Updated · Built and checked by the PlinzoTools team, formulas tested against published sources
Calculate retaining wall blocks and drainage backfill gravel from your wall's dimensions.
4 course(s) x 15 blocks per course.
This calculator sizes a segmental retaining wall (built from individual concrete retaining-wall blocks, not poured concrete) -- computing block count from courses and blocks-per-course, plus a drainage backfill gravel volume behind the wall, which is a genuinely important requirement specific to retaining walls: without it, water pressure builds up behind the wall and can cause it to fail.
Enter your wall's length and height. In "Advanced," adjust the block's length and height if your specific product differs from the common 16in x 6in default, and set your backfill gravel depth (12 inches behind the wall is standard practice).
Blocks per Course = ROUNDUP((Wall Length in ft × 12) ÷ Block Length in)
Courses = ROUNDUP(Wall Height in ÷ Block Height in)
Backfill Gravel (cu yd) = (Wall Length × Wall Height × Backfill Depth) ÷ 27
Retaining walls above about 3-4 feet, or walls holding back significant slope or water pressure, often need engineered design (geogrid reinforcement, proper drainage pipe, and a compacted base) beyond what a simple block count and backfill volume capture -- consult a structural engineer or experienced contractor for taller or load-bearing walls.
The tool estimates two things: face blocks and drainage backfill. It counts blocks as courses multiplied by blocks per course, then adds waste. It does not count cap blocks, the buried base course, geogrid, drainage pipe, filter fabric, or the compacted base material. Those are all part of a proper retaining wall and should be added to your materials list separately.
Many segmental wall systems recommend burying part of the first course so the wall is anchored in the ground. Entering only the visible height under-counts the blocks. A common approach is to add the buried depth to your wall height before calculating, but the amount to bury depends on the wall height and the system, so follow the block manufacturer's installation manual.
Suppose a hypothetical 30-foot wall stands 3 feet (36 inches) tall above grade, and you plan to bury one 6-inch course, so you enter a height of 42 inches. Blocks measure 16 by 6 inches. Blocks per course: ROUNDUP(360 / 16) = 23. Courses: 42 / 6 = 7. Total 161 blocks, and with 10 percent waste about 178. Backfill gravel at 12 inches deep on a 3.5-foot wall is 30 x 3.5 x 1 / 27, about 3.9 cubic yards.
Total Blocks = Blocks per Course × Courses × (1 + Waste)
Backfill (cu yd) = (Length × Height × Depth in ft) ÷ 27
The top of the wall is usually finished with cap blocks, which are separate from the face blocks. Count the cap by dividing the wall length by the cap's length. Walls with corners or curves need extra cuts and may need more waste than a straight wall. Increase the waste percentage if your design is complicated.
Gravel backfill is only part of drainage. Many designs also use a perforated drain pipe at the base that carries water to a safe discharge point, and fabric between soil and gravel to keep fines from clogging the drainage layer. Water is the main enemy of a retaining wall, so give drainage as much attention as the blocks.
An engineer can specify reinforcement and drainage, and your local building department can tell you whether a permit is required.
Backfill gravel lets water drain through rather than building up hydrostatic pressure behind the wall, which is one of the most common causes of retaining wall failure over time.
Many jurisdictions require an engineered design above roughly 3-4 feet, though this varies by local building code -- always check your specific local requirements before building a taller wall.
It depends on your location and the wall's height. Contact your local building department to find out the rules that apply to you.
Follow the block manufacturer's instructions, which vary with wall height and design.
No. Base gravel sits under the first course and is compacted. Drainage backfill is placed behind the wall and is calculated separately here.
Many segmental blocks rely on gravity and interlock, while caps are often glued. Follow the specific product's guidelines.