Block calculator: concrete blocks, CMU (cinder block) and bricks for a wall

How does the block calculator work?

It counts by area. Each block or brick in the wall takes up its own face plus one mortar joint along the top and one at the end. That face-plus-joint rectangle is called the module, and the wall is a grid of modules.

• Module: (unit length + joint) × (unit height + joint). A 390 × 190 mm block with a 10 mm joint is 400 × 200 mm, or 0.08 m².
• Area to lay: wall length × wall height, less the width × height of every door and window.
• Blocks or bricks: area to lay ÷ module, rounded up.
• To order: that count × 1.05, rounded up. The 5% covers cut units at the ends and around openings, and breakage.
• Courses: wall height ÷ (unit height + joint). In one course: wall length ÷ (unit length + joint).
• Mortar: the order count ÷ the number of units one bag lays, as the bag's maker states it, rounded up to whole bags.

In feet and inches the calculator works in millimetres inside (1 in = 25.4 mm) and gives areas in square feet. A US block is sold by its nominal size, which already includes the joint: an 8 × 8 × 16 in CMU is 15⅝ × 7⅝ in, and the 3/8 in joint makes it 16 × 8 in in the wall.

Worked example: a 10 m block wall with a door and a window

A wall of 390 × 190 mm blocks, 10 m long and 2.4 m high, with a 900 × 2100 mm door and a 1200 × 1200 mm window. These are the numbers the calculator opens with.

1. Wall area: 10 × 2.4 = 24 m².
2. Openings: 0.9 × 2.1 = 1.89 m² and 1.2 × 1.2 = 1.44 m², so 3.33 m².
3. Area to lay: 24 − 3.33 = 20.67 m².
4. Module: 0.4 × 0.2 = 0.08 m², so 1 ÷ 0.08 = 12.5 blocks per m².
5. Blocks: 20.67 × 12.5 = 258.4, rounded up to 259.
6. To order: 259 × 1.05 = 271.95, so 272 blocks.
7. Courses: 2400 ÷ 200 = 12, with 10000 ÷ 400 = 25 blocks in a full course.
8. Mortar: Cement Australia Trade Mortar covers about 10 masonry blocks per 20 kg bag, so 272 ÷ 10 = 27.2, or 28 bags.

Worked example: a CMU (cinder block) wall, 40 × 8 ft, with a door

Switch the calculator to Feet and inches and it opens on this wall: standard 8 × 8 × 16 in blocks, 40 ft long and 8 ft high, with a 3 × 7 ft door.

1. Wall area: 40 × 8 = 320 ft². Door: 3 × 7 = 21 ft². Area to lay: 299 ft².
2. Module: 16 × 8 = 128 in², and 144 ÷ 128 = 1.125 blocks per ft².
3. Blocks: 299 × 1.125 = 336.4, rounded up to 337.
4. To order: 337 × 1.05 = 353.9, so 354 blocks.
5. Courses: 96 ÷ 8 = 12, with 480 ÷ 16 = 30 blocks in a full course.
6. Mortar: QUIKRETE Mortar Mix lays up to 13 standard 8 × 8 × 16 in blocks per 80 lb bag, so 354 ÷ 13 = 27.2, or 28 bags.

A 10 × 10 ft wall with no openings is 100 × 1.125 = 112.5, so 113 blocks and 119 to order. "Cinder block", "concrete block" and "CMU" (concrete masonry unit) are the names people use for the same standard block in the US; the count only depends on its face size and the joint.

Worked example: brick walls

Bricks are counted the same way, with a smaller module.

• Australia, 230 × 76 mm bricks, a garden wall 6000 × 1800 mm. Module 240 × 86 mm = 0.02064 m², so 48.45 bricks per m². 6 × 1.8 = 10.8 m², and 10.8 × 48.45 = 523.3, so 524 bricks and 551 to order. 1800 ÷ 86 = 20.9 courses: 21 courses come to 1806 mm, so set the wall height to whole courses or cut the top one. Mortar at about 20 bricks per 20 kg bag of Cement Australia Trade Mortar: 551 ÷ 20 = 27.6, so 28 bags.
• UK, 215 × 65 mm bricks, a wall 5000 × 900 mm. Module 225 × 75 mm, so 59.26 per m². 5 × 0.9 = 4.5 m², and 4.5 × 59.26 = 266.7, so 267 bricks and 281 to order, in exactly 12 courses. Tarmac Mortar Mix lays 1800 to 2000 bricks per 75 bags, so at 24 bricks a bag: 281 ÷ 24 = 11.7, or 12 bags.
• US, modular bricks 7⅝ × 2¼ in, a wall 20 × 4 ft. Module 8 × 2⅝ in = 21 in², so 6.857 per ft². 80 ft² × 6.857 = 548.6, so 549 bricks and 577 to order. QUIKRETE Mortar Mix lays up to 37 standard bricks per 80 lb bag: 577 ÷ 37 = 15.6, so 16 bags.

