River Rock Calculator

Measure each area in feet, pick a depth, and get river rock in cubic yards, an honest tonnage range, and how far a ton actually goes. The cited depth is different over landscape fabric than on bare soil — and for coarse stone it is set by the size of the stone.

How this is calculated

Shape areas are summed and multiplied by depth: volume = total area × depth (at depth d inches, cubic yards = square feet × d ÷ 324).
Tonnage = volume × 1.5–1.7 tons per cubic yard, published as a range because the figure is an aggregate-class conversion rather than a river-rock-specific one.
Coverage per ton = your area ÷ tonnage at the depth you chose — the same cited range, expressed the way stone yards quote it.
No compaction or waste factor is applied, because none is published for washed rounded stone.

Data used, with sources

Value	Used as	Source
Tonnage conversion 1.5–1.7 tons/cu yd	tonnage and coverage-per-ton shown as a range — WisDOT statewide conversion factor for the coarse sand / sand-and-gravel aggregate class — not a river-rock-specific producer figure, and a wider reach than for pea gravel: washed, uniformly sized rounded stone carries more void space than the graded material this class describes, so treat the low end as the likelier one and the delivered scale ticket as the number that governs. Order by volume where you can.	Wisconsin Department of Transportation / Wisconsin Highway Research Program
Depth over landscape fabric: 2 in	depth preset — the fabric blocks the light, so the stone only has to cover it	University of Nevada, Reno Extension
Depth on bare soil, stone ½–3 in across: 3–6 in	depth preset 4 in — the coarse-material band, set by particle size	Utah State University Extension ; Colorado State University Extension

Tons here means US short tons (2,000 lb).

Depth is set by stone size, not by habit

The usual "2 to 3 inches of rock" advice comes from guidance written about material smaller than half an inch. Utah State Extension separates the two cases explicitly, by particle size: 1 to 2 inches for fine, dense material under ½ inch, and 3 to 6 inches for coarse material between ½ and 3 inches across — which is the river-rock size band. The reason is geometric rather than horticultural. A 2-inch layer of 2-inch stone is one stone deep, with gaps to the soil at every contact point, and light through a gap is a weed.

Landscape fabric changes the arithmetic, which is why this page presets both. If the fabric is blocking the light, the stone only has to cover the fabric — Nevada Extension puts that at 2 inches. Choosing between them is a real decision with a cost attached: at 4 inches you are buying twice the stone you would over fabric, and over fabric you are buying a layer that will be unpleasant to lift when it silts up.

What river rock is actually for

It is worth being honest that river rock is chosen for appearance and drainage, not for the things mulch is usually bought to do. Colorado State Extension sets a half-inch ceiling on stones "used for mulch" if you want water conservation and weed suppression; above that size, the benefit is landscape interest. Nevada Extension agrees that smaller rock retains soil water better, and warns that rock mulches "are hot and heavy, and become unsightly when trash gets caught in them" — the heat is a real consideration next to a wall or over the root zone of a plant that did not evolve for it. Stone under that half-inch line is what the pea gravel calculator covers.

Where river rock earns its place is where water moves: dry creek beds, splash zones under downspouts, French drain covers, and paths that need to shed water rather than hold it. The graded stone that goes inside the drain itself is a different product, sized by the crushed stone calculator. Those are the jobs the size and the roundness are good at, and none of them are mulch.

Common mistakes

Ordering 2 inches for bare soil. That figure is cited for rock over fabric. On open soil, coarse stone needs 3 to 6 inches to actually cover.
Buying river rock to suppress weeds. The cited size limit for that benefit is half an inch. Big rounded stone looks good and lets light through.
Adding a compaction allowance. No measured expansion exists for rounded stone; borrowing the graded-aggregate figure would inflate an order by up to half.
Treating tons as the reliable unit. Rounded, uniform stone stacks loosely, so a ton covers less than a graded product would. Order by volume where the yard allows it.
Putting it where it has to be moved again. Rock is permanent in a way bark is not — Nevada Extension notes that once fabric is under a permanent mulch such as rock, treat it as permanent too.

