Concrete Block Calculator (CMU wall: blocks, courses, mortar bags)

Understanding the concrete block calculator — concept and principle

This calculator estimates how many concrete blocks you need for a wall, with a waste allowance, plus the number of courses, blocks per course and rough mortar bags. Use it to price a garden wall, a shed base wall or a retaining course before ordering from a builders' merchant.

Blocks for a wall: with the default inputs the result is 237 blocks (with 5% waste · 9 courses).

How it is calculated

Each block covers its face plus one mortar joint. Korean KS F 4002 block: (390 + 10) × (190 + 10) mm = 0.40 × 0.20 m = 0.08 m². US 8 × 16 in nominal block: 0.4064 × 0.2032 m.
Net wall area = length × height − openings.
Blocks without waste = net area ÷ area per block.
Blocks to buy = blocks × (1 + waste %), rounded up.
Courses = height ÷ block height with joint, rounded up; blocks per course = length ÷ block length with joint.
Mortar (standard sizes only): about 30 blocks per 70 lb bag of masonry cement, about 62 per 94 lb bag of portland cement used with lime.

The principle

Blocks are laid with a mortar joint on one end and one bed, so each block occupies a "module" slightly bigger than its face. Standard blocks are made 10 mm (or 3/8 in) smaller than a round module so that block plus joint gives tidy dimensions such as 400 × 200 mm or 16 × 8 in. That makes counting simple: blocks per square metre is 1 ÷ module area. A waste allowance covers cut blocks at ends and openings and breakage on site; the US masonry industry guide uses 5%.

Worked example

Default inputs: wall 10 m long, 1.8 m high, no openings, KS F 4002 block, 5% waste.

Area per block = 0.40 × 0.20 = 0.08 m², so 12.5 blocks per m².
Net area = 10 × 1.8 = 18 m².
Blocks = 18 ÷ 0.08 = 225; with waste 225 × 1.05 = 236.25, rounded up to 237.
Courses = 1.8 ÷ 0.2 = 9; blocks per course = 10 ÷ 0.4 = 25.
Masonry cement: 237 ÷ 30 = 7.9, so 8 bags; portland cement: 237 ÷ 62 = 3.8, so 4 bags (sand and lime extra).

Each point is the calculator re-run with only “Wall length (m)” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Wall length (m)	Blocks to buy
2.5	60
5	119
7.5	178
10	237
12.5	296
15	355
20	473

How to read the result

Order the "blocks to buy" figure. Increase waste to about 10% if the wall has many corners, piers or openings that need cut blocks. Corners use the same block count but need corner or half blocks, so check the course layout. The mortar figures are rough guides for hollow blocks with mortar on the face shells only.

Common mistakes

Using the block's actual size without the joint, which overestimates the count.
Forgetting to subtract doors and windows.
Ordering exactly the net number with no spare blocks.
Using the mortar figures for odd sizes or fully bedded blocks.

Keep in mind: Block walls, especially retaining walls and anything above garden height, need foundations, reinforcement and ties designed to local building codes. Have a qualified professional check any structural wall.

Frequently asked questions

How many blocks are in one square metre?

12.5 for 390 × 190 mm blocks with 10 mm joints, and about 12.1 for US 8 × 16 in blocks.

How much waste should I allow?

5% is the usual allowance for simple walls; add more for many cuts or corners.

How many blocks does one bag of mortar lay?

About 30 hollow blocks per 70 lb bag of masonry cement, or about 62 per 94 lb bag of portland cement with lime.

Sources: Concrete Masonry & Hardscapes Association, TEK 04-02A "Estimating Concrete Masonry Materials" · KS F 4002, Concrete blocks (390 × 190 mm face size)

Checked 9 Nov 2026. Concrete Masonry & Hardscapes Association, TEK 04-02A “Estimating Concrete Masonry Materials” (8/9 ft² per 8 × 16 unit; 119 units per 100 ft² incl. 5% waste; ≈ 30 hollow units per 70 lb bag of masonry cement and ≈ 62 per 94 lb bag of portland cement with lime, face-shell bedding); KS F 4002 hollow concrete block 390 × 190 mm face.

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