Most kidney stones pass on their own within one to six weeks, but the timeline depends almost entirely on the stone’s size. A stone smaller than 4 millimeters has about an 80% chance of passing on its own in roughly 31 days. A stone between 4 and 6 millimeters takes closer to 45 days, with about a 60% chance of passing without intervention. Stones larger than 6 millimeters rarely pass on their own, with only about a 20% success rate even over 12 months.
Passage Time by Stone Size
Size is the single biggest predictor of whether a stone will pass and how long it will take. Here’s what the numbers look like:
- Under 4 mm: About 31 days on average. These are the stones most likely to pass without help, with roughly 4 in 5 making it through.
- 4 to 6 mm: About 45 days on average. Your odds drop to around 60%, meaning roughly 2 in 5 stones this size will eventually need a procedure.
- Over 6 mm: These can take many months and only about 1 in 5 will pass on their own. Most stones this size require medical intervention.
These are averages. Some people pass a small stone in a few days, while others wait several weeks for the same size. The shape, surface texture, and exact location of the stone all play a role. A smooth, round 3 mm stone may slide through with minimal trouble, while an irregularly shaped one of the same size can get stuck along the way.
Where Stones Get Stuck
A kidney stone doesn’t travel through a smooth, wide-open tube. The ureter, the narrow passage connecting each kidney to the bladder, has three natural bottleneck points where stones commonly lodge. The first is right where the kidney meets the ureter at the top. The second is in the mid-section where the ureter crosses over blood vessels near the pelvis. The third, and most common spot for a stone to stall, is at the very bottom where the ureter enters the bladder wall.
When a stone gets stuck at one of these points, that’s typically when pain flares up. The stone may sit in one spot for days before fluid pressure behind it eventually pushes it forward. Once a stone reaches the bladder, the worst is over. Passing it from the bladder out through the urethra usually takes hours at most and causes far less discomfort.
What the Pain Actually Feels Like
The pain from a kidney stone, called renal colic, comes in waves rather than staying constant. Each wave typically lasts 20 to 60 minutes, with the worst intensity hitting one to two hours after the episode begins. Between waves, you might feel a dull, lingering ache or nothing at all. Some people describe days of feeling completely fine between flare-ups, then suddenly being hit with another round of sharp, intense pain.
The pain usually starts in your back or side, below the ribs, and can radiate down toward your groin as the stone moves lower in the ureter. This shifting location is actually a useful signal. Pain that migrates downward over days or weeks suggests the stone is making progress. Pain that stays in one place may mean the stone is lodged.
How Doctors Decide When to Intervene
Current guidelines from the American Urological Association recommend giving a stone approximately 30 days to pass before considering surgical options. During that window, doctors often prescribe an alpha-blocker medication that relaxes the smooth muscle lining the ureter, making it easier for the stone to move through. For stones between 6 and 10 mm, this medication reduces passage time by about 6 days on average. For smaller stones, the benefit is more modest, shaving off roughly 2 days.
If a stone hasn’t passed after four to six weeks, or if complications develop, the wait-and-see approach typically ends. A stone that blocks urine flow for an extended period can cause the kidney to swell with backed-up fluid, a condition called hydronephrosis. Left unchecked, this pressure can permanently damage kidney tissue. That’s why ongoing monitoring matters even when the plan is to let a stone pass naturally.
What You Can Do While Waiting
You may have heard that drinking large amounts of water will flush a stone out faster. This is a common belief, but there’s no strong evidence that loading up on fluids speeds stone passage. What high fluid intake does help with is prevention of future stones. For that purpose, aim for 2.5 to 3 liters of fluids daily as a long-term habit.
While waiting for a stone to pass, staying reasonably hydrated is still important so your kidneys continue producing urine and maintaining some pressure behind the stone. Over-the-counter pain relievers like ibuprofen can help manage discomfort between episodes. Gentle physical activity, like walking, is generally fine and may help the stone move along, though no controlled studies confirm this definitively. Many people find that applying heat to the affected side provides some relief during pain waves.
Signs You Shouldn’t Keep Waiting
Certain symptoms signal that a stone is causing problems beyond normal discomfort. Get medical attention promptly if you experience any of the following:
- Fever or chills: This can indicate an infection behind the blockage, which can become dangerous quickly.
- Pain so severe you can’t sit still or find a comfortable position: This level of pain often means the stone is fully blocking urine flow.
- Nausea and vomiting alongside pain: Persistent vomiting can lead to dehydration and may indicate a more severe obstruction.
- Inability to urinate: Complete blockage requires urgent treatment.
- Visible blood in your urine: Some blood-tinged urine is common with kidney stones, but heavy or persistent bleeding warrants evaluation.
A fever combined with a blocked stone is the most urgent scenario on this list. An infection trapped behind an obstruction can escalate to a body-wide infection within hours, so this combination always warrants emergency care regardless of stone size or how long you’ve been managing at home.