River Rock Calculator: How Much You Need

This river rock calculator works out the volume and weight of river rock you need for a landscaping project. You enter the length and width of the area in feet, the desired depth in inches, and it returns the volume in cubic yards and cubic feet. Use that volume to order the right amount of rock from a supplier, accounting for the fact that rock is sold by the cubic yard.

Formula

The calculator uses the standard volume formula for a rectangular prism:

Volume (ft³) = Length (ft) × Width (ft) × Depth (ft)

Since depth is entered in inches, it is first converted to feet by dividing by 12. The result in cubic feet is then converted to cubic yards by dividing by 27 (because 1 yard = 3 feet, so 1 yd³ = 27 ft³). Combining both conversions gives:

Volume (yd³) = (Length × Width × Depth in inches) / 324

Variables:

• Length (l) – the longest side of the area, in feet
• Width (w) – the shortest side of the area, in feet
• Depth (d) – the desired thickness of the rock layer, in inches

Worked Example

Using the default values: length = 40 ft, width = 25 ft, depth = 3 in.

1. Convert depth to feet: 3 in ÷ 12 = 0.25 ft
2. Calculate volume in cubic feet: 40 × 25 × 0.25 = 250 ft³
3. Convert to cubic yards: 250 ÷ 27 = 9.26 yd³ (rounded to 2 decimals)

The calculator shows 9.26 yd³ and 250.0 ft³, matching the manual calculation.

When to Use This Calculator

Use this calculator for any project that involves spreading a layer of river rock, pebbles, gravel, or similar loose aggregate over a rectangular area. Common uses include:

• Creating a decorative rock bed around trees or shrubs
• Laying a base for a walkway or patio
• Filling a dry creek bed or drainage swale
• Mulching a garden area where you want a permanent, weed-suppressing ground cover

The same formula works for other materials like crushed stone, pea gravel, or even soil, as long as you know the depth you want. For irregular shapes, break the area into rectangles, calculate each one, and add the volumes together.

How to Choose the Right Depth

The depth you choose depends on the rock size and the purpose of the layer. As a general guide:

• 1–2 inches – decorative top dressing over existing soil or mulch; fine rock (1/4 to 1/2 inch)
• 2–3 inches – typical for most landscaping beds; medium rock (1/2 to 1 inch)
• 3–4 inches – walkable paths, areas with foot traffic, or where weeds are a concern
• 4–6 inches – under heavy objects like boulders or for drainage layers

Larger rocks (2–4 inches) need more depth to look even and stay in place. If you are using river rock over landscape fabric, 2–3 inches is usually sufficient. For areas where you expect erosion, go deeper.

Common Mistakes and How to Avoid Them

One of the most frequent errors is mixing units. Always measure length and width in feet, and depth in inches, then let the calculator handle the conversion. If you measure depth in feet, multiply by 12 before entering it.

Another common mistake is underestimating the volume because you forget that rock settles. Freshly spread rock has air gaps that compact over time, so the actual depth after settling will be slightly less than what you installed. To compensate, add 5–10% to your order.

Finally, remember that the calculator gives volume, not weight. River rock is heavy: a cubic yard of most river rock weighs about 2,500–3,000 pounds (1.25–1.5 tons). Check with your supplier for the exact weight per cubic yard, especially if you are having it delivered and need to know the truck load.

How to Order River Rock

River rock is sold by the cubic yard (or ton) from landscape supply yards, and by the bag (usually 0.5 cubic feet) from home improvement stores. Here is how to use the calculator result:

• Bulk order (cubic yards): Round your cubic yard result up to the nearest whole number. If the calculator says 9.26 yd³, order 10 yd³ to be safe. Most suppliers will deliver in fractions of a yard, but a full yard is often the minimum.
• Bagged rock: Multiply the cubic feet result by 1.1 (for 10% extra) and divide by the bag size in cubic feet. For example, 250 ft³ × 1.1 = 275 ft³ ÷ 0.5 ft³ per bag = 550 bags.
• Weight-based ordering: If the supplier sells by the ton, multiply the cubic yards by the weight per yard (ask the supplier) and add 10%.

Always confirm the coverage with your supplier, as different rock sizes and shapes have different void spaces. A 3-inch layer of 1-inch rock covers about 100 square feet per cubic yard, but larger rock covers less area because of bigger gaps.

Cost Considerations

River rock is generally more expensive than crushed gravel or pea gravel because it is naturally rounded and often sourced from riverbeds. The price varies widely by region and availability. On average, bulk river rock costs $30–$80 per cubic yard, while bagged rock costs $4–$8 per 0.5-cubic-foot bag. Delivery fees are often separate, and some suppliers require a minimum order (often 1–2 cubic yards).

To get an accurate estimate, contact two or three local suppliers and ask for a quote based on the cubic yardage from this calculator. Mention the rock size you want, as larger rock costs more per yard. Also ask about the weight per cubic yard, because delivery trucks are limited by weight, not just volume.

Frequently Asked Questions

How many square feet does a cubic yard of river rock cover?

At a depth of 3 inches, one cubic yard covers 108 square feet (since 1 yd³ = 27 ft³, and 27 ft³ ÷ 0.25 ft depth = 108 ft²). At 2 inches deep, it covers 162 ft²; at 4 inches, it covers 81 ft². Use the formula: coverage (ft²) = 27 ÷ depth in feet.

Can I use this calculator for pea gravel or crushed stone?

Yes, the volume calculation is the same for any loose aggregate. The only difference is the weight per cubic yard and the recommended depth. Pea gravel is often used at 2–3 inches, while crushed stone for a base may be 4–6 inches.

How do I calculate for a circular area?

For a circle, use the formula for the area of a circle (π × r²) to get the square footage, then multiply by the depth in feet. For example, a 10-foot diameter circle (radius = 5 ft) has an area of π × 25 ≈ 78.54 ft². At 3 inches deep, that is 78.54 × 0.25 = 19.64 ft³, or 0.73 yd³.

Do I need landscape fabric under river rock?

Landscape fabric is recommended to prevent weeds from growing up through the rock. It also keeps the rock from sinking into the soil over time. Use a permeable fabric so water drains properly, and overlap seams by at least 6 inches.

How much river rock do I need for a 10×10 area at 3 inches deep?

Using the calculator: 10 ft × 10 ft × 3 in ÷ 324 = 0.93 yd³. That is just under one cubic yard, so order 1 yard to be safe. In cubic feet, it is 100 × 0.25 = 25 ft³.

Is it better to buy river rock by the bag or in bulk?

For small projects (under 10 cubic feet), bags are convenient and avoid delivery fees. For larger areas, bulk is almost always cheaper per cubic yard and requires less handling. Compare the total cost, including delivery, before deciding.

Related

• Pea Gravel per Square Foot: How Much You Need
• Mulch per Yard: Cost & Coverage Guide
• Sand Calculator: How Much Sand Do I Need?

in l | Lawn length | ft | 40 | 0 | 100000
in w | Lawn width | ft | 25 | 0 | 100000
in d | Depth | in | 3 | 0 | 120
out yd | Volume | yd³ | l * w * d / 324 | 2
out ft3 | Volume | ft³ | l * w * d / 12 | 1