Audited 27 Jul 2026·Last updated 28 Oct 2026·6 citations·Tier 1·0 uses

Retaining Wall Block Calculator

Count retaining wall blocks and caps by course, buried rows and waste included, plus base gravel, drainage stone and drain pipe for a block wall.

Retaining Wall Block Calculator

Wall length

Exposed wall height

Block face length

Block height

Buried courses

Cap length

Waste allowance

Base trench width

Base gravel depth

Drainage aggregate width behind wall

Drainage pipe allowance

Wall blocks to order

171

Wall units to buy: the net block count increased by your waste allowance and rounded up to a whole block.

Exposed courses

Total courses including buried courses

Blocks per course

Wall blocks before waste

162

Cap blocks to order

Base gravel volume (cu ft)

Base gravel volume (cu yd)

1.3333

Base gravel to order (cu yd)

1.4

Drainage aggregate volume (cu ft)

108

Drainage aggregate volume (cu yd)

Drainage aggregate to order (cu yd)

4.2

Drain pipe length (ft)

37.8

Background.

A 30-inch garden wall looks like five rows of block. Build it with 6-inch units and you will lay six, because the first row sits below the finished grade. Before that row goes down you will have dug a trench, filled it with compacted stone, and planned for a foot-wide band of clean gravel rising behind every course. Most of what keeps a segmental wall standing never shows.

Segmental retaining walls are dry-stacked concrete units, normally laid without mortar on a compacted gravel pad rather than a poured footing. They stay put through their own weight, the small setback built into each course and free-draining stone behind. Water is what usually brings them down: backfill that stays saturated presses on the wall far harder than dry soil, which is why manufacturers' installation manuals spend as many pages on drainage stone and pipe as on stacking block.

The estimate follows the takeoff a block maker's worksheet uses: courses from height, blocks per course from length, caps along the top, and the stone as two volumes, a trench under the first course and a drainage zone behind the full stack of block.

For the 36-foot example wall that comes to 171 wall blocks, 38 caps, 1.33 cubic yards of base stone and 4 cubic yards of drainage aggregate. There is three times as much stone behind the wall as beneath it, and people who price only the block are regularly caught out by the gravel bill.

None of this tells you whether a wall will stand. Height, soil, slopes above or below, a driveway or patio near the top, and frost all change what is needed, and taller walls are usually built with layers of geogrid reinforcement designed for the site. Use the counts to price and order; take the construction details from the block manufacturer's installation guide, or from an engineer when the wall is tall or carries a load.

What is retaining wall block calculator?

A retaining wall material takeoff is the list of quantities needed to build a segmental block wall: wall units, cap units, base aggregate, drainage aggregate and drain pipe. A course is one horizontal row of block. Buried courses are the rows set below finished grade to anchor the toe of the wall; manufacturers call this embedment. The base, sometimes called a leveling pad, is a trench of compacted crushed stone that the first course is laid on. The drainage zone is the band of clean stone directly behind the units that lets water fall to the pipe instead of building up against the wall. Caps are the thinner finishing units glued along the top course. Every count here assumes a straight run of one block size laid at the face length you enter.

How to use this calculator.

Enter the wall length in feet, measured along the front face, and the exposed height in inches from finished grade at the base to the top of the last block. Leave the cap out of that height; caps sit on top and are counted separately.
Take the block face length and block height from the product sheet for the unit you will buy. Face length is the width you see from the front, not the depth into the hill.
Set buried courses from the manufacturer's embedment rule. Allan Block asks for at least 6 inches, or 1 inch per foot of wall height when that is more: one course of 6-inch block covers it on walls up to 6 feet, but 4-inch block needs two.
Enter the cap length as the distance along the wall each cap covers as laid, and a waste allowance. Allan Block's worksheet recommends 5 percent extra; add more for curves, corners, steps and cut ends.
Fill in the base trench width and gravel depth, the width of drainage stone behind the block, and a pipe allowance large enough to reach the point where the pipe will discharge.
Read the block and cap counts as order quantities, and use the two "to order" stone volumes, which already carry your waste allowance; ask the supplier how many tons each cubic yard of their stone weighs.

