River Rock Calculator

Order (yd³) = length × width × (depth ÷ 12) × (1 + wastage ÷ 100) ÷ 27

L

Length of the bed, border or channel, in feet

W

Width, in feet

D

Depth of the rock layer, in inches

w

Wastage allowance, as a percentage of the net volume

ρ

Bulk density of river rock, in kilograms per cubic metre

Use it before you order rock for a landscaping bed, a tree ring, a border along a path, or a dry creek bed. The allowance is the part worth thinking about: a bed with a hard edge and a flat subgrade needs little, while rock running up to a wall, around shrubs or down a channel between stones needs a good deal. Buy for the shape you are actually building, not for the rectangle you measured.

1. A 20 ft by 12 ft bed, three inches deep, with 10% allowed for wastage

   1. Area: 20 × 12 = 240 square feet
   2. Depth in feet: 3 ÷ 12 = 0.25 ft
   3. Net volume: 240 × 0.25 = 60 cubic feet = 2.22 cubic yards
   4. With 10% allowed: 60 × 1.1 = 66 cubic feet = 2.44 cubic yards
   5. River rock at 1,500 kg per m³: 66 ft³ = 1.869 m³ = 2,803 kg
   6. That is 2.80 tonnes, or 3.09 short tons

   The two volume lines are the useful pair: 2.22 yd³ is what the bed measures, 2.44 yd³ is what you order. Watching the gap open and close as you change the allowance is quicker than working out what 10% of a cubic yard is.

2. A dry creek bed 30 ft long and 4 ft wide, six inches of rock, 15% for wastage

   1. Area: 30 × 4 = 120 square feet
   2. Depth in feet: 6 ÷ 12 = 0.5 ft
   3. Net volume: 120 × 0.5 = 60 cubic feet = 2.22 cubic yards
   4. With 15% allowed: 60 × 1.15 = 69 cubic feet = 2.56 cubic yards
   5. River rock at 1,500 kg per m³: 69 ft³ = 1.954 m³ = 2,931 kg
   6. That is 2.93 tonnes, or 3.23 short tons

   Same net volume as the first example — 60 cubic feet — and a different order, because a channel between rocks eats more than a bed with a hard edge. The extra 0.12 cubic yards is a third of a wheelbarrow; the point is that nothing in the geometry tells you which allowance to use, so it stays a decision rather than a constant.

The density is a typical value, not a specification: river rock runs near 1,500 kg per m³, and a real load moves with the quarry, the grading, the moisture and how much it has settled in transit. The wastage allowance is a judgement too — nothing publishes a standard for it, and the right figure depends on the shape you are filling, the subgrade and how evenly the rock can be spread, which is why it is an input with a range from 0 to 50 rather than a hidden 10%. The depth is a loose depth, rounded stone does not lock together the way crushed stone does, and a layer spread three inches will read as roughly three. The area is treated as a rectangle; a curved bed or a tapering border takes less. And the tonnage is the rock alone — delivery, a geotextile membrane under a creek bed, and any minimum-load charge are separate.

How much area does a cubic yard of river rock cover?

It depends on the depth: a cubic yard spread two inches deep covers about 162 square feet, and at three inches about 108. That is why river rock coverage is quoted per depth — at one inch a cubic yard reaches 324 square feet, at four inches only 81. Enter your depth and the page returns the yards rather than the other way round.

How much wastage should I allow for river rock?

Ten percent is a fair default for a bed with a defined edge, five if the ground is flat and the shape is simple, and fifteen to twenty for a channel, a slope, or rock tucked around plants and walls. Nothing publishes a standard for it, so it is your call — which is why the calculator asks rather than assuming.

Why is the order bigger than the volume I measured?

Because rock is spread, not placed. The subgrade is never perfectly flat, the edges of a bed lose material, and rounded stone leaves gaps that have to be filled from above. The net volume line shows the measurement and the main result shows the order, so the difference between them is visible instead of buried in a hidden multiplier.

Can I buy river rock by weight instead of by volume?

Yes, and the calculator returns both: 2.44 cubic yards of river rock is about 3.09 short tons or 2.80 tonnes, with a density of 1,500 kg per m³ behind it. Treat the weight as a typical figure and the volume as the quantity — the density moves about 10% with moisture, and that is the number the tonnage depends on.

Is river rock the right landscaping rock here?

River rock is rounded and reads as a decorative surface: beds, borders, tree rings, dry creek beds and the margins of a pond. Crushed stone locks together and is the better choice underfoot or under a vehicle, and pea gravel sits between the two. The depth and the allowance are the same question in all three cases.

How deep should river rock be in a bed?

Two to three inches over a weed membrane is the usual decorative depth, and four to six for a dry creek bed that has to read as water. Deeper is not better: past about four inches the top layer is the only one you see, and the rock below it is weight you paid for. Keep the layer shallow and the area honest instead.

• Approximate Conversions from U.S. Customary Measures to Metric — the chart behind the cubic yard, pound and kilogram figures used here — National Institute of Standards and Technology
• The International System of Units (SI) brochure — the kilogram and the cubic metre, the two units the density on this page is defined in — Bureau International des Poids et Mesures (BIPM)