What Size Kidney Stone Will Pass? Understanding the Limits

Passing a kidney stone depends largely on its size, but there’s a lot more to it than most doctors explain. In this blog, I’ll walk you through the true measurements that matter, how imaging mistakes can mislead you, and what steps you can take to improve your chances of passing a stone naturally. If you’ve ever wondered what size kidney stone will pass, this is everything you need to know.

Key Takeaways

• Imaging results can overestimate the size of your kidney stone.

• Stones under 3mm pass easily, but 3-6mm stones take longer and need help.

• Stones up to 9mm can still pass naturally, but it requires patience and the right approach.

• Anti-inflammatories and diuretics can help stones pass faster and with less pain.

One of the questions I get asked the most is what size kidney stone will pass? It’s a great question, but the answer is a little more complicated than just a number.

The Problem With Stone Size Measurements

First, we have to talk about the big margin for error that comes from imaging. Whether it’s an X-ray, CT scan, or ultrasound, these are all 2D images of a 3D object.

The truth is, what matters most is the mean diameter of the stone. But many times, doctors report the width, not the true diameter that determines how easily it can travel through your ureter.

I’ve personally experienced this when I was told a stone was 9mm wide—only to find out later that its diameter was actually much smaller and easier to pass than expected. So always remember: flat imaging can exaggerate size and make it seem scarier than it is. 

Defining Small Stones

Small stones are considered less than 3 millimeters in size.

According to the data, about 80% of small stones pass naturally without needing any intervention at all.

On average, it takes around 30 days for a small stone to pass. Most people don’t even realize they had a stone once it’s passed—except for the initial discomfort when it detaches from the kidney.

Medium Sized Stones: 3mm to 6mm

Medium stones fall between 3 and 6 millimeters. Now we’re starting to talk about stones that can cause a little more trouble.

Your ureter is about 3-4mm wide naturally. That means anything over 3mm can start rubbing against the walls and causing painful pressure.

Even though it’s tougher, about 60% of medium stones still pass unassisted. But expect it to take longer—around 45 days on average.

There’s more friction, more swelling, and sometimes more help needed from medications or natural strategies. 

Large Stones: 6mm to 9mm

Now we get to the big ones—6 to 9 millimeters in size.

This is the point where most doctors immediately recommend surgery. But let me tell you from experience: it is still possible to pass these stones naturally.

I’ve passed stones that were 8, 9, and even 10 millimeters wide without surgery. It’s slower, it’s painful, but it can be done.

Only about 20% of people pass large stones naturally, but the main barrier is that many give up early and choose surgery instead of waiting it out.

Passing a large stone naturally will usually take months, and you’ll need to be very intentional with your approach.

How to Improve Your Chances of Passing a Kidney Stone

If you’re determined to pass a kidney stone on your own, there are a few key strategies that can dramatically boost your odds.

First, increase your water consumption. Aim for 2 to 3 liters of water per day at a minimum.

Second, reduce inflammation. Stones cause swelling in your ureter, which can trap them even tighter. Using a natural or prescription anti-inflammatory can ease that swelling and create more room for the stone to move.

Third, add a diuretic to the mix. A natural or pharmaceutical diuretic increases urinary output, putting extra pressure behind the stone to push it out faster.

When you combine hydration, anti-inflammatory action, and a diuretic, you give yourself the best shot at passing the stone without surgery. 

Final Thoughts on What Size Kidney Stone Will Pass

In conclusion, stones less than 9mm in diameter can pass naturally—but it’s not always easy.

It takes patience, time, and the right strategies to help move the stone along safely.

Always be aware that imaging may overestimate your stone’s true challenge. Focus on hydration, inflammation reduction, and boosting urine output to speed the process.

And most of all, stay positive. Passing a kidney stone naturally is tough, but it’s absolutely possible.