River Rock Calculator

Two numbers decide the whole order: area and depth

Landscape rock is sold by volume or weight, and both come from the same two measurements you take with a tape measure: the area you are covering and the depth you want. Everything else is arithmetic.

• Area in square feet = length × width. Measure the bed at its widest, and split an L-shaped bed into two rectangles and add them.
• Depth in inches, 2 to 3 inches for decorative cover, 3 to 4 for paths, 4 to 6 for dry creek beds.
• Cubic feet = area × (depth ÷ 12).
• Cubic yards = cubic feet ÷ 27, or more directly (area × depth) ÷ 324. The 324 is 12 × 27, and it is the constant to remember.
• Tons = cubic yards × density. River rock and pea gravel run about 1.35 tons per cubic yard.

That last multiplier is where suppliers differ, so ask before you order. Some sell by the cubic yard and quote a yard as roughly 1.35 tons; others sell strictly by weight and the same yard becomes 1.2 or 1.4 tons depending on the stone and how wet it is. The calculator above lets you pick the size so the density matches what you are buying.

Worked example: a 20 ft by 10 ft bed at 3 inches

The classic front bed: 20 feet long, 10 feet wide, 3 inches of 1 to 1.5 inch river rock.

• Area: 20 × 10 = 200 sq ft
• Cubic feet: 200 × (3 ÷ 12) = 200 × 0.25 = 50 cu ft
• Cubic yards: 50 ÷ 27 = 1.85 cu yd (or 200 × 3 ÷ 324 = 1.85)
• Tons: 1.85 × 1.35 = 2.5 tons
• Bags: 50 cu ft ÷ 0.5 = 100 bags of 0.5 cubic foot rock

With the 10% cushion you would order about 2.04 cubic yards or 2.75 tons. If your supplier sells in whole cubic yards, that rounds to 2 yards, close enough, and you can always top up with a couple of bags. If they round up, 2.5 yards leaves you with spare stone for the edge of the driveway.

Cost check: at a mid-range $85 per cubic yard, 2 yards is about $170, plus delivery. The same 50 cubic feet in 0.5 cubic foot bags is 100 bags at, say, $5 each, $500, and a hundred trips from the car. That gap is why anything past about half a cubic yard is usually worth buying in bulk.

Worked example: a 40 ft by 3 ft path at 2 inches

A path is the other common shape: 40 feet long, 3 feet wide, 2 inches of pea gravel.

• Area: 40 × 3 = 120 sq ft
• Cubic yards: 120 × 2 ÷ 324 = 0.74 cu yd
• Cubic feet: 0.74 × 27 = 20 cu ft
• Tons: 0.74 × 1.35 = 1.0 ton
• Bags: 20 ÷ 0.5 = 40 bags

This one is right on the boundary. One cubic yard or one ton is a typical minimum delivery, so ordering 0.74 yards means paying for a full yard or picking up bags. Many people split the difference: buy one cubic yard (1.35 tons) and use the extra to widen the path or dress the bed beside it.

Compare the two examples and the pattern is clear. Depth dominates small areas, going from 2 inches to 3 inches on this path adds 50% more rock for the same footprint. Area dominates everything else: doubling the bed doubles the order. If your estimate feels expensive, reducing the depth by an inch usually saves more than shopping around for a cheaper stone.

Coverage at a glance

How far one cubic yard or one ton of river rock goes, using a density of 1.35 tons per cubic yard:

An easy sanity check while you are standing in the yard: a 10 by 10 foot square at 3 inches is almost exactly one cubic yard. So a bed that is about three times that size, 300 square feet at 3 inches, needs roughly 3 cubic yards, or 4 tons.

Tons, yards and bags: what you are buying

The three units are not interchangeable, and suppliers quote in whichever one suits them.

• By the cubic yard. The most common bulk unit. One yard is 27 cubic feet. Ask whether the yard is a struck measure or a heaped one, it makes a visible difference.
• By the ton. Common for delivery. At 1.35 tons per cubic yard, a 3-ton load is about 2.2 cubic yards, which covers roughly 240 square feet at 3 inches.
• By the bag. Usually 0.5 cubic foot, about 50 pounds. Fine for a small bed or a quick top-up, expensive per cubic foot for anything larger, and 54 bags make a cubic yard.

Notice the last two rows are identical in volume: 500 square feet at 3 inches is the same amount of rock as 300 square feet at 5 inches. Volume is what you are buying, and depth is how you spread it.

Choosing a depth, and the mistakes that cost money

Depth is the decision that changes your order most, and it is also where people go wrong.

• 1 inch is not enough for a new bed. It looks thin, weeds come through, and you will be back at the yard next spring. Extension guidance for gravel used as mulch often specifies a 1 inch layer, but that assumes fabric underneath and a neat existing bed.
• 2 to 3 inches is the sweet spot for decorative cover. Deep enough to hide the fabric, shallow enough to walk on.
• 3 to 4 inches for paths so the stone does not migrate into the soil under foot traffic.
• 4 to 6 inches for dry creek beds and drainage, where the stone is doing a job. If the area handles stormwater, treat it as a drainage feature rather than decoration, the EPA's Soak Up the Rain guidance covers how rock and gravel are used in rain gardens and drainage swales.

Four mistakes that show up again and again: ordering without the 10% cushion, then running short with a half-finished bed; laying rock straight onto bare soil so it sinks within a year; using round river rock where a crushed angular stone would lock together better (paths and driveways want angular, beds look better with river rock); and forgetting that the first delivery is the hardest part, 2.5 tons is a lot of wheelbarrow trips, so have the truck drop it as close to the bed as it can get.

For the ground-cover side of the maths, Mississippi State University Extension's mulches guide sets out how application rates are calculated for gravel, stone and organic mulches alike. Everything above is an estimate from the measurements you enter, confirm the exact coverage and weight with your supplier before you pay for a delivery.