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 input means:

• Wall Length and Wall Height: the finished wall, measured from the ground it will sit on. Feet or metres, your choice on each field.
• Block Length and Block Height: the face dimensions of the unit you are buying, in inches or centimetres. Take a tape to a sample at the yard rather than trusting a spec sheet, this one included. Most standard retaining wall blocks land somewhere between 0.67 and 1.0 sq ft of face area, which is a wide enough range to change the order.

Two things the calculator does not ask for, and why:

Setback. Each course steps back 0.5 to 1.5 inches from the one below, which is what leans the wall into the bank. It changes the base width and the finished lean, but not how many blocks you buy, so it stays out of the arithmetic.

Block depth. It does not affect the count either, but it does affect the base trench. The rule is a base twice the depth of the block, so a 12-inch-deep unit wants a 24-inch trench. The base gravel figure below assumes a 12-inch trench, which suits a small garden-wall block. Double it for a full-size unit.

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.

Note what the rounding did there. Five courses of 8-inch block is 40 inches of wall, not the 36 you asked for. That extra half course is not waste, it goes into the ground: dig the base trench 4 inches deeper and the finished height comes out where you wanted it. Trying to end on a part course instead means cutting block, which on a split-face unit looks exactly as bad as it sounds.

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, in a 12-inch-wide trench): 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, because compacting it is what takes the volume out. Use dense-grade base, not clean stone, since this layer needs to lock up rather than drain. The Gravel Weight Calculator turns that into a delivery weight.

Bury depth by wall height:

• Walls under 3 ft: bury at least 6 inches. On most units that is one full course, but check yours, a 4-inch garden block needs two.
• Walls 3 to 6 ft: bury 10% of the finished height. The old carpenter’s version of this is an inch per foot, which comes out a shade under 10% and is close enough on a wall this size.
• The buried course is what stops the base sliding out under lateral soil pressure. It is the course people are tempted to skip because nobody will see it.

Is your wall mortared? Then this is the wrong page. Everything here assumes a segmental block dry-stacked with a lip that hooks the course below, which is what almost every garden retaining wall is. A wall laid in mortar out of standard 8 by 8 by 16 CMU is a different build with different materials, and the concrete block wall calculator covers it: mortar bags, rebar sticks and grout, none of which exist here, plus caps that do not exist there. The block counts themselves agree, since geometry is geometry. This page adds 10% for waste against that page’s 7%, because cutting for curves, corners and a cap course wastes more than a straight mortared run does.

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.