Easy Garden Calc

How many blocks for a 40 ft retaining wall, 3 ft high?

Short answer

A 40 ft retaining wall 3 ft high in 16 × 6 in blocks needs about 221 blocks: 7 courses (6 above ground plus 1 buried) of 30 blocks each is 210, plus 5%. You also need 32 caps and about 5.04 cubic yards of drainage stone.

How to work it out

  1. 40 ft ÷ 16 in = 30 blocks per course
  2. (36 in − 3 in cap) ÷ 6 in = 6 courses above grade
  3. Buried: max(6 in, 10% of 42 in) ÷ 6 in = 1 course
  4. 7 courses × 30 + 5% = 221 blocks
  5. Caps: 40 ft ÷ 16 in + 5% = 32 caps; adhesive 40 ft ÷ about 20.7 ft per tube = 2 tubes
  6. Above ground: 6 × 6 in + 3 in cap = 39 in
  7. Built height: 7 × 6 in + 3 in cap = 45 in
  8. Trench: 6 in pad + 1 × 6 in buried = 12 in deep, 24 in wide
  9. Pad: 40 ft × 24 in × 6 in = 1.48 cu yd compacted
  10. 1.48 cu yd compacted × 1.25 = 1.85 cu yd loose × 1.5 tons/cu yd = 2.78 tons
  11. + 5% waste = 2.92 tons
  12. Effective rate: ≈ 1.97 tons per cu yd compacted
  13. Drainage stone: 40 ft × 12 in × (45 in − 8 in soil cap) = 4.57 cu yd in place
  14. 4.57 cu yd in place × 1.05 = 4.8 cu yd loose × 1.4 tons/cu yd = 6.71 tons
  15. + 5% waste = 7.05 tons
  16. Effective rate: ≈ 1.54 tons per cu yd in place
  17. Filter fabric: 40 ft × (37 in up the back + 12 in over the top) = 163 sq ft
  18. Pipe: 40 ft + 1 × 10 ft to daylight = 50 ft
  19. Geogrid (rough): 3 layers × 40 ft × 2.5 ft (60% of 45 in, rounded up) = 300 sq ft

Three feet of exposed wall sounds modest, but it is the size where the rules change. Many block makers’ gravity-wall limits, most permit thresholds and the point where geogrid starts to appear on the plans all sit around here. Read the numbers below before you order two pallets of block.

Adjust for your project

ft
Along the face. 20 ft 6 in = 20.5
in
What shows above the ground in front, in inches. 2 ft = 24
Face size, length × height, from the pallet label
in
Thickness of one cap stone — often 3–4 in
in
More options: Buried · Block depth · Corners …
Blank = auto: enough courses to bury at least 6 in of block, or 10% of the wall’s total height if that’s more
in
Sets the pad width (block depth + 6 in in front and behind). Small garden blocks are often shallower — check the label
Each outside corner takes a corner block in every course
%
Allan Block suggests ordering about 5% extra
in
Width of clean stone behind the blocks — Belgard and Allan Block both say 12 in
in
The drainage stone stops this far below the top; soil over fabric fills the rest (Allan Block: 8 in)
Compactable crushed stone. Your supplier’s weight per yard is better than ours
%
Extra loose gravel to reach the compacted pad depth. Blank = typical: crusher run 25%, #57 stone 5%, your own material 25%
%
For the pad and drainage stone · 5% is typical
Where the drain pipe leaves the wall to run downhill
ft
From the wall to where it can drain. 10 ft is a starting guess — measure yours
Hollow blocks get their cores filled with the same stone as behind the wall
in
Rough estimate only — Allan Block’s manual puts layers on about 16 in centers. Your block maker’s chart or engineer sets the real layout
ft
From your supplier. Grid is cut across the roll and laid with the roll direction running back from the wall
ft
Allan Block notes rolls come up to about 13 ft wide and 250 ft long, in different strengths — use the grid your chart or engineer names
$
Blank to skip
Wall blocks221 blocks
Courses (incl. buried)
7 courses
Buried courses
1 course
Blocks per course
30 blocks
Cap stones
32 caps
Cap adhesive (approx.)
2 tubes · 10 oz tube
Height above ground, as built
39 in
Built height (bottom block to top of cap)
45 in
Dig the trench about
12 in
Drainage stone behind the wall (to order)
5.04 cu yd (136 cu ft)
Drainage stone (#57, approx.)
7.05 tons (14,101 lb)
Filter fabric behind the stone (if your plan calls for it)
163 sq ft (18.1 sq yd)
Leveling pad (crusher run)
2.92 tons (5,833 lb)
Wall face area
120 sq ft
Perforated drain pipe
50 ft
Geogrid
Likely needed above about 3 ft. Follow your block maker’s chart or an engineer’s design
Geogrid layers (rough estimate)
3 layers
Geogrid (rough estimate)
300 sq ft (33.3 sq yd)
  • Blocks come in whole courses, so this wall shows 39 in above the ground, not the 36 in you entered. Options: one course fewer shows 33 in; or set the whole wall 3 in lower (dig the trench that much deeper) so 36 in shows; or pick a thinner cap or a different block height.
  • Geogrid: at 39 in showing, many blocks need geogrid layered into the backfill. Belgard recommends it above a block’s gravity height limit, with a qualified engineer’s design; Allan Block’s manual says grid typically runs back at least 60% of the total wall height, in layers about 16 in apart. Rough estimate only: 3 layers (first course, then every 16 in), each 2.5 ft back. Follow your block maker’s chart for your block and soil; past its gravity limit, or anywhere a permit is needed, follow an engineer’s plan instead of this number. Enter the roll size under More options to count rolls.
  • Filter fabric: Belgard’s typical section puts filter fabric between the drainage stone and the soil behind it. Allan Block’s spec doesn’t suggest it with clay (cohesive) soils, where it can clog — follow your block maker’s guide. Add the seam overlaps the fabric maker asks for.
  • Measured from the bottom of the leveling pad to the top of the cap, this wall is about 4.3 ft tall. The model residential code (IRC R105.2) only exempts walls up to 4 ft measured that way — and none that hold back a slope, driveway or other surcharge — so expect to need a permit.
  • Lay perforated drain pipe at the back of the pad inside the drainage stone and run it out to daylight at the low end — water pressure behind the wall is what pushes walls over.
  • Cap adhesive is approximate: Allan Block’s worksheet allows one 29 oz tube per 60 ft of wall, and we scale that to your tube size. Glue the caps to the top course and to each other at the ends.
Check your codeWalls over 4 ft — measured from the bottom of the footing or leveling pad to the top of the wall — or any wall holding back a slope, driveway or other load generally need a permit (IRC R105.2) and often an engineered design. Check your local code and the block maker’s height limit before you build.