These counts are for one skin, half a brick thick. A double-brick or cavity wall has two skins: work out each skin and add them.

How many blocks or bricks per square metre?

One square metre (or square foot) divided by the module. These are the sizes in the calculator, each with a 10 mm or 3/8 in joint:

Where the sizes come from: Angelus Block lists an 8 × 8 × 16 in unit at 7.625 × 7.625 × 15.625 in, its nominal size being that plus the joint (Angelus Block, dimensions and sizes); Firth's 20 Series blocks in New Zealand are 390 × 190 mm on the face (Firth 20 Series flyer, PDF); the Brick Industry Association lists the modular brick at 7⅝ × 2¼ in with a 3/8 in joint (BIA Technical Note 10, Table 1, PDF); Littlehampton Bricks gives the standard Australian brick as 230 mm long and 76 mm high, about 48.5 to the square metre with mortar (Littlehampton Bricks); and the UK brick is 215 × 65 mm on the face, 225 × 75 mm with 10 mm joints, about 60 to the square metre (Designing Buildings, brick sizes). Checked on 1 November 2026.

Published round figures can differ a little from the table. The BIA's Table 4 gives 675 modular bricks per 100 ft², because it lays three courses to 8 in (2⅔ in each) rather than the 2⅝ in that a 2¼ in brick and a 3/8 in joint make; the calculator gives 685.7. Check your own unit's size before you order.

How much mortar for 100 blocks?

It depends on the mortar, the unit and the joint, so the calculator uses each maker's own figure for its premixed mortar and the unit it names:

Sources: QUIKRETE Mortar Mix data sheet, “Yield” (PDF); Cement Australia Trade Mortar sheet (PDF); Tarmac Mortar Mix data sheet, “Yield” (PDF, via Wickes). Checked on 1 November 2026. For Tarmac, 1500 ÷ 75 = 20 blocks and 1800 ÷ 75 = 24 bricks a bag, the low end of its range; its sheet gives 1000 blocks per m³ for 150 mm blocks, so thicker blocks take more mortar. The calculator works mortar out from the order count, with the 5% in it, because “up to” and “about” are best-case figures. Mixing your own from cement, lime and sand is a different sum, and the calculator doesn't do it.

Common mistakes

• Dividing by the block without the joint. 390 × 190 mm alone is 0.0741 m², which gives 13.5 blocks per m² instead of 12.5: 8% too many.
• Using the nominal US size as the block. A CMU sold as 8 × 8 × 16 in is already the module. Don't add another 3/8 in joint to it.
• Counting corners twice. Where two walls meet, measure one wall to the outside corner and the other to the inside face, or measure both along their centre lines.
• Forgetting the second skin. A cavity or double-brick wall is two walls for counting.
• Not taking out openings. A 900 × 2100 mm door is 1.89 m², which is 24 blocks of 390 × 190 mm.
• Ordering the exact count. Cut blocks at the ends and around openings, and the odd broken one, use more than the area says. Order the figure with the 5% in it, or more on a wall with many short returns.

Questions people ask

What this calculator doesn't do

• One skin of a straight wall at a time, in stretcher (running) bond. No double-skin, cavity or collar-jointed walls in one go: work out each skin.
• No core fill, grout or reinforcement in hollow blocks, and no bond beams, lintels or special shapes.
• No footing: the concrete calculator works out the strip footing under the wall.
• Mortar only for the six standard sizes, from the makers' published yields. No mortar for a custom size, and no cement, lime and sand split. For Indian bricks in a 9-inch or 4½-inch wall, with cement bags and sand for 1:6 or 1:4 mortar, use the brick calculator for India.
• The 5% is fixed. It doesn't decide what the wall needs structurally: that's set by the drawings, your local building code and your council or building department.

The footing under the wall

A block or brick wall stands on a footing, and that's concrete. For a strip footing, use the length of the wall along its centre line, the footing width and the depth from the drawings: 20 m of footing 300 mm wide and 400 mm deep is 2.4 m³, and 2.64 m³ to order. The concrete calculator on this site does that sum in metric or US units, with bag counts and an optional cost, and how much concrete a strip footing needs goes through it by hand. The same volume maths is in MeisterCalc's Concrete calculator (Home ▸ Concrete ▸ Slab), which is free to use and works offline on site, on Android 7.0 or later.

Related guides

• Concrete for slabs, footings and post holes, worked out in m³ or cubic yards
• How much concrete do I need for strip footings?
• How much concrete do I need for a slab?
• How do I set out a building or slab square?
• More guides on the blog