πͺ¨ Kidney Stone Passage Calculator
What your stone's size says about passing it.
For informational purposes only, not medical advice. Consult a healthcare professional.
Whether a kidney stone passes on its own comes down mostly to size. Stones under 5 mm usually pass within days to weeks. Stones 5 to 10 mm pass about half the time. Stones over 10 mm rarely pass without a procedure. Location refines the odds: lower ureter stones pass more readily than upper ones.
Enter the stone's largest diameter in millimeters from the CT report and where it sits. The calculator gives the passage outlook, the fluid target that best prevents the next stone, and the red flags that mean the emergency department instead of waiting.
How to Use This Calculator
Find the stone size in millimeters on the CT report (the largest dimension). Pick the location: kidney, upper ureter, or lower ureter/UVJ. Read the passage band and outlook. Strain your urine through the provided filter to catch the stone for analysis, which guides prevention.
Enter the stone size in mm from the CT report.
Pick the stone location.
Read the passage outlook for your size band.
Strain urine, manage pain per your doctor's plan, and watch for red-flag symptoms.
Why Size Dominates
5-10 mm: about half pass
Over 10 mm: procedure likely
The ureter is about 3 to 4 mm wide, so a stone under 5 mm can usually pass through it with enough time and urine flow. Published series consistently show spontaneous passage rates around 68% or higher for stones under 5 mm, near 47% for 5 to 10 mm, and under 10% above 10 mm, with lower ureter position adding roughly 10 to 20 percentage points at each size. Medical expulsive therapy with tamsulosin helps mainly for 5 to 10 mm distal stones. These are group rates from urology literature, presented as bands here rather than false-precision percentages, because individual anatomy varies.
Worked Example
A 4 mm stone in the lower ureter: Small stone, and High: most stones under 5 mm pass on their own, often within weeks. Location matters too: 4 mm at this size, and the prevention target afterward is 2.5-3 L urine/day (roughly 3+ L intake). A 7 mm stone in the same spot drops to the medium band, where about half pass and the rest need a procedure.
What Your Band Means
Small stone: waiting is reasonable with pain control, fluids, and straining; most pass within 2 to 4 weeks. Medium: a trial of passage is common, often with tamsulosin, but have a urology follow-up date set rather than waiting indefinitely. Large: plan the procedure discussion early (shockwave lithotripsy, ureteroscopy, or percutaneous approaches depending on size and location). Any band plus fever, vomiting that prevents fluids, unrelenting pain, or a single kidney means urgent care: an infected obstructed kidney is an emergency.
Stones Plus Infection Are an Emergency
Fever with a known stone, inability to urinate, severe pain unrelieved by prescribed medication, or persistent vomiting all warrant the emergency department, not patience. After the stone passes or is removed, get the stone analyzed and ask about a 24-hour urine metabolic workup: prevention is far better than repeat episodes. For informational purposes only, not medical advice. Consult a healthcare professional.
Related tools: kidney failure risk (KFRE) and water intake calculator.
For informational purposes only, not medical advice. Consult a healthcare professional.
Frequently Asked Questions
Will a 4 mm kidney stone pass on its own?
Very likely. Most stones under 5 mm pass spontaneously, often within a couple of weeks, especially in the lower ureter.
How long does it take to pass a kidney stone?
Small stones often pass within days to 4 weeks. If a stone has not passed in about 4 to 6 weeks, urologists usually intervene.
Does drinking water help pass a kidney stone?
Good hydration supports passage and is the best prevention, but chugging gallons during an acute episode does not force a large stone through. Aim for pale-yellow urine steadily.
What is the best pain relief for kidney stones?
NSAIDs like ibuprofen are first-line and often outperform opioids for renal colic. Follow your doctor's plan, especially with kidney issues.
Does tamsulosin help pass kidney stones?
It helps mainly for 5 to 10 mm stones in the lower ureter, relaxing ureteral smooth muscle. Evidence for smaller stones is weak.
Should I strain my urine?
Yes. Catching the stone lets the lab analyze its composition, which directly guides prevention (calcium oxalate versus uric acid need different strategies).
What foods cause kidney stones?
High sodium, high animal protein, and low fluid intake raise risk for calcium stones. Spinach-level oxalate loads matter mainly for known stone formers.
When is a kidney stone an emergency?
Fever with a stone, unrelenting pain, vomiting preventing fluids, no urine output, or a single kidney: go to the emergency department.
Can drinking water really help a kidney stone pass?
Yes, and it is the single most useful home measure. High fluid intake, enough for 2.5 to 3 liters of urine a day, keeps the ureter flushed and dilates it slightly, which helps small stones move. Plain water does the heavy lifting; citrate-rich fluids like lemonade may add a small benefit against stone growth, but volume matters more than the type.
Do I need a strainer to catch the stone?
If your doctor asks you to, yes. Straining urine through the fine mesh filter the clinic gives you lets you catch the stone for lab analysis, which identifies its type: calcium oxalate, uric acid, struvite, and so on. Knowing the type guides prevention, like diet changes or medication, so catching it is worth the awkwardness.