Retaining Wall Block Calculator

A retaining wall holds back soil on a slope or elevation change. Calculating block quantity requires knowing the wall’s length, height, and the specific block type being used, since different blocks have different face dimensions and coverage per block.

Core formulas: Rows = Wall Height ÷ Block Height, rounded up Blocks per Row = Wall Length ÷ Block Length, rounded up Total Blocks = Rows × Blocks per Row Add 10% waste: Order = Total Blocks × 1.10

Working from block length and height rather than face area is the more reliable route, because it tells you how the courses actually stack. Face area gets you the same total on paper but hides whether the top course lands flush.

Base course: The bottom course is buried below grade for stability. Bury roughly 10% of the total wall height, and never less than one full course. That buried course is part of the total above, not an extra to add on.

What each variable means:

• Wall Face Area — the visible surface area of the finished wall in square feet.
• Face Area per Block — varies by manufacturer. Most standard retaining wall blocks cover 0.67–1.0 sq ft of face area.
• Base Courses (buried) — blocks embedded below the finished grade surface. These provide stability but are not visible. The rule of thumb is to bury 10% of total wall height.
• Setback: each course typically steps back 0.5"–1.5" from the previous course. This affects total base width for taller walls.

Common retaining wall block sizes:

Manufacturers revise their line-ups, so measure a sample block at the yard rather than trusting any published table, this one included.

Worked example: A wall 40 feet long by 3 feet tall, built from 12 inch by 8 inch units.

Rows = 36 in ÷ 8 in = 5 rows (the calculator rounds up to whole courses) Blocks per row = 480 in ÷ 12 in = 40 Total = 5 × 40 = 200 blocks With 10% waste: 220 blocks, which is the figure to order

Of those 5 rows, about one sits below grade at the 10% burial rule, leaving 4 showing.

Cap blocks are a separate product, solid and usually 3 to 4 inches tall, laid on top of the finished wall. One per block in the top row, so 40 caps here. They are not part of the 200.

Base gravel volume (a 6-inch compacted layer beneath the wall): Base Gravel (cu ft) ≈ Wall Length × 1 ft × 0.5 ft = 0.5 × Length

So a 40-foot wall needs 20 cubic feet of compacted gravel base, about 0.74 cubic yards. Order 10% extra for compaction loss.

Bury depth by wall height:

• Walls under 3 ft: bury at least 6 inches, meaning one full block height
• Walls 3 to 6 ft: bury about 1 inch per foot of wall height, which is the same 10% rule stated another way
• The buried portion carries the wall’s lateral load into the ground

Safety note: Walls over 4 feet tall typically require an engineer’s stamp and permits in most jurisdictions. Drainage (gravel backfill and perforated drain pipe) is critical to prevent hydrostatic pressure failure. For walls over 4 feet, deadman anchors or geogrid reinforcement may be required to resist lateral soil pressure.