Riprap Sizes and Weight: Classes, Tons, and Coverage

Riprap sizes and weight depend on the class of stone, which is chosen based on how much water force the rock must resist. Riprap is large, angular rock placed to protect slopes, shorelines, ditches, culvert outlets, and stream banks from erosion. Light classes might average around 6 inches, while heavy classes include boulders a foot or two across. Riprap typically weighs roughly 1.4 to 1.6 tons per cubic yard as placed, but your supplier’s figures and project specs come first.

Volume and tonnage calculator →

Typical riprap classes

State transportation agencies and suppliers define classes differently, so names and numbers vary. A common pattern:

General class	Typical size range	Example uses
Light or Class 1	About 4 to 12 inches	Ditches, small culvert outlets, light slope protection
Medium or Class 2	About 6 to 18 inches	Drainage channels, moderate slopes, small ponds
Heavy or Class 3	About 12 to 24 inches	Stream banks, shorelines, larger culverts
Extra heavy	Boulders 2 feet and larger	High-energy shorelines, large channels

Designers often specify a median stone size, called D50, meaning half the rock by weight is smaller and half larger.

Layer thickness

Riprap is usually placed in a layer about 1.5 to 2 times the D50 size, or at least as thick as the largest stones. For example, a riprap with a 9-inch median size might be placed about 14 to 18 inches thick.

Weight and tonnage math

Area: length x width in square feet.
Volume: area x layer thickness in feet, divided by 27 for cubic yards.
Tons: cubic yards x about 1.4 to 1.6 tons per yard.

Example: a 6-foot-wide ditch lining 50 feet long at 12 inches thick: 300 x 1 = 300 cubic feet, divided by 27 = 11.1 yards, times 1.5 = about 16.7 tons.

Area	12 inches thick	18 inches thick
100 sq ft	About 5.2 to 5.9 tons	About 7.8 to 8.9 tons
500 sq ft	About 26 to 30 tons	About 39 to 44 tons

The volume and tonnage calculator can run these numbers.

Underlayment matters

Riprap needs a filter layer so soil does not wash out through the gaps. Common choices:

Geotextile fabric rated for riprap.
Bedding stone, a graded gravel layer under the rock.
Both, for higher-flow areas.

Without a filter, riprap can slump as the soil underneath erodes.

Placement tips

Grade the slope smooth and stable first.
Place larger stones at the toe of the slope, often keyed into a trench.
Mix sizes so smaller stones fill gaps between larger ones.
Avoid dumping from height on fabric, which can tear it.
Check permits: work in streams, wetlands, or shorelines often requires local, state, or federal approval.

Bottom line

Riprap sizes and weight range from light, fist-to-head-sized stone to heavy boulders, with most riprap weighing about 1.4 to 1.6 tons per cubic yard placed. Choose the class for the water force, place it about 1.5 to 2 times the median stone size thick over fabric or bedding, and check permits for work near water.

Questions people ask

How much does a yard of riprap weigh?

Roughly 1.4 to 1.6 tons per cubic yard as placed, depending on rock type and size.

What size riprap do I need?

It depends on water velocity and slope. Light riprap may be 4 to 12 inches; heavier classes run 12 to 24 inches or more.

How thick should riprap be placed?

Usually about 1.5 to 2 times the median stone size, or at least as thick as the largest stones.

Do you need fabric under riprap?

Usually. Geotextile or bedding stone keeps soil from washing out between rocks.

Do I need a permit to place riprap?

Often, if the work is in or near streams, wetlands, or shorelines. Check local and state rules.

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