The formula.

N = Bpc × Tc × (1+w)

N · wall blocks to orderBpc · blocks per course: wall length in inches ÷ face length, rounded upTc · total courses: exposed rows (height ÷ block height, rounded up) plus buried rowsw · waste allowance, as a fraction (5% = 0.05)

Exposed courses = ⌈H ÷ h⌉, where H is the exposed height and h the block height, both in inches. Total courses Tc = exposed courses + buried courses. Blocks per course Bpc = ⌈12L ÷ f⌉, with L the wall length in feet and f the face length in inches. Wall blocks to order N = ⌈Bpc × Tc × (1 + w)⌉, where w is the waste allowance as a fraction. The rounding happens in three places — rows, blocks per row and the final order — and each one only ever pushes the count up.

Caps = ⌈(12L ÷ c) × (1 + w)⌉, with c the cap length; the caps are not rounded per row first, so a part cap is absorbed into the waste before the final round-up. Base gravel = L × (B ÷ 12) × (D ÷ 12) cubic feet, for a trench B inches wide and D inches deep. Drainage aggregate = L × (A ÷ 12) × (Tc × h ÷ 12) cubic feet, for a stone zone A inches wide: its height is the full stack of block including buried rows, but not the base below or the cap above. Both volumes are divided by 27 to give cubic yards, since a cubic yard is 3 × 3 × 3 feet, and each cubic-yard figure is also shown multiplied by (1 + w) as the quantity to order. Pipe = L × (1 + p), where p is the pipe allowance.

Three things are simplified. The drainage figure is a plain rectangle: it does not include stone poured into the hollow cores of units that have them, which Allan Block's manual fills with the same wall rock. Corners, curves and steps in the base are not modelled; they are what the waste allowance has to cover. And the waste percentage is applied to the stone as a plain multiplier, the same one used for blocks, which is only a rough stand-in for the extra loose material that compacted stone takes.

A worked example.

Example

Take a straight wall 36 feet long showing 30 inches of block above grade, built from units 16 inches long and 6 inches high with one buried course. The height gives 30 ÷ 6 = 5 exposed courses, and the buried row makes 6. The length is 36 × 12 = 432 inches, and 432 ÷ 16 = 27 blocks per course, a whole number, so nothing is lost to rounding. Six courses of 27 is 162 blocks; adding the 5 percent allowance gives 162 × 1.05 = 170.1, which rounds up to 171 wall blocks to order. The 12-inch caps need 432 ÷ 12 = 36 to cover the top, and 36 × 1.05 = 37.8 rounds up to 38. The base trench, 24 inches wide and 6 inches deep, holds 36 × 2 × 0.5 = 36 cubic feet, or 36 ÷ 27 = 1.33 cubic yards. The drainage zone is 12 inches wide and as tall as six 6-inch courses, which is 3 feet, so it holds 36 × 1 × 3 = 108 cubic feet, exactly 4 cubic yards. The pipe comes to 36 × 1.05 = 37.8 feet. The stone figures above are before waste. Applying the same 5 percent gives 1.4 cubic yards of base stone and 4.2 cubic yards of drainage stone to order, which the calculator shows as separate results.

block Height In6

base Gravel Depth In6

wall Length Ft36

block Face Length In16

cap Length In12

buried Courses1

waste Percent5

exposed Wall Height In30

drainage Aggregate Width In12

base Trench Width In24

drain Pipe Allowance Percent5

Frequently asked questions.

How many courses of retaining wall block go below ground?

Follow the embedment rule for your block system. Allan Block's estimating worksheet sets the buried depth at a minimum of 6 inches, or 1 inch for each foot of wall height when that is greater. With 6-inch-high units, one buried course meets that until the wall passes 6 feet. With 4-inch units, one course gives only 4 inches, so you need two: on the 36-foot example with 12-inch-long, 4-inch-high block, that takes the order from 341 to 378 units.

