Kidney stones smaller than 5 to 6 mm generally pass on their own, while stones larger than 6 mm increasingly require some form of surgical intervention. But size alone doesn’t tell the whole story. Where the stone is located, whether it’s actively moving, and how long it’s been stuck all factor into the decision.
Passage Rates by Size
The likelihood of a kidney stone passing on its own drops steadily as the stone gets bigger. Research tracking stones measured on CT scans found these approximate passage rates:
- 1 to 3 mm: 72% to 87% pass without intervention
- 4 to 5 mm: 60% to 72% pass
- 6 to 7 mm: 47% to 72% pass
- 8 to 10 mm: 27% to 56% pass
The numbers make it clear that even at 5 mm, about 4 in 10 stones won’t pass. Once a stone reaches 7 mm or larger, the odds of it passing naturally drop below 50%. At 10 mm, only about 1 in 4 stones will make it out on their own.
How Long You Can Safely Wait
If your doctor recommends watching and waiting, the timeline depends on the stone’s size. Stones under 4 mm typically pass within about 31 days. Stones between 4 and 6 mm may take 45 days or longer, often with medication to help relax the ureter and ease the stone along. Stones larger than 6 mm are unlikely to pass on their own and usually need a procedure.
Waiting too long carries real risk. A stone that stays lodged in the ureter blocks urine flow, and prolonged obstruction can damage the kidney. If you’ve been managing a stone at home for several weeks with no progress, that’s typically when your urologist will recommend moving to a procedure.
Where the Stone Sits Matters
A kidney stone can be in the kidney itself or somewhere along the ureter, the narrow tube connecting the kidney to the bladder. Stones in the lower part of the ureter (closer to the bladder) have a better chance of passing because they’ve already traveled most of the way. Stones stuck higher up, near the kidney, face a longer and narrower path, so they’re more likely to need intervention at the same size.
Location also matters for stones still sitting in the kidney. Stones in the lower pole of the kidney, a cup-shaped area at the bottom, are harder to clear because gravity works against them. The American Urological Association treats lower pole stones more aggressively: stones over 1 cm (10 mm) in that location generally warrant a more involved procedure, while stones elsewhere in the kidney can often be managed with less invasive options up to about 2 cm.
The Three Main Procedures
When surgery is needed, the procedure your urologist recommends depends largely on the stone’s size and location.
Shock Wave Lithotripsy (SWL)
This is the least invasive option. A machine sends focused sound waves through your body to break the stone into smaller fragments that you then pass naturally. It works best for stones under 1 cm in the kidney or upper ureter. It’s not recommended as the first choice for lower pole kidney stones over 1 cm or for any kidney stone over 2 cm, because the clearance rates drop significantly for larger or harder stones. Recovery typically takes two to three days before you’re back to normal activities.
Ureteroscopy (URS)
A thin, flexible scope is passed through the urethra and bladder into the ureter, where a laser breaks the stone apart. No incisions are involved. This is the go-to option for most ureteral stones up to about 10 mm and a common choice for kidney stones between 1 and 2 cm. It’s a same-day procedure, and most people return to normal activities within two to three days.
Percutaneous Nephrolithotomy (PCNL)
For large kidney stones over 2 cm, PCNL is the recommended first-line treatment. The surgeon makes a small incision in your back and passes a scope directly into the kidney to break up and remove the stone. It has the highest stone-free rates for large stones but is the most involved procedure. You’ll typically stay in the hospital for one to two days and need one to two weeks before resuming normal activities. A smaller version called mini-PCNL is available at some centers for stones up to 3 cm and offers a similar success rate with a slightly smaller incision.
When Size Doesn’t Matter
Certain situations call for urgent intervention regardless of how big the stone is. A stone of any size that causes a fever with chills suggests an infection behind the blockage, which can become dangerous quickly. Similarly, a stone blocking your only functioning kidney, or one causing your kidney function to decline on blood tests, needs prompt treatment. Uncontrollable pain or vomiting that prevents you from staying hydrated also moves the timeline up. In these cases, even a 3 mm stone may require emergency drainage or removal.
Stone density plays a role too. Very hard, dense stones are more resistant to shock wave lithotripsy, which means your urologist may recommend ureteroscopy or PCNL even for a stone that would otherwise be a candidate for the less invasive approach. Your CT scan gives your doctor information about the stone’s density, and this factors into the treatment plan alongside size and location.