Blocks, caps, base stone and drainage gravel in one pass — including the buried course almost every online estimate forgets.
Exposed height is what you see above finished grade. The buried course is added below.
Measure a sample block rather than trusting the catalogue name — nominal sizes often differ from actual ones.
%
Bury one course for walls up to about 4 ft, and roughly one course for every 8 in of wall above that. It is what stops the base sliding forward.
Base & drainage
A 12 in column of clean angular stone behind the blocks, full height, with a perforated pipe at the bottom. Set to 0 to leave it out.
Price (optional)
Wall blocks needed
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Blocks per course—
Courses (incl. buried)—
Cap units—
Total wall height—
Wall face area—
Base stone—
Drainage gravel—
Estimated material cost—
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The calculation
Blocks are counted as a grid: blocks per course × number of courses. A 20 ft wall built from 12 inch blocks needs 240 ÷ 12 = 20 per course. For 3 ft of exposed height in 8 inch blocks that is 36 ÷ 8 = 4.5, rounded up to 5 courses, plus one buried course — 6 courses, 120 blocks before waste.
The buried course is not optional
The first course sits below finished grade, on compacted stone, and it is what stops the wall walking forward under load. The usual rule is one buried course for walls up to about 4 ft, then roughly an extra inch of burial for every additional foot of height. Estimates that ignore it come up a full course short and, worse, suggest the wall can just sit on the surface.
Base and drainage
Element
Typical spec
Base trench width
Twice the block depth, minimum
Base stone depth
6 in compacted, more on soft ground
Drainage column
12 in of clean angular stone, full height
Drain pipe
Perforated, at the base, daylighted to an outlet
Water is what destroys retaining walls. Saturated soil behind a wall can double the pressure on it, so the drainage stone and the pipe matter more than the blocks do.
A dry-stacked segmental retaining wall is one of the most forgiving structures a homeowner can build, right up until it is too tall — at which point it becomes an engineering problem with real consequences. Knowing which side of that line you are on is the first thing to establish, before counting a single block.
The four-foot rule
Most jurisdictions treat 4 ft of exposed height as the threshold above which a wall needs an engineered design, and often a permit and inspection. That is not bureaucratic caution: the lateral pressure on a wall rises with the square of its height, so a 6 ft wall is carrying far more than one and a half times the load of a 4 ft one. Above that line you will typically need geogrid tied back into the retained soil every few courses.
Terracing instead of going tall
Two 3 ft walls set apart with a planted bench between them will usually be simpler, cheaper and better looking than a single 6 ft wall — provided they are separated by at least twice the height of the lower wall. Any closer and the upper wall surcharges the lower one, and you are back to an engineered design.
Order the base stone with the blocks
The base trench takes more stone than people expect, and running out mid-build means an uncompacted joint you will never be able to fix. Add the base and drainage gravel to the same order, and buy the geotextile fabric at the same time if the retained soil is fine or silty.
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Frequently asked questions
How many blocks do I need for a retaining wall?
Divide the wall length in inches by the block length to get blocks per course, then divide the wall height by the block height and round up for the number of courses — remembering to add the buried course. A 20 ft long, 3 ft high wall in 12 × 8 in blocks needs about 126 blocks with 5% waste.
How deep should a retaining wall base be?
Six inches of compacted crushed stone in a trench about twice the block depth wide, with the entire first course buried below finished grade. Soft or clay ground may need 8 to 12 inches of base.
Do I need drainage behind a retaining wall?
Yes for anything retaining real soil. A 12 inch column of clean angular stone running the full height, with a perforated pipe at the base draining to an outlet, is the standard detail. Water pressure behind a wall is the most common cause of failure.
How high can I build a retaining wall without an engineer?
Commonly 4 feet of exposed height, though some jurisdictions set the limit at 3 feet. Above the threshold you will normally need an engineered design with geogrid reinforcement and a permit — check your local building department before ordering.
How many courses should be buried?
One full course for walls up to about 4 feet. As a general rule bury roughly one inch of wall for every foot of height, rounded up to a whole course, and always set that course on compacted base stone rather than soil.