Introduction
Rip rap is rock armor. Contractors use it on streambanks, shorelines, drainage channels, and slopes where water or erosion will move anything smaller. Order too little and you’re short on the day the truck arrives. Order too much and you’re paying for stone that sits in a pile behind the shop for a year.
The sizing problem is the void ratio. Rip rap stones don’t pack tight like gravel. A cubic yard of rip rap holds roughly 70% stone and 30% air. If you need a 12-inch layer over a 40 ft × 8 ft area, the solid volume is 320 cubic feet, but you’ll need closer to 457 cubic feet of delivered stone to fill that layer because of the voids. That’s the difference between ordering 12 cubic yards and 17.
This calculator handles that adjustment, then multiplies through to the weight in tons so you can compare it against a supplier’s quote. Rip rap pricing is almost always per ton, not per yard, and the two don’t convert simply. Weight depends on the stone’s density. Most rip rap runs near 100 lb/ft³ solid, but limestone, granite, and basalt each sit a little differently. The density field accepts four units so you can plug in whatever your supplier specifies.
Class selection (R-1 through R-6) works by size range. R-1 is 4 to 8-inch stone for light erosion control. R-6 is 36 to 48-inch stone for heavy wave action or fast-moving water. All classes currently preset the density to 100 lb/ft³. If your supplier quotes a different density, switch to Custom and enter the value.
How to Use the Rip Rap Calculator
Choose your currency from the dropdown at the top if you want the cost output in something other than US dollars.
Enter the length and width of the area you’re covering. Pick feet, inches, yards, meters, centimeters, or millimeters.
Enter the depth or thickness of the rip rap layer. Nine to 24 inches is typical for streambank work.
Select a rip rap class from the dropdown. R-1 through R-6 covers the standard size ranges. Pick Custom if you want to enter your own density.
Confirm the stone density. The default is 100 lb/ft³, which fits most rip rap. Switch the density unit to kg/m³, lb/yd³, or ton/yd³ if that’s what your supplier uses.
Set the void ratio. Thirty percent is a reasonable default for angular rip rap. Rounded river rock packs tighter, closer to 20%.
Add a waste factor. Ten percent covers spillage, uneven subgrade, and stone that walks off the truck.
Enter the price per unit weight and pick the unit: per ton, per pound, or per kilogram.
Add delivery cost if it’s not baked into the stone price. Choose total, per ton, or per load (assuming 20 tons per load).
Add fabric or geotextile cost, either as a total or per square foot or square meter.
Enter any labor cost as a lump sum.
Hit Calculate. The Summary tab shows the headline numbers. The Detailed Breakdown tab lists every intermediate value.
How the Rip Rap Calculator Formula Works
The tool converts every dimension to feet and every density value to lb/ft³ before running the numbers.
Formula: Surface Area (ft²) = Length (ft) × Width (ft)
Formula: Solid Volume (ft³) = Length × Width × Depth (ft)
Formula: Volume with Voids (ft³) = Solid Volume ÷ (1 − Void % ÷ 100)
Formula: Volume with Waste (ft³) = Volume with Voids × (1 + Waste % ÷ 100)
Formula: Weight (lbs) = Solid Volume × (1 + Waste % ÷ 100) × Density (lb/ft³)
Formula: Weight (tons) = Weight (lbs) ÷ 2000
Formula: Rip Rap Cost = Weight (lbs) × Price per lb
Formula: Total Cost = Rip Rap Cost + Delivery + Fabric + Labor
The void adjustment runs on volume, not weight. A 30% void ratio means the delivered stone occupies 30% more space than its solid volume because of gaps between rocks. The math divides solid volume by 0.70 to get the real space the stone fills.
Weight works differently. You’re paying the supplier by weight, and that weight reflects solid stone. The void ratio doesn’t change the weight because air doesn’t weigh anything. So weight = solid volume × density, with the waste factor applied. That keeps the tonnage honest.
The three density units each convert to lb/ft³ before use. kg/m³ multiplies by 0.062428. lb/yd³ divides by 27. ton/yd³ multiplies by 2000 then divides by 27. Whatever you enter, the internal calculation runs in the same units.
Price units follow the same pattern. Per ton divides the entered price by 2000 to get per-lb. Per kg multiplies by 0.453592. Per lb is 1:1. Delivery cost supports three modes: a flat total, a per-ton rate multiplied by total tonnage, or a per-load rate that assumes 20 tons per truck.
Worked Example
You’re armoring a drainage channel. The area is 40 ft long and 8 ft wide. You need a 9-inch layer of R-3 rip rap. Your supplier charges $48 per ton for the stone, with delivery included. You’re adding 10% waste and using the standard 30% void ratio.
Step 1: Convert depth to feet.
9 inches ÷ 12 = 0.75 ft.
Step 2: Surface area.
40 × 8 = 320 ft².
Step 3: Solid volume.
40 × 8 × 0.75 = 240 ft³.
Step 4: Apply void ratio.
240 ÷ (1 − 0.30) = 240 ÷ 0.70 = 342.86 ft³.
Step 5: Add 10% waste.
342.86 × 1.10 = 377.14 ft³.
In cubic yards: 377.14 ÷ 27 = 13.97 yd³.
Step 6: Calculate weight.
240 × 1.10 × 100 = 26,400 lbs.
In tons: 26,400 ÷ 2000 = 13.20 tons.
In kilograms: 26,400 × 0.453592 = 11,975 kg.
Step 7: Calculate cost.
13.20 tons × $48 = $633.60.
The Summary tab shows 320 ft² of surface area, 240 ft³ solid volume, 377.14 ft³ with voids and waste (13.97 yd³), 13.20 tons of stone, and $633.60 total cost. The breakdown tab lists every intermediate step.
If you switch to R-5 (24–36 inch stone) without changing anything else, the weight stays the same because the calculator uses density, not stone size, to compute tonnage. The volume with voids also stays the same. Size class changes what the stone looks like in the field, not how much it weighs.
