How much river rock do I need?
River rock is sold by the cubic yard at the landscape yard and by the ton at the quarry, and by the half cubic foot bag at the store, and none of those is the square feet you measured. Here is the arithmetic that connects them, the depth each size of stone wants, and the honest comparison with mulch, which is cheaper this year and dearer by the fifth.
River rock calculator
How much river rock do you need?
Working out a whole yard rather than one bed? Sketch it to scale in the planner and every area is measured, costed and totalled together.
The formula
cubic yards = (square feet × depth in inches) ÷ 324
Then tons is cubic yards times about 1.4, which is what a yard of rounded stone weighs with the air between the pieces counted in. A 150 square foot bed at 3 inches is (150 × 3) ÷ 324 = 1.4 cubic yards, or about 2.0 tons. The gravel guide goes into why that weight moves with the stone; for river rock, which is a single rounded size with no fines, it moves less than for most.
One cubic yard covers:
| Depth | One cubic yard covers | Which stone |
|---|---|---|
| 2 in | 162 sq ft | Pea gravel and pebbles under an inch |
| 3 in | 108 sq ft | The common 1 to 2 inch stone |
| 4 in | 81 sq ft | 2 to 4 inch cobbles, dry creek beds |
How deep
The rule is a layer about twice the size of the stone, so that no ground shows through the gaps and the pieces lock against each other rather than rolling loose. Small pebbles cover at two inches; the one to two inch stone most beds use wants three; the big cobbles in a dry creek bed want four or more, and they are usually set on a bed of the smaller size rather than laid straight on the ground.
Deeper than that buys nothing. Rock does not break down, so there is no topping up to allow for, and a thick layer is just more to walk across and more to pay for. What does matter is what is under it.
What goes underneath
Landscape fabric under the stone is the difference between a bed that stays clean and one that grows weeds through it in two summers, and it also keeps the stone from working its way down into the soil, which is where a three inch layer goes to become a one inch layer. Use a woven fabric, not plastic sheeting: water and air have to get through to the roots of anything planted in the bed.
Edging holds the shape. River rock is round, which is why it looks the way it does and also why it travels: every footstep and every downpour moves a few stones towards the lawn. A steel or paver edge around the bed keeps the depth where you put it. How much edging works out the run.
What it costs
Loose, river rock runs about $70 – $150 per cu yd, which is towards the top of the range for landscape stone: it is dug, washed and screened rather than crushed, and the rounded, coloured stone people want is not found everywhere. By the ton, divide the yard price by about 1.4. Bagged it is $3.50 – $9 for half a cubic foot, which is $189 – $486 a cubic yard once you carry 54 of them, so anything over a yard is worth having tipped.
Delivery is charged per load, $50 – $150, and most yards have a minimum of 2 cubic yards on a delivery. A bed that works out at a yard and a half is better ordered with the gravel for a path on the same truck.
Rock or mulch
The same bed in mulch costs about $30 – $65 per cu yd, a third or less of the stone. But mulch is gone in two or three years and gets topped up every one of them, and the stone is there in ten. Over five years the two cost about the same; over ten the stone is cheaper, and it never needs the truck again.
Cost is not the whole comparison. Mulch feeds the soil as it breaks down and keeps it cool; stone does neither, and a bed of dark rock in full sun runs hot enough to stress the plants in it. Rock suits beds of shrubs and grasses that are planted once and left, paths, dry creek beds and the strip along the house. Mulch suits anything replanted every year. The low-water front yard example puts river rock and decomposed granite where a lawn would be, and its material list shows the whole trade.
Quick checklist
- Measure the bed in square feet, and choose the depth by the stone size: twice its size, three inches for the common stone.
- Convert to cubic yards with the 324 rule, and to tons at about 1.4 a yard if the quarry quotes by weight.
- Lay woven landscape fabric under it, and edging around it, before the stone arrives.
- Bag it under a cubic yard; have it tipped over one, and fill the truck with the base for anything else.
- Against mulch, count the years: stone costs more once and mulch costs less every year.
Paint the bed with river rock at the depth you have chosen and the planner gives the cubic yards, the tons and the cost for your state, beside everything else in the yard. Paint it with mulch instead to see the difference on the same shape.