Why does the quarry's tonnage not match my cubic metres?

Because a rock blanket is mostly stone and partly air, and the two trades measure different things. This figure is the volume the blanket occupies on the slope — stone plus the voids between the stones — while the quarry weighs what leaves the yard. Converting between them needs the PLACED density of that gradation, which is well below the density of the rock itself and varies with how well graded the material is: a single-size stone leaves more void than a graded one that lets small pieces bed into the gaps. Ask the supplier for the placed density of the specific gradation rather than assuming a rock figure, and remember the layer thickness itself is normally specified as a multiple of the median stone size, so a coarser gradation makes a thicker blanket carrying the same protection.

How is riprap volume calculated?

It's the slope surface area multiplied by the average stone layer depth — the standard method for estimating bulk riprap stone quantity for erosion protection.

What stone layer depth should I use?

A commonly used average stone layer depth for D50-sized riprap is about 0.45 m, but check your project's design depth requirement rather than assuming this default.

Is the slope area the horizontal footprint or the sloped face?

It's the slope surface area — measured along the sloped face that will actually be covered with riprap, not its horizontal projection.

Heavy Civil & Infrastructure

Riprap Slope Protection Volume Calculator

Calculate the stone volume needed for a riprap slope protection layer.

Answers as you typeNo submit button
Every formula citedNamed published standards
Calculated in your browserNo account, nothing uploaded

SettingsSettings for this calculationUS

Market

Imperial · sales tax

Riprap Slope Area

The slope surface area to be covered with riprap.

Measure the area along the sloped face itself, not the horizontal footprint beneath it.

Average Stone Layer Depth

The average thickness of the placed riprap stone layer.

A commonly used average layer depth for D50-sized riprap is about 0.45 m — check your project's design depth rather than assuming this default.

Riprap volume needed

47.78 yd³

High confidence

Then change the inputs to see how far the answer moves.

Show calculation logic

How this was calculated

Formula source(s)

Riprap volume = slope surface area × average stone layer depth, the standard method for estimating bulk riprap stone quantity for erosion protection

Inputs used

Riprap Slope Area: 860 sq ft
Average Stone Layer Depth: 18 in

Final result47.78 yd³

What this calculation does not cover

This is a quantity take-off, not a riprap design. It does not size the stone or set the blanket thickness, and it makes no check against flow velocity, wave exposure, bank height or slope angle. Both numbers you type have to come from a design or a project specification.
Toe keys, launch aprons and crest anchor trenches are not included. Those sit outside the slope face you measured, and on a bank or channel job the toe trench is often the largest single extra over the blanket itself.
The answer is the stone blanket alone. Bedding, a granular filter course or geotextile underlayment called for beneath the riprap is separate material and separate volume.
No allowance is made for stone lost into a soft or over-excavated subgrade, or for the thickness overrun that comes from placing rock on a rough graded slope. Placed riprap generally takes more stone than the nominal blanket.
This is volume, not weight. Converting to tonnes or tons needs the placed density of the specific gradation from your supplier, not a solid-rock density, and the difference is large enough to change a delivery count.

Estimated cost — your price

Price per yd³ ($)

This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.

Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.

Sources checked 2026-09-06 · in the site-wide review of 2026-09-06 · v1.0.1

Regulatory standards & verification citations1

Riprap volume = slope surface area × average stone layer depth, the standard method for estimating bulk riprap stone quantity for erosion protection

Cite this page

Your workspace

Most jobs need more than one number. Add the calculators you need next and they open right here, underneath this one — your figures stay on screen and nothing is lost to a page change.

Now that you have the number

These guides cover the work this quantity is for — the first ones run this calculator inside the section that raises the question.

Building a Gabion Walluses this calculator
A gabion is a container, not a structure. The trade sits in mesh aperture, in the fill's placed bulk density, and in what the bottom course beds on.
Erosion and Sediment Controluses this calculator
Every erosion and sediment control on a jobsite does one of two things: it slows water down, or it catches the soil that water is already carrying.
Grading a Site for Drainageuses this calculator
Trace every drop from the divide above the pad to the permitted outlet, and size the grade at each transition it crosses.

Still deciding? Riprap vs Erosion Control Blanket — the factors that actually differ, with no invented prices.

How to calculate riprap slope protection volume in 3 steps

Riprap Slope AreaThe slope surface area to be covered with riprap.
Average Stone Layer DepthThe average thickness of the placed riprap stone layer.
Riprap volume neededThe tool computes the riprap volume needed from those figures and shows the formula, its sources, and a confidence rating alongside it.

Riprap volume needed by riprap slope area

Page defaults, not your figures above.

Riprap Slope Area	Riprap volume needed (yd³)
400 sq ft	21.9
600 sq ft	32.8
800 sq ft	43.7
1,000 sq ft	54.7
1,200 sq ft	65.6
1,400 sq ft	76.6
1,600 sq ft	87.5

Frequently asked questions

Why does the quarry's tonnage not match my cubic metres?: Because a rock blanket is mostly stone and partly air, and the two trades measure different things. This figure is the volume the blanket occupies on the slope — stone plus the voids between the stones — while the quarry weighs what leaves the yard. Converting between them needs the PLACED density of that gradation, which is well below the density of the rock itself and varies with how well graded the material is: a single-size stone leaves more void than a graded one that lets small pieces bed into the gaps. Ask the supplier for the placed density of the specific gradation rather than assuming a rock figure, and remember the layer thickness itself is normally specified as a multiple of the median stone size, so a coarser gradation makes a thicker blanket carrying the same protection.
How is riprap volume calculated?: It's the slope surface area multiplied by the average stone layer depth — the standard method for estimating bulk riprap stone quantity for erosion protection.
What stone layer depth should I use?: A commonly used average stone layer depth for D50-sized riprap is about 0.45 m, but check your project's design depth requirement rather than assuming this default.
Is the slope area the horizontal footprint or the sloped face?: It's the slope surface area — measured along the sloped face that will actually be covered with riprap, not its horizontal projection.

Preliminary estimate, not certified engineering. This tool produces an indicative quantity calculation for planning purposes only — it is not a certified structural analysis, a guaranteed material takeoff, or a substitute for building department approval. Always verify measurements on-site and have a licensed contractor or structural engineer review any load-bearing, code-sensitive, or safety-critical work before purchasing materials or starting construction. Spotted an arithmetic or standards error? Report it to contact@craftquantities.com with your inputs — a confirmed fix gets a permanent check of its own, so the same mistake cannot come back.