Frequently asked

How deep should river rock be?

It depends on what is underneath, and the two cited answers differ for a reason. Over landscape fabric, University of Nevada Reno Extension says "a layer of landscape fabric under a two inch layer of rock is sufficient" — the fabric is doing the light-blocking, so the stone only has to cover it completely. On bare soil the stone has to block the light itself, and Utah State Extension sets that thickness by particle size: "1 to 2 inches of fine, dense material (less than ½ inch diameter) or 3 to 6 inches of coarse, fluffy material (between ½ inch and 3 inches in diameter)". River rock sits in that coarse band, so 3 to 6 inches — we preset 4, the depth Colorado State Extension cites for weed suppression.

How much area does a ton of river rock cover?

At 2 inches, roughly 95–108 square feet per ton; at 4 inches, roughly 48–54. Both come from the same cited tonnage range rather than a coverage claim of our own, which is why they are ranges. Treat them as planning figures: rounded stone of a uniform size stacks loosely and unpredictably, and the number that settles your invoice is the scale ticket.

Is river rock a good mulch?

Not for the reasons mulch is usually chosen. Colorado State Extension is direct: "Any stones used for mulch should be smaller than a half-inch in diameter for the benefits of water conservation and weed suppression" — larger stone provides landscape interest rather than function. Nevada Extension adds that "a mulch of smaller rocks is more effective at retaining soil water than are larger ones", and that "rock mulches are hot and heavy, and become unsightly when trash gets caught in them". River rock is bought for how it looks and how it drains. If you want the mulch benefits, the cited guidance points at something under half an inch — pea gravel or an organic mulch.

Do I need landscape fabric under river rock?

The sources genuinely disagree, so here is both sides. Nevada Extension recommends it: "for optimum weed control, it is advisable to place a layer of landscape fabric (not sheet plastic) on the soil surface before installing rocks", and it lets you use a 2-inch layer instead of a deeper one. Colorado State Extension takes the opposite view for landscape beds — that fabrics "are not recommended beneath mulch in landscape areas", because weeds germinate in the debris on top and root through, and the fabric interferes with water and air exchange. The practical trade: fabric buys you a thinner, cheaper stone layer now and a harder cleanup in five years.

How many cubic yards of river rock do I need?

Area in square feet × depth in inches ÷ 324. A 1,000-square-foot bed at 4 inches is 12.35 cubic yards. Buying by volume rather than by weight is the safer way to order rounded stone, because the volume is exactly what you measured while the weight depends on how loosely the stone happens to stack.

Why is there no compaction factor here?

Because nobody has published one for washed rounded stone, and we do not invent factors. The measured 38–52 % compacted-to-loose expansion in the engineering report behind our tonnage figure was taken on graded sand-and-gravel, a material that packs together in a way uniform river rock does not. Applying it here would inflate your order by half for no reason. River rock is placed loose and stays loose.

Sources

Weight-Volume Relationships and Conversion Factors for Soils and Aggregates of Wisconsin (WHRP G22-05) — Wisconsin Department of Transportation / Wisconsin Highway Research Program, accessed Aug 11, 2026.
Mulches for Nevada Landscapes (A. M. O’Callaghan, SP-06-10, 2006) — University of Nevada, Reno Extension, accessed Aug 11, 2026.
Using Mulches in Utah Landscapes and Gardens (R. Koenig, K. Farrell-Poe, B. Miller, December 2010) — Utah State University Extension, accessed Aug 11, 2026.
CMG GardenNotes #245: Mulching — Colorado State University Extension, accessed Aug 11, 2026.

Quantities on this page are planning estimates, not engineering advice or a purchase guarantee — product sizes vary by producer, so confirm coverage with your supplier before ordering. Rock placed in a drainage channel, swale, or anywhere water is being directed is a sizing question governed by flow, not by this estimate.