Rip rap calculator for Hawaii
Calculate shoreline and slope tonnage using armor-layer depth, not shallow landscaping coverage.
Enter your project dimensions
About 17 square feet is covered by one ton of riprap. The number is accurate. Because armor stone is placed 12 inches deep or more, a ton reaches only about one-tenth the area of landscape rock. A pile may appear modest beside the amount of bank it protects. This calculator uses installed depth, 3,100 lb per cubic yard, and rock generally ranging from 4 to 12 inches and larger.
How to measure an armored bank
- Use the sloped face for your measurement, rather than the level area above it. Measure down the bank with a tape, then multiply by the channel or shoreline length.
- For the rock layer, allow 1.5 to 2 times the diameter of the largest stone. A 12-inch depth is the minimum used for erosion control.
- Divide cubic feet by 27 for cubic yards, then multiply by 1.55 for tons.
Tons and coverage
| Layer thickness | 1 ton covers | Tons per 100 sq ft |
|---|---|---|
| 12 inches | ≈ 17 sq ft | 5.7 |
| 18 inches | ≈ 12 sq ft | 8.6 |
| 24 inches | ≈ 9 sq ft | 11.5 |
Installation details for stable riprap
- Start with fabric or bedding: place non-woven geotextile beneath the rock, or use 3-4 inches of #4 stone. On saturated windward clay, rock laid straight on soil may look settled for a season before runoff undermines the bank and the stone drops.
- Lock in the toe by burying the lowest row below the waterline or anticipated scour depth. Riprap typically breaks down from the toe upward.
- Rain runoff needs a stable face, so shape the slope to 2:1 or flatter before placing stone. A steeper face needs engineering, not additional rock.
- For the exposed top layer, use a machine or hand placement to lock the stones together rather than leaving them where they roll.
- For culvert outlets, carry the apron 4-6 pipe diameters past the outlet downstream. That is the area water scours.
Example calculation: 50 feet of pond bank
A shoreline slope 6-foot high and 50 feet long has 300 square feet of face. For an 18-inch layer: 300 x 1.5 = 450 cubic feet = 16.7 cubic yards, or about 26 tons of riprap and 27 with a small allowance. Include a geotextile roll, or use about 4 tons of #4 stone if rock bedding is preferred. Before ordering, walk the truck access route. The last 20 feet to a pond edge can bog down loaded trucks, and relocating stone twice costs twice.
After a major storm
After the water recedes, walk along the bank. Check the toe for displaced rock, exposed soil between stones, visible fabric, and any scouring at the outfall apron. Put shifted pieces back in place and use reserve stone to fill sparse areas. A brief check a couple of times each year, especially after heavy rain, is the ongoing upkeep.
Stone sizes at a glance
Select stone size for the force of the water. Four to eight inch riprap is suited to pond banks, gentle streams, and downspout outfalls. Use eight to twelve along lake edges exposed to boat wake and in faster channels. Highway and spillway projects have engineered classes and layer requirements, so follow their drawings instead of this page. When choosing between sizes, use the larger rock and a thicker layer. Greater stone weight allows more margin.
Riprap quantity, sizing and installation
How can I determine the rip rap quantity I need?
What area will 1 ton of riprap cover?
What does a 1 ton pile of rip rap look like?
What is the cost of 1 ton of rip rap?
What depth should a rip rap layer have?
Is hand placement an option for rip rap?
Other calculators
Protect exposed banks with armor stone.
Order RipRap by the ton with free delivery, along with #4 bedding stone for underneath it.