Why did two extra inches of height add a whole row of block?

Courses come in whole units, so the exposed height is rounded up to the next full row. With 6-inch block, 30 inches is exactly 5 rows, but 32 inches needs 6, and the example order jumps from 171 to 199 blocks. In practice you would let the wall show 36 inches, lower the grade at the base, or bury one more course instead of trying to cut a 2-inch row.

How tall can I build before I need a permit?

That is set locally. The model International Residential Code exempts retaining walls up to 4 feet, measured from the bottom of the footing to the top of the wall, unless the wall supports a surcharge such as a driveway or a slope. Note the measuring point: the example wall's six courses on a 6-inch base already stand 42 inches before the cap goes on. Ask your building department how they measure a gravel-based block wall.

Do I need the perforated drain pipe at all?

Allan Block's residential manual requires a drain pipe for any wall reinforced with geogrid, for unreinforced walls over 4 feet tall, and on any site with poor drainage. It goes at the lowest possible point toward the back of the base trench and is vented to daylight every 50 feet. The pipe length shown covers the run behind the wall; use the allowance for the extra pipe from the wall to its outlet.

What goes behind the drainage stone?

Beyond the stone zone, the hill is backfilled with compacted soil, on-site material if it is suitable. Allan Block's manual also has you finish behind the last course with organic soil rather than stone, so lawn or planting can run to the cap. If your system's detail caps the drainage zone that way, the full-height stone volume here runs slightly high for that top row.

How do I convert the cubic yards into tons for the stone yard?

Ask the supplier for the weight per cubic yard of the specific stone you are buying, because crushed rock varies with the rock type and size grading, then multiply. Order a little more than the volume shown: the plain base and drainage volumes are net, but the "to order" results add your waste percentage (1.4 and 4.2 cubic yards on the example), and stone that is compacted in lifts with a plate compactor settles into less space than it took up loose on the truck.

How do I measure a curved wall or one with corners and steps?

Measure along the front face at the base. On a curve the setback changes each course's length slightly, with upper rows getting shorter on an outside curve and longer on an inside one, so the base measurement plus a larger waste allowance covers it. Corners often need special corner units, and Allan Block's worksheet notes that step-ups and stairs need extra block; count those separately.

How many caps do I need, and how are they fixed?

Divide the wall length by the length each cap covers as laid, then add waste; the example's 36 feet of 12-inch caps comes to 38. Caps are bonded to the top course with a masonry adhesive rather than mortar. Allan Block's worksheet allows one 29-ounce tube for every 60 feet of capped wall, so the example wall would use part of a single tube.

When does a block wall need geogrid?

Geogrid is a polymer mesh laid between courses and run back into compacted soil so that the wall and a block of earth behind it act together. Whether you need it depends on the block, the height, the soil, any slope above or below and any load near the top, and each manufacturer publishes its own limits. Geogrid is not counted here; if your wall needs it, the layout and lengths should come from the manufacturer's chart or an engineer.

Does a smaller block make the wall cheaper?

Not always, because smaller units mean more of them. Switching the example from 16-by-6-inch block to 12-by-4-inch block lifts blocks per course from 27 to 36 and exposed courses from 5 to 8, since 30 inches is seven and a half 4-inch rows. Even with one buried course the order roughly doubles to 341 units, and you lay every one by hand.

References& sources.

How this page was produced

Published by: Quanta Calculator
Overseen by: Daniel, Quanta Calculator Researcher
Primary sources: 6 cited below
Method: N = Bpc × Tc × (1+w)
Published: 8 June 2026
Last verified: 27 July 2026

Built with AI assistance and verified by automated tests against the cited sources — every worked example on this page is computed by the same code that runs the calculator. How we build and check calculators.

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