River rock calculator estimates tons, cubic yards, bags, and cost using area × depth × waste, then converts volume by density. Enter square feet, depth, rock size, and price.
Depth Is the Part Most People Get Wrong
Ordering river rock is straightforward once you have the right area and depth. The part that trips people up is the layer thickness — specifically, whether the depth you’re planning is actually enough to hide the landscape fabric or bare soil beneath. A 2-inch layer of 3″-5″ boulders isn’t coverage; it’s scattered rocks with gaps. This calculator checks that automatically and flags it before you commit to a quantity.
Three Ways to Enter Your Area
Not all landscaping shapes are rectangles. The Project Shape dropdown gives you three paths:
Rectangle / Bed is the default — enter length and width and the tool multiplies them. Works for straight-sided beds, walkways, and borders.
Circle / Planter takes a single diameter measurement and calculates area using the full π × r² formula. Useful for circular tree rings, round fountain surrounds, and planter bowls where measuring a diameter is easier than estimating an equivalent rectangle.
If you already have your coverage area from a landscape plan or measurement app, I Know My Area skips the geometry entirely and accepts square feet or square meters directly.
All three modes then run through the same volume-to-weight chain: area × depth = net cubic footage, scaled up by your waste percentage, then multiplied by the bulk density of your selected stone size to get pounds and short tons.
Why Larger Stone Actually Weighs Less Per Cubic Foot
This is the one thing that surprises most people. The density values in this tool run downward as stone size increases: small 3/8″–1/2″ pea gravel sits at 105 lb/ft³, medium 1″–2″ rounds at 95, and large 3″–5″ river rock at just 85 lb/ft³. That’s nearly a 20% weight difference per cubic foot between the smallest and largest grades.
The reason is void space. Rounded river stones don’t pack — each stone’s smooth surface keeps adjacent stones from nesting tightly together. Larger stones leave proportionally bigger air gaps between them. So a cubic yard of 4-inch cobble contains considerably more air than a cubic yard of 3/4-inch gravel, even though the individual stones are far heavier.
Order by volume, and this matters enormously. Order a cubic yard expecting the weight of small gravel and your cost estimate for large cobble will run high by several hundred pounds.
The Coverage Depth Check
Card 4 in the results does something different from a straightforward volume calculation. It compares your entered placement depth against the maximum stone size of your selected grade and reports one of three states:
Too Thin fires when your depth is less than the maximum stone diameter — meaning individual stones will sit proud of the layer and bare ground will show. The alert turns amber and tells you exactly how much more depth you need.
Adequate means your depth meets or exceeds the stone’s maximum size. The layer will close. This is the minimum acceptable threshold for full coverage.
Optimal triggers when your depth is at least twice the maximum stone size — the point where stones have room to settle and shift without exposing gaps beneath. For a decorative bed, this is the depth that stays looking finished after rain and foot traffic.
Bags or Bulk: The Delivery Decision
Card 5 shows two numbers side by side: how many 50 lb bags your quantity equals, and what fraction of a 10-cubic-yard dump truck that represents. Both are useful, but for opposite reasons.
Under about 0.5 truck loads (5 cubic yards), bagged stone from a home center is often more practical — no minimum order, no scheduling a delivery, and you only buy what fits your vehicle. Above that threshold, bulk delivery from a landscape supplier almost always costs less per ton and cuts down on packaging waste. The bag count the tool shows rounds up with Math.ceil, so it always gives you the safe whole-bag number rather than a partial. If you’re near a bag boundary — say, 20.1 bags — you’ll see 21, not 20.
Example: Circular Fountain Surround, 3/4″ River Rock
A backyard fountain had a circular gravel surround that needed refreshing. The ring measured 14 feet in diameter. The plan was 3/4″ rounded river rock at a 4-inch depth, with 10% waste factored in for an irregular edge border.
Switching to Circle / Planter mode and entering a 14 ft diameter gave a coverage area of 153.94 sq ft (π × 7²). With 4 inches of depth and 10% waste, final volume came to 6.30 cu yd (170.08 cu ft). At 100 lb/ft³ for 3/4″ rock, weight worked out to 17,008 lbs → 8.50 short tons.
The coverage check showed Optimal — 4 inches placed against a 0.75-inch maximum stone size clears the 2× threshold with room to spare. Bag equivalent: 341 bags. That number made the delivery decision easy — at $52/ton, total stone cost came to $441.99, and no one was hauling 341 fifty-pound bags from a home center.
Frequently Asked Questions
The 1″–2″ and 1″–3″ sizes show the same density — does it matter which one I pick?
Yes, significantly. Both options use a bulk density of 95 lb/ft³, so your weight and cost outputs will be identical. But the Coverage Depth Check uses the maximum stone size, not the density. Selecting 1″–3″ tells the tool that your stones go up to 3 inches, so a 2-inch depth layer will read as Too Thin — because some stones will poke through. Selecting 1″–2″ at the same 2-inch depth reads as Adequate. Pick the option that matches the actual top end of your stone’s size range, not just the average.
When I use Circle mode, is the area exactly correct or is it an approximation?
It’s an exact calculation. The tool uses Math.PI × (diameter ÷ 2)², which is the full floating-point value of π — not a rounded approximation like 3.14. For a 10-foot diameter circle that means 78.5398 sq ft, not 78.5. Over a large circular planter, that precision prevents meaningful underestimates in both volume and cost.
The bag count jumped to the next whole number even though I’m just barely past a threshold. Is that right?
That’s intentional. The bag equivalent uses ceiling rounding — if your total weight works out to 1,501 lbs, the tool shows 31 bags (not 30.02). This reflects the reality that bags come in whole units, and the last partial bag still needs to be purchased and accounted for. If you want to reduce the bag count, try lowering your waste percentage slightly — even a 1% reduction can bring you back under a bag threshold on smaller jobs.
What counts as a valid depth in the ft+in dual-input mode?
When using Feet & In for depth, the inches field must be less than 12. Entering 0 ft and 12 in — or any inch value of 12 or above — will fail the validation and clear the results. The tool expects you to carry inches over into the feet field yourself: 1 foot 0 inches, not 0 feet 12 inches. The same constraint applies in Meters & cm mode, where the centimeter field must stay below 100.