Measuring for river rock comes down to three numbers: the length of your area, the width, and how deep you want the rock. Multiply those together, divide by 27, and you have the cubic yards needed to place your order. The details that trip people up are choosing the right depth for your project, handling odd-shaped beds, and knowing how much extra to order so you don’t end up short.
The Basic Formula
Every river rock calculation uses the same core formula:
Cubic yards = (Length in feet × Width in feet × Depth in feet) ÷ 27
There are 27 cubic feet in a cubic yard, which is why you divide by 27. Most suppliers sell river rock by the cubic yard or by the ton, so this is the number you’ll need when ordering. The one mistake to watch for: depth is usually something you think about in inches, but the formula needs feet. Divide your depth in inches by 12 before plugging it in. For example, a 3-inch depth becomes 0.25 feet.
Say you have a rectangular bed that’s 20 feet long, 10 feet wide, and you want 3 inches of rock. That’s 20 × 10 × 0.25 = 50 cubic feet. Divide by 27 and you need roughly 1.85 cubic yards.
Choosing the Right Depth
Depth depends on your rock size and what the area is for. Smaller rocks can go shallower; larger rocks need more depth to look right and stay in place.
- Small river rock (0.5 to 1.5 inches): 2 to 3 inches deep. Best for pathways and accent areas.
- Medium river rock (1.5 to 3 inches): 2 to 4 inches deep. Handles most general landscaping, drainage, and ground cover.
- Large river rock (3 to 5 inches): 3 to 5 inches deep. Works for decorative borders, dry creek beds, and focal points.
Going deeper gives you better weed suppression and keeps soil from showing through, but it also costs more and can overwhelm a small space. For most general ground cover projects, 3 inches is the sweet spot. Drainage areas benefit from 4 inches, which gives water enough room to flow between the stones. Lay landscape fabric underneath regardless of depth to block weeds while still letting water drain through.
Measuring Irregular Shapes
Not every garden bed is a neat rectangle. If your area has curves or angles, break it into simpler shapes, calculate each one, and add them together.
Circular Beds
Measure the radius (the distance from the center to the edge). Then use: Area = 3.14 × radius × radius. A circular bed with a 6-foot radius has an area of about 113 square feet.
Semicircle Beds
Measure the straight edge and divide by two to get the radius. Calculate as a full circle (3.14 × radius × radius), then multiply by 0.5.
Triangular Beds
Measure the base and the height (the perpendicular distance from the base to the opposite point). Area = base × height × 0.5.
Freeform or Kidney-Shaped Beds
For truly irregular shapes, use the segment method from Iowa State University Extension. Find the longest line through the shape. This is your length line. Then measure several width lines perpendicular to it, spaced equally apart. Add up all those width measurements and multiply by the distance between them. The more width lines you measure, the more accurate your result. This works surprisingly well for kidney bean shapes, curved borders, and other organic forms that don’t fit a simple formula.
Once you have the total square footage, multiply by your depth in feet and divide by 27 to get cubic yards.
How Rock Size Affects Coverage
Bigger rocks leave bigger gaps between them, so they cover less ground per ton. This matters when you’re budgeting and placing orders:
- Small river rock (0.5 to 1.5 inches): covers 100 to 120 square feet per ton at 2-inch depth.
- Medium river rock (1.5 to 3 inches): covers 80 to 100 square feet per ton.
- Large river rock (3 to 5 inches): covers 60 to 80 square feet per ton.
If your supplier prices by the ton rather than the cubic yard, these coverage rates let you estimate how many tons you need directly from your square footage. For a 200-square-foot area using medium river rock at standard depth, plan on roughly 2 to 2.5 tons.
How Much Extra to Order
Always order more than your formula says. River rock is rounded, and rounded stones settle after installation as they shift and nestle together. Plan for 10 to 15 percent extra material to account for settling, irregular edges, and waste during installation. Complex layouts with lots of curves or tight corners may need up to 20 percent extra.
River rock specifically tends to need at least 5 to 10 percent additional material just for settling alone, on top of whatever you add for edge fitting and waste. Running short mid-project is a bigger hassle than having a small pile left over, especially if your supplier charges a delivery fee. A little extra rock can always fill in thin spots after a few months of settling.
Putting It All Together
Here’s the full process from start to finish. Grab a tape measure, a calculator, and a notepad.
First, measure your area in feet. For rectangles, that’s length and width. For curves and odd shapes, break them into simpler pieces and use the formulas above. Write down the total square footage. Next, choose your depth based on rock size and purpose. Convert that depth to feet by dividing inches by 12. Multiply your square footage by your depth in feet to get cubic feet, then divide by 27 for cubic yards. Finally, add 10 to 15 percent to that number.
As a quick example: a curved garden border measures roughly 150 square feet after you add up its segments. You want 3-inch medium river rock for drainage. That’s 150 × 0.25 = 37.5 cubic feet, or about 1.4 cubic yards. Add 15 percent and you’re ordering 1.6 cubic yards. At the coverage rate for medium rock, that’s roughly 1.5 to 2 tons, depending on how your supplier measures it.
If you’re covering multiple areas around your yard, calculate each one separately and add the totals. This keeps your math clean and lets you double-check each section if the final number looks off.