For this job · affiliate link
Landscape block adhesive32 cap stones need gluing down — about 2 tubes of landscape-block adhesive in a caulk gun; the tube’s label says how far it goes.
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40 ft × 3 ft walls, by block size

40 ft × 3 ft walls, by block size
Block faceWall blocksCourses (incl. buried)Drainage stone behind the wall (to order)
12 × 4 in462 blocks11 courses5.31 cu yd
16 × 6 in221 blocks7 courses5.04 cu yd
18 × 6 in199 blocks7 courses5.04 cu yd
18 × 8 in171 blocks6 courses5.85 cu yd

What to know about a 40 ft retaining wall, 3 ft high

  • It shows 39 in, not 36. Blocks come in whole courses: 33 in of block above the ground (under the 3 in cap) needs six 6 in courses, so the cap ends up 39 in above the lawn. Drop a course for 33 in, or set the wall 3 in lower.
  • It is over 4 ft to the code. The model residential code (IRC R105.2) measures from the bottom of the footing, not from the lawn. Seven 6 in courses are 42 in, the 3 in cap makes 45 in, and a 6 in leveling pad takes it to 51 in. Expect a permit, and if there is a slope, driveway or patio load above the wall, expect one regardless of height.
  • One buried course is enough here. A 6 in block buried one course deep meets the 6 in minimum, and 10% of the 42 in of block is only about 4 in. The trench is 12 in deep: 6 in of pad and one 6 in course.
  • Geogrid and an engineer. Belgard’s guide recommends geogrid (plastic mesh layered into the backfill) for walls beyond the block’s gravity height limit, designed by a qualified engineer. Look up the limit for your exact block before assuming this wall can be built without it. At 39 in showing, the calculator adds a rough geogrid estimate (three layers, 2.5 ft back), but the chart or the engineer decides.
  • Drainage is a truckload. About 5.04 cubic yards of clean stone behind the wall weighs about 7 tons, and the leveling pad takes another 2.9 tons of crusher run. Allan Block requires a drain pipe on gravity walls over 4 ft; lay 40 ft of perforated pipe at the back of the pad and bring it out to daylight at the low end — 50 ft in all with one 10 ft outlet run.
  • Handling. 16 × 6 in blocks are heavy, and 221 of them plus 32 caps usually arrive as more than one pallet. Plan where the pallets land so you are not wheeling each block the full 40 ft.

Belgard’s typical section puts filter fabric between the drainage stone and the soil behind it, which keeps soil from clogging the stone (Allan Block doesn’t suggest fabric with clay soils); the landscape fabric calculator sizes the rolls. For a lower wall well under the permit line, see How many blocks for a 20 ft retaining wall, 2 ft high.

Different size? Use the full Retaining Wall Block Calculator.

What you’ll need

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Frequently asked questions

How many 12 × 4 in blocks for a 40 ft wall, 3 ft high?

462 blocks: 11 courses (9 above ground, 2 buried) of 40 blocks, plus 5%, and 42 caps. Small blocks more than double the count and are usually rated for lower walls, so check the maker’s height chart.

How many blocks for a 40 ft retaining wall, 2 ft high?

158 blocks in 16 × 6 in blocks: 5 courses of 30 blocks, plus 5%. With the pad and cap it stays under the 4 ft code line, at 33 in of block and cap.

How much drainage stone for a 40 ft wall with an 18 in gravel zone?

About 7.55 cubic yards (10.6 tons) of clean stone for a 3 ft wall on one buried course, stopping 8 in below the top for a soil cap.

Other common sizes

Sources