Concrete Block Calculator

Quick answer: standard 8x8x16 concrete blocks cover 1.125 blocks per square foot of wall face. A 20 ft long, 4 ft high wall is 6 courses of 15 blocks, so 90 blocks, or about 95 with 5% extra, plus roughly 3 bags of 80 lb mortar mix.

Concrete block calculator

Wall length (ft)

Wall height (ft)

Openings to subtract (sq ft)

Block width 6" (6x8x16)8" (8x8x16)10" (10x8x16)12" (12x8x16)

Vertical rebar spacing None24"32"48"

Extra for waste and cuts 3%5%10%

Blocks are 7-5/8" x 15-5/8" on the face; with a 3/8" mortar joint each one covers 8" x 16", so 1.125 per sq ft for every width. Mortar use varies with the mason and the joints; the range shown assumes about 30 to 35 blocks per 80 lb bag of mortar mix. Rebar and grout layout for a structural wall must come from your plans or local code.

Blocks for common wall sizes

How the calculation works

1. Courses (rows) = wall height in inches ÷ 8.
2. Blocks per course = wall length in inches ÷ 16.
3. Blocks = courses × blocks per course, minus openings at 1.125 blocks per sq ft, plus waste.
4. Vertical bars = one at each end, plus one at every chosen spacing in between.

Engineer's note: block walls are strong when squeezed but weak when pushed sideways. A garden wall, a retaining wall and a house foundation wall carry very different sideways loads, so the steel, grout and wall thickness are not interchangeable. Anything holding back soil more than about 4 ft high, or supporting a building, needs a design, not just a material count. The footing under the wall matters too; work out its concrete with the concrete slab calculator using the footing's width, length and depth.

Common questions

How many blocks in a square foot?

1.125 standard 8x16 blocks, or 112.5 blocks per 100 sq ft of wall.

How many blocks for a 10 x 8 ft wall?

12 courses of 7.5 blocks, so 90 blocks, about 95 with 5% extra.

How much mortar per block?

A common estimate is one 80 lb bag of mortar mix for about 30 to 35 standard blocks. Masons working from a mixer usually buy masonry cement and sand instead; ask your supplier for their ratio.

Checked by: Engr. Abdul Latif, civil and structural engineer. Block counts are geometry based on standard modular units; mortar yields are typical estimates. For planning only.