Concrete Block Calculator

Estimate how many concrete blocks (CMU, cinder blocks or besser blocks) you need for a wall, retaining wall or foundation. Enter the wall length and height, subtract openings, pick the block size used in your region and the mortar joint, and the calculator gives blocks per square foot or square metre and the total with waste.

By Rashid MehmoodLast updated 1 November 2026Free, no sign-up

How to use the concrete block calculator

1. Measure the wall. Enter the total wall length and height. For a foundation, use the perimeter as the length.
2. Subtract openings. Enter the area of doors, windows and vents.
3. Choose the block size. Pick 8 × 8 × 16 in (US/Canada), 440 × 215 mm (UK) or 390 × 190 mm (Australia/metric). The size is the face, not the thickness.
4. Set joint and waste. A 3/8 in or 10 mm joint is standard. Add 5–10% waste for cuts and breakages.

Concrete blocks are larger than bricks, so a block wall goes up faster and needs far fewer units. Counting blocks uses the same method as bricks: find how many fit in each square foot or square metre, then multiply by the wall area. This calculator does that for the standard block sizes used in North America, the UK and Australia.

The formula explained

Blocks per area = 1 ÷ [(Block length + Joint) × (Block height + Joint)]
Net area = Length × Height − Openings
Blocks = ⌈ Net area × Blocks per area × (1 + Waste ÷ 100) ⌉

Nominal vs actual. In the US a block called 8 × 8 × 16 actually measures 7⅝ × 7⅝ × 15⅝ in. With a 3/8 in joint it takes up exactly 8 × 16 in of wall. The calculator uses actual sizes plus the joint so the arithmetic is exact.

Worked examples

Example 1: US garage wall

A wall 40 ft long and 4 ft high, 8 × 8 × 16 in CMU, 3/8 in joints, 5% waste:

1. Module: 16 in × 8 in = 128 in² = 0.889 ft², so 1.125 blocks per ft².
2. Area: 40 × 4 = 160 ft².
3. Blocks: 160 × 1.125 = 180.
4. With 5% waste: 189 blocks.

Example 2: UK inner leaf

A 12 m² inner leaf of 440 × 215 mm blocks with 10 mm joints:

1. Module: 0.45 × 0.225 = 0.10125 m², so 9.88 blocks per m².
2. Blocks: 12 × 9.88 = 119; with 5% waste, 125.

Example 3: Australian retaining wall

A 12 m² wall of 390 × 190 mm blocks with 10 mm joints:

1. Module: 0.4 × 0.2 = 0.08 m², so 12.5 per m².
2. Blocks: 150; with 5% waste, 158.

Block sizes by region

Half blocks, corner blocks and bond beam blocks are used at corners and openings; they are counted as full blocks in an area estimate, and the waste allowance covers cuts.

Counting by courses

An alternative check is to count courses and blocks per course. A 4 ft high wall of 8 in courses has 6 courses; a 40 ft wall with 16 in modules has 30 blocks per course, so 6 × 30 = 180 blocks, matching the area method. Counting by courses is useful when the wall height is not an exact number of courses: round up to the next whole course, because you cannot lay part of one without cutting every block lengthwise. If your design height falls between courses, consider adjusting it to suit the block module.

Regional notes: US, UK, Australia and Canada

United States. The National Concrete Masonry Association publishes TEK notes on design, estimating and construction with CMU. Common widths are 4, 6, 8, 10 and 12 in. Reinforced walls have rebar in grouted cores; see the rebar calculator for slab steel.

United Kingdom. Aggregate, lightweight and aerated blocks are sold in 440 × 215 mm face sizes, in thicknesses of 75–215 mm. In cavity walls, blocks usually form the inner leaf.

Australia. The Concrete Masonry Association of Australia publishes guides. “Besser” blocks (after a manufacturer) are typically 390 × 190 mm in 90–190 mm widths. Retaining walls over certain heights need engineering under local rules.

Canada. Metric 390 × 190 mm and imperial sizes are both available, and CSA standards apply to concrete masonry.

Tips and common mistakes

• Use the face size, not thickness, in the calculator.
• Plan the height in whole courses. 8 in or 200 mm courses avoid cutting blocks lengthwise.
• Order specialty blocks (corners, lintels, bond beams) in addition to the count.
• Footings first. Block walls need a solid footing; the concrete calculator can work out the volume.
• Weight adds up. A standard hollow 8 in CMU weighs roughly 35–40 lb, so plan deliveries and handling.
• Retaining walls hold back significant loads. Get engineering advice for anything beyond a low garden edge.

Related tools

For brickwork, use the brick calculator. For a concrete slab under the wall, use the concrete calculator.

Frequently asked questions

Related calculators

Sources and references