River Rock Calculator

How to use the River Rock Calculator

Stone size sets the depth, so pick that first. The depth has to suit the stone: a 3 to 5 inch river rock cannot be spread three inches deep, because a single stone is thicker than that. The rule is roughly twice the largest stone dimension, and the calculator warns you if the depth is short.

For a dry creek, measure along the curves rather than end to end, a winding channel is considerably longer than the distance it spans. Enter the top and bottom widths separately, because a creek bed is a shallow trapezoid, not a rectangle.

River Rock Cost prices by the ton or the yard. Stone size affects both, since larger stone needs more depth to cover the ground, the same bed costs noticeably more in 3 to 5 inch rock than in one inch.

River Rock formula

Beds are rectangles:

• volume = length × width × depth

Creek beds are trapezoidal channels:

• cross-section = (top width + bottom width) ÷ 2 × depth
• volume = cross-section × channel length

Then, for both:

• cubic yards = cubic feet ÷ 27
• tons = cubic yards × 2,700 ÷ 2,000

River rock runs about 2,700 lb per cubic yard, or 1.35 tons.

Worked example

A 30 ft × 6 ft border in 1 to 3 inch rock at 3 inches deep, 5% waste:

• Area: 180 sq ft. Volume: 180 × 0.25 = 45 ft³
• Cubic yards: 1.67, plus 5% = 1.75 yd³, about 2.36 tons

A 40 ft dry creek, 4 ft across the top, 2 ft across the bottom, 8 inches deep, in 3 to 5 inch rock:

• Cross-section: (4 + 2) ÷ 2 × 0.667 = 2.00 sq ft
• Volume: 40 × 2.00 = 80 ft³ = 2.96 yd³, plus 10% = 3.26 yd³ (about 4.40 tons)

Treating that creek as a plain 40 × 4 ft rectangle would have given 3.95 yd³ against the channel’s 2.96, a third more rock than it actually holds.

What the result means

Depth is set by the stone, not by taste. Spread large rock too thin and you get gaps, visible fabric and stones that roll when stepped on. Twice the largest dimension is the working rule, and it is why a dry creek in 3 to 5 inch rock takes several times the material a 1 inch bed does over the same footprint.

River rock is rounded and smooth, which is exactly why it is used and exactly why it will not stay on a slope without help. On any real grade it needs check dams, larger anchor stones, or a buried edge.

It is also among the more expensive landscape stones by the yard, and heavy, a single cubic yard is well over a ton. Small jobs go in a truck bed; anything past two yards is a delivery.

The volumes are exact. Tonnage is an estimate, and rounder, larger stone packs with more air in it than the density charts assume.

Common mistakes

• Spreading large stone too thin. You cannot lay 3 to 5 inch rock at three inches. Match the depth to the stone or buy smaller stone.
• Measuring a creek as a rectangle. A trapezoidal channel holds noticeably less than its top width suggests, about a third less in the example above.
• Measuring a winding creek in a straight line. Walk the centreline with a tape. The curves are the whole point and they add real length.
• Skipping the fabric. River rock sinks into soft ground and is miserable to weed once it does. Geotextile underneath, always.
• Using it on a slope without anchoring. Rounded stone rolls downhill. Larger stones, check dams or a buried edge restraint.
• Underestimating the weight. A cubic yard is around 1.35 tons, more than most pickup beds are rated for.

Frequently asked questions