Get blocks, caps, base and drainage gravel, and a cost range for a block retaining wall.
Quick answer: A standard 12 in by 4 in block covers about one third of a square foot of wall face, so a 20 ft wall 3 ft high, or 60 sq ft, needs roughly 180 blocks. Add one cap per block along the top course, base gravel 4 to 6 in deep and drainage gravel behind the wall.
ft
ft
Measure from the base course, not from the soil in front of it.
%
5% covers cuts at corners and steps.
$
$
$
Walls taller than about 3–4 ft, walls carrying a surcharge (a driveway or slope above them) and tiered walls usually need an engineered design, a permit and geogrid reinforcement. The maths here sizes material only.
Blocks needed
284
90 sq ft of wall face
Estimated materials
$1,650
30 caps · 4.26 cu yd gravel
Materials & earthwork
Blocks (12 × 4 × 8 in)284
Caps30
Base gravel (6 in compacted)0.93 cu yd
Drainage gravel behind wall3.33 cu yd
Gravel weightabout 6.0 tons
Estimated materials$1,650
Do not forget
Perforated drain pipe30 linear ft, plus outlets
Filter fabric108 sq ft to keep soil out of the gravel
Construction adhesive2 tubes for caps and corners
Compactionplate compactor for base and each course of backfill
Block counts come from the block face area, and the gravel figures assume a 6 in compacted base plus a 12 in drainage column behind the wall. Drainage is not optional — most retaining wall failures are water, not weight.
Core facts
Blocks per sq ft
About 3 for a 12 by 4 in face
Base gravel
Typically 4–6 in deep
Drainage
About 12 in of gravel behind
Permits
Often required over 4 ft tall
What is Retaining Wall Calculator?
A segmental block retaining wall is counted in face area. The block face — commonly 12 in wide by 4 in high — covers 48 square inches, and there are 144 square inches in a square foot, so you need about three blocks per square foot of wall. Multiply the wall length by the wall height to get that area, then add waste for cuts at the ends and on the top course. On top you need one cap per block along the length, a compacted gravel base typically 4 to 6 in deep and wider than the block, and drainage gravel backfilled behind the wall with a perforated pipe at the base. Taller walls need geogrid, engineering and a permit.
Common uses for Retaining Wall Calculator
Count blocks for a garden or terrace wall before ordering a pallet
Work out base and drainage gravel in cubic yards
Compare a 3 ft and a 4 ft wall for cost and permit needs
Check a contractor's block count against your own wall measurement
Plan the drainage stone and pipe behind a wall on a slope
Why the base and first course decide the wall
A block wall holds because of its mass and its base, not because the blocks are glued together. The trench is dug below grade, lined with compacted gravel, and the first course is leveled in both directions so every course above inherits that level. A base that is too shallow or not compacted shows up later as a leaning wall, and fixing it means rebuilding the wall, not just the base.
Drainage, the part everyone skips
Saturated soil pushes with far more force than dry soil, and that pressure is what topples most failed walls. The standard fix is a gravel zone behind the wall, landscape fabric to keep fines out of it, and a perforated drain pipe at the base that carries water away. If there is nowhere downhill for the pipe to discharge, plan a dry well or a French drain before you build.
Height, geogrid and permits
Walls up to about 3 ft often work as a simple gravity wall. As height increases, the design usually calls for geogrid layers extending back into the retained soil, and taller walls may need stamped engineering — many jurisdictions set that threshold at 4 ft measured from the bottom of the footing, and some go lower. Surcharge from a driveway, a slope above the wall or a fence post changes the calculation, so check the rules first.
Frequently Asked Questions
How many blocks do I need per square foot?
Divide 144 by the face area of one block in square inches. A block with a 12 in by 4 in face is 48 square inches, so 144 divided by 48 gives three blocks per square foot. Wider or taller blocks cover more, which is why the calculator asks for your block dimensions instead of assuming a size.
How deep should the base be?
Four to six inches of compacted gravel is the usual base course for a low block wall, placed in a trench wider than the block and dug below the local frost depth where that matters. The first course is often set partly below grade so the wall cannot slide. Check your local frost depth before you dig.
Do I need drainage behind the wall?
Yes. Water is the main reason retaining walls fail. A typical detail is about 12 in of clean gravel behind the blocks for the full height, filter fabric between the gravel and the soil, and a perforated pipe at the base draining to daylight or a dry well. The calculator estimates gravel from a 12 in wide zone.
When does a retaining wall need a permit?
Many U.S. jurisdictions require a permit for a retaining wall over 4 ft tall, measured from the bottom of the footing to the top of the wall, and some set the threshold lower or add rules for walls near a slope or property line. Walls above that height often need stamped engineering. Confirm with your local building department.
What does a block retaining wall cost?
Published U.S. averages for a professionally installed segmental block wall commonly land in the $20 to $60 per square foot range, with materials alone for a DIY build typically $8 to $25 per square foot. Cost moves with height, access, drainage requirements and whether geogrid or engineering is needed. Get local quotes.