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Concrete block wall calculator

Count standard 8×8×16 concrete blocks and 80 lb bags of mortar mix for a wall, after taking out doors and windows. It also tells you whether your wall height works out to whole courses.

Figures checked October 1, 2026

ft²

Width × height of each opening, added up. A 3 × 7 ft door is 21 ft².

%
95 blocks

8×8×16 in, including 5% for breakage and cuts

Wall area after openings
80 ft²
Blocks before extra
90
80 lb bags of mortar mix
8
Courses (8 in each)
6

Mortar figures are for pre-mixed 80 lb bags laid with ⅜ in joints. Grout or concrete to fill the cores is extra.

The formula

Wall area = length × height − openings Blocks = wall area × 1.125, plus extra, rounded up Mortar bags = blocks ÷ blocks per bag, rounded up

Why 1.125? A standard block is 7⅝ × 7⅝ × 15⅝ in. With a ⅜ in mortar joint it covers 8 × 16 in of wall — 128 in², or 0.889 ft² — so each square foot of wall takes 1 ÷ 0.889 = 1.125 blocks.

Worked example: a 20 ft wall, 4 ft high

  • Wall area: 80 ft². Blocks: 80 × 1.125 = 90.
  • With 5% for breakage and cuts: 94.5, so 95 blocks.
  • Mortar at 12 blocks per bag: 95 ÷ 12 = 7.9, so 8 bags of 80 lb mortar mix.
  • Courses: 48 in ÷ 8 in = 6 — a whole number, so no cut row.

Mortar: pre-mixed bags or cement and sand

The calculator counts pre-mixed 80 lb bags, which only need water. If you mix masonry cement with sand instead, the figures vary more: per 100 blocks, Martinsville Concrete’s estimating guide says 3 bags of masonry cement, while Quikrete’s and Sakrete’s product figures work out to about 2.2–2.6 bags. Order the higher number and keep the receipt.

Planning the layout

Block walls lay out on an 8 in grid. Lengths in multiples of 8 in use whole and half blocks; heights in multiples of 8 in avoid a cut course. Openings placed on the grid save the most cutting.

Questions people ask

How many blocks in 100 square feet of wall?

112.5 standard 8×16 blocks, so buy 113 before adding any extra for breakage and cuts.

How many blocks does one bag of mortar lay?

Sakrete says 12 blocks per 80 lb bag of its Type S or Type N mortar mix; Quikrete says up to 13 for its mortar mixes. That works out to about 8–9 bags per 100 blocks.

Why does the calculator warn about courses?

Blocks are 8 in tall including the joint, so walls lay out cleanly in multiples of 8 in. A 4 ft wall is 6 courses; a 3 ft 6 in wall is 5¼, which means cutting a whole row. Adjusting the height by a few inches can save a day of cutting.

Does this include filling the cores?

No. Grout or concrete in the block cores — and any steel in them — is extra and depends on your plans.

Sources for the numbers on this page

  1. Martinsville Concrete — Masonry estimating guide (8×8×16 block, 1.125 blocks per sq ft) · checked October 1, 2026
  2. Quikrete — Mortar Mix No. 1102 — technical data sheet (blocks per bag) · checked October 1, 2026
  3. Sakrete — Mortar Mix Type S (blocks per bag) · checked October 1, 2026

Corrections and updates are logged on the sources page.