How to calculate how much rip rap you need
- Measure the length of bank or ditch you are protecting.
- Measure down the slope face, from the top of the protection to the toe.
- Multiply the two to get the area, then by the layer thickness in feet for cubic feet.
- Divide by 27 for cubic yards and multiply by about 1.5 for tons.
Worked example. 40 ft of bank with a 12 ft slope is 480 ft². An 18 inch layer is 480 × 1.5 = 720 ft³, or 26.67 yd³. At 1.5 tons per yard, order about 40 tons.
How thick should rip rap be?
A common rule is a layer about twice the average stone size, so the stones overlap instead of leaving gaps. Projects near streams, lakes or roads may need an engineered design or permit that sets the size and thickness.
| Average stone size | Layer thickness | One ton covers |
|---|---|---|
| 6 in | 12 in | 18 ft² |
| 9 in | 18 in | 12 ft² |
| 12 in | 24 in | 9 ft² |
How much does rip rap weigh?
Placed rip rap usually weighs 1.4 to 1.6 tons per cubic yard, depending on the rock type and how tightly it packs. Granite and basalt sit at the heavy end. Quarries quote by the ton, so use their figure when you have it.
Do you need fabric under rip rap?
Yes, in most cases. A filter fabric between the soil and the stone stops water from washing soil out through the gaps. The calculator adds 10 percent to the area for overlaps.
Questions about rip rap quantities
How many square feet does a ton of rip rap cover?
At 1.5 tons per yard, a ton is 18 ft³: about 18 ft² at 12 in thick, or 12 ft² at 18 in.
Why measure along the slope?
The stone lies on the sloped face, which is longer than the flat distance across. Using the flat distance under-orders.
Should I order extra?
Add about 10 percent for uneven ground and gaps, and more if the toe of the slope is buried below grade.
Sources
Last reviewed November 2, 2026. Found a wrong figure? Tell us.