Rip Rap Calculator (stone tonnage & layer thickness for slopes and channels)

Understanding the riprap calculator — concept and principle

This calculator estimates the volume and weight of riprap (loose rock armour) needed to protect a slope, shoreline or channel from erosion, plus the filter fabric or gravel filter beneath it. Landowners and contractors use it to order stone.

Thick enough for the stones: with the default inputs the result is 30.6 tonnes (at 1.7 t/m³).

How it is calculated

Area = length × width along the slope.
Layer thickness = your value, or by rule 1.5 × the largest stone and at least 15 cm (6 in).
Volume = area × thickness; tonnes = volume × in-place density.
Median stone size d50 ≈ largest stone ÷ 1.5.
Filter: geotextile area = area; gravel filter = area × 0.15 m.

The principle

Riprap protects soil by covering it with stones too heavy for the water to move. The layer must be thick enough that the largest stones are fully embedded and the smaller ones fill the gaps; a common rule is 1.5 times the largest stone and never thinner than 6 in. In a well-graded mix the largest stone is about 1.5 times the median size. A filter beneath the stone stops fine soil being washed out through the gaps, which would otherwise undermine the layer. The in-place density, supplied by the quarry, already allows for voids between stones.

Worked example

Default inputs: area 10 × 4 m, largest stone 30 cm, thickness by rule, density 1.7 t/m³, geotextile.

Thickness by rule = max(1.5 × 30, 15.24) = 45 cm.
Area = 10 × 4 = 40 m²; volume = 40 × 0.45 = 18 m³.
Weight = 18 × 1.7 = 30.6 tonnes.
Median stone d50 = 30 ÷ 1.5 = 20 cm.
Geotextile = 40 m²; gravel filter not selected (—).

Each point is the calculator re-run with only “Length of the area (m)” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Length of the area (m)	Riprap (tonnes)
2.5	7.6
5	15.3
7.5	22.9
10	30.6
12.5	38.3
15	45.9
20	61.2

How to read the result

Order a little extra for uneven ground and toe trenches at the base of the slope. The stone size itself must come from a hydraulic design for the flow, waves or ice on your site; this calculator only works out quantities for a size you choose.

Common mistakes

Using rock that is too small for the water forces.
Leaving out the filter layer.
Placing a layer thinner than 1.5 × the largest stone.
Using bulk density of loose stone in a truck instead of in-place density.

Keep in mind: Work in or near watercourses often needs permits. Erosion protection must be designed for local flow conditions by a qualified professional.

Frequently asked questions

How thick should riprap be?

At least 1.5 times the largest stone and not less than 6 in (15 cm), under the North Carolina practice standard.

How many tonnes of riprap for 40 m² at 45 cm?

About 30.6 tonnes at 1.7 t/m³.

Do I need filter fabric under riprap?

A filter (geotextile or gravel) is needed to stop soil washing out under the stone.

Sources: North Carolina Erosion and Sediment Control Planning and Design Manual, Practice Standard 6.15 "Riprap"

Checked 9 Nov 2026. North Carolina Erosion and Sediment Control Planning and Design Manual, Practice Standards and Specifications 6.15 “Riprap” (rev. 2025 draft): thickness at least 1.5 × the maximum stone diameter and not less than 6 in; largest stone about 1.5 × d50; each filter layer at least 6 in thick.

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