Easy Garden Calc

How many blocks for a 20 ft retaining wall, 2 ft high?

Short answer

A 20 ft retaining wall 2 ft high in 12 × 4 in blocks needs 168 blocks: 8 courses (6 above ground plus 2 buried) of 20 blocks each is 160, plus 5% for breakage. Add 21 caps on top and about 1.84 cubic yards of drainage stone behind it.

How to work it out

  1. 20 ft ÷ 12 in = 20 blocks per course
  2. (24 in − 3 in cap) ÷ 4 in = 6 courses above grade
  3. Buried: max(6 in, 10% of 32 in) ÷ 4 in = 2 courses
  4. 8 courses × 20 + 5% = 168 blocks
  5. Caps: 20 ft ÷ 12 in + 5% = 21 caps; adhesive 20 ft ÷ about 20.7 ft per tube = 1 tube
  6. Above ground: 6 × 4 in + 3 in cap = 27 in
  7. Built height: 8 × 4 in + 3 in cap = 35 in
  8. Trench: 6 in pad + 2 × 4 in buried = 14 in deep, 24 in wide
  9. Pad: 20 ft × 24 in × 6 in = 0.74 cu yd compacted
  10. 0.74 cu yd compacted × 1.25 = 0.93 cu yd loose × 1.5 tons/cu yd = 1.39 tons
  11. + 5% waste = 1.46 tons
  12. Effective rate: ≈ 1.97 tons per cu yd compacted
  13. Drainage stone: 20 ft × 12 in × (35 in − 8 in soil cap) = 1.67 cu yd in place
  14. 1.67 cu yd in place × 1.05 = 1.75 cu yd loose × 1.4 tons/cu yd = 2.45 tons
  15. + 5% waste = 2.57 tons
  16. Effective rate: ≈ 1.54 tons per cu yd in place
  17. Filter fabric: 20 ft × (27 in up the back + 12 in over the top) = 65 sq ft
  18. Pipe: 20 ft + 1 × 10 ft to daylight = 30 ft

A two-foot wall is a typical garden terrace or the retaining edge of a raised planting bed. Small 12 × 4 in blocks are easy to carry and set by hand, and at this height the job is well within reach of a weekend or two. What keeps it straight after a few winters is not the blocks, it is the gravel pad under them and the drainage stone behind.

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
$
Blank to skip
Wall blocks168 blocks
Courses (incl. buried)
8 courses
Buried courses
2 courses
Blocks per course
20 blocks
Cap stones
21 caps
Cap adhesive (approx.)
1 tube · 10 oz tube
Height above ground, as built
27 in
Built height (bottom block to top of cap)
35 in
Dig the trench about
14 in
Drainage stone behind the wall (to order)
1.84 cu yd (49.6 cu ft)
Drainage stone (#57, approx.)
2.57 tons (5,145 lb)
Filter fabric behind the stone (if your plan calls for it)
65 sq ft (7.22 sq yd)
Leveling pad (crusher run)
1.46 tons (2,917 lb)
Wall face area
40 sq ft
Perforated drain pipe
30 ft
  • Blocks come in whole courses, so this wall shows 27 in above the ground, not the 24 in you entered. Options: one course fewer shows 23 in; or set the whole wall 3 in lower (dig the trench that much deeper) so 24 in shows; or pick a thinner cap or a different block height.
  • 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.
  • 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 adhesive21 cap stones need gluing down — about 1 tube of landscape-block adhesive in a caulk gun; the tube’s label says how far it goes.
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2 ft high walls in 12 × 4 in blocks, by wall length

2 ft high walls in 12 × 4 in blocks, by wall length
Wall lengthWall blocksCap stonesDrainage stone behind the wall (to order)
10 ft84 blocks11 caps0.92 cu yd
15 ft126 blocks16 caps1.38 cu yd
20 ft168 blocks21 caps1.84 cu yd
25 ft210 blocks27 caps2.3 cu yd
30 ft252 blocks32 caps2.76 cu yd

What to know about a 20 ft retaining wall, 2 ft high

  • Two buried courses, not one. With 4 in tall blocks, a single buried course puts only 4 in of block below grade. Belgard’s and Allan Block’s guides both ask for at least 6 in, which with these blocks means two buried courses: 168 blocks instead of the 147 you would get with one. The trench is then 14 in deep — 6 in of compacted pad plus 8 in of buried block — and 24 in wide for a 12 in deep block.
  • The cap is part of the 2 ft. Measured from the ground in front to the top of a 3 in cap, 24 in leaves 21 in of block above grade, which rounds up to six 4 in courses. The top ends up about 27 in above grade; if you want exactly 2 ft, bury the bottom course a little deeper.
  • Height to the code. Measured the way the model code measures it (IRC R105.2), from the bottom of the 6 in pad to the top of eight 4 in courses and the cap, this wall is 41 in. That is under the 4 ft line, so in many places no permit is needed as long as nothing heavy sits above it: no driveway, shed or steep slope.
  • Stone in bulk or bags. The 12 in drainage zone, stopping 8 in below the top for a soil cap, takes about 1.84 cubic yards of clean 3/4 in stone to order (roughly 2.6 tons), and the leveling pad about 1.5 tons of crusher run. That is past the point where bags make sense; a small bulk delivery is easier.
  • Caps and adhesive. 21 caps glued with landscape-block adhesive finish the top. Allan Block’s rule of one 29 oz tube per 60 ft works out to one 10 oz tube for 20 ft; buy a spare.

A wall like this often goes along the uphill side of a patio: see How many pavers for a 12×16 patio. For a longer, taller wall that crosses the permit line, compare How many blocks for a 40 ft retaining wall, 3 ft high.

Different size? Use the full Retaining Wall Block Calculator.

What you’ll need

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Frequently asked questions

How many blocks for a 20 ft retaining wall 3 ft high?

231 blocks in 12 × 4 in blocks: 11 courses (9 above ground, 2 buried) of 20 each, plus 5%. At 3 ft, check the block maker’s height limit and your local permit rules.

How many 16 × 6 in blocks for a 20 ft wall, 2 ft high?

79 blocks: 5 courses (4 above ground, 1 buried) of 15 blocks, plus 5%. Bigger blocks mean far fewer to set, but each one is much heavier.

How many blocks for a 20 ft retaining wall with an L-shaped corner?

168 blocks plus 8 corner blocks for the 90° corner, one in each of the 8 courses. If both legs add up to 20 ft, the straight-block count stays the same.

Other common sizes

Sources