are waiting for nature to take its course.