Last updated: 2026-07-02

Mortar Bags for Block

A concrete block wall is a grid of masonry units bedded in mortar. Estimating the job comes down to two numbers: how many blocks cover the wall face, and how many bags of mortar mix lay that many blocks. Working both out before the delivery truck arrives avoids a stalled job and a pallet of leftover block.

Counting the blocks

Each block covers a rectangle of wall equal to its nominal length times its nominal height, joint included:

Dividing the wall area by that face area gives the base block count. Multiplying by one plus a waste allowance covers the cuts, breakage, and spares, and the result is rounded up to a whole block:

Bags of mortar

Mortar yield is quoted as blocks laid per bag. Dividing the block count by that figure and rounding up gives the number of bags:

A standard 80 lb (36 kg) bag of mortar mix lays roughly 25 to 30 blocks, so a middle figure of about 27 per bag is a reasonable default. Thicker joints and larger units lay fewer blocks per bag.

Worked example

Take a wall of 100 square feet9.29 square metres built with standard blocks nominally 16 inches long by 8 inches high40.6 cm long by 20.3 cm high, at 27 blocks per bag and a 10% waste allowance.

One block face is 16 in × 8 in = 128 in², or about 0.888 ft². Dividing 100 ft² by 0.888 ft² gives 112.5 blocks, and applying 10% waste gives 112.5 × 1.1 = 123.75, which rounds up to: One block face is 40.6 cm × 20.3 cm ≈ 0.0825 m². Dividing 9.29 m² by 0.0825 m² gives about 112.5 blocks, and applying 10% waste gives 112.5 × 1.1 = 123.75, which rounds up to:

Then the mortar comes to 124 blocks at 27 per bag:

Openings, bond, and yield

Subtract large openings such as doors and windows from the wall area before estimating, so you are not paying for block you will never lay. A plain running bond on a rectangular wall wastes little, while many corners, curves, and cut ends push the waste allowance toward 15% or more.

Mortar yield is the figure most sensitive to site conditions. Joint thickness, block size, the mix you buy, and how much mortar falls off the trowel all move the blocks-per-bag figure up or down. When a job runs to either extreme, buy a spare bag rather than run short mid-course.

For related material estimates, see the Wall Framing Stud Calculator for framed walls and the Paver Patio Calculator for ground-level masonry.