How to Measure Stone and Rip Rap Boulder Sizes

Planning to Order Stone or Rip Rap Boulders? Here's How Sizes Are Actually Measured

When you're planning a landscape, shoreline, or hardscaping project, knowing how stone sizes are measured is more important than most people realize. Whether you're ordering rip rap, large boulders, or decorative stone, size matters - and it’s easy to misunderstand what those measurements actually mean.

In the stone industry, stones and boulders are sized based on their narrowest side, not their length.

This might seem confusing at first, but it follows a consistent standard - one that’s very similar to how rip rap and other aggregates are measured.

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Think of It Like a Giant Sieve

If you’ve ever ordered rip rap or gravel, you’ve likely heard terms like “6-inch minus” or “Type C rip rap.” These sizes are based on what the stone can pass through - just like using a sieve.

If a stone can fit through a 12-inch opening, it’s a 12-inch stone - even if it’s 3 feet long in the other direction.

This applies to everything from rip rap and erosion stone to the larger boulders used in retaining walls, creek beds, or landscaping features.

Correct way to measure a boulder or riprap stone: by the narrowest vertical height.

Incorrect example: measuring by the longest side does not reflect industry standards.

How We Measure Stones and Rip Rap Boulders

For Smaller Stones: We use calipers or simple measuring tools to measure the narrowest cross-section. This mimics how the stone would behave in a sieve test - and determines the listed size.

For Large Boulders and Rip Rap: We measure with a tape measure, laying the boulder flat in its natural resting position. The shortest vertical height is used to define its size - not the longer horizontal length.

So if a boulder is 48 inches long and 24 inches tall, it’s typically classified as a 24-inch boulder, because that's the size opening it would fit through - just like rip rap grading.

Why This Matters to You

If you're ordering a “24-inch boulder” or “Type C rip rap,” you might picture the longest dimension - but that’s not what those sizes refer to.

For example:

• A 24-inch rip rap rock might be up to 36 or 48 inches long, but it’s called 24-inch because that’s the shortest side.
• A 36-inch boulder might be 60 inches long, but it’s labeled as 36-inch.

Ordering based on the wrong assumption can lead to stones that are too large to handle - or too small for your project’s needs.

Stone Shapes Vary - And That Affects How Size Is Perceived

It’s also important to understand that not all stones are shaped the same.

Some are more squared, meaning the difference between their length and width is minimal - like a block. Others are more elongated or irregular - meaning one side is much longer than the other.

Here’s how that affects sizing:

• Squared stones will appear close to the listed size no matter which side you look at.
• Elongated or irregular stones may seem much larger than the listed size - even though they’re measured correctly by the shorter side.

So if you get a 24-inch stone that’s long and narrow, it might actually be 40 inches long - but still correctly classified as 24-inch based on the industry standard.

When Size Ranges Are Used (Like 12–24 Inches)

Rip Rap Boulders are sold in a range, such as 12–24 inch rip rap. This means:

• You’ll receive a mix, some closer to 12 inches, others closer to 24 inches - and many in between.
• Both squared and elongated stones can pass through the sieve and fall within this range.
• But it doesn’t mean all the stones will feel oversized or undersized.
• Only when a stone is on the upper end of the range and also very elongated might it feel bigger than expected.

In most cases, the variation is normal and expected - and the range gives you a practical idea of what to expect, especially for rip rap, landscape boulders, or bulk orders.