Concrete Block Calculator

Concrete Block Calculator

Work out how many concrete blocks, breeze blocks or cinder blocks (CMU) you need for a wall. Enter the wall size, subtract doors and windows, pick your block size, and get the block count plus the mortar, cement and sand to lay them.

UK, metric & US block sizesOpenings deductedMortar includedLast updated: November 2, 2026

Calculate blocks for a wall

m
m
m²
Total area to deduct. A standard door ≈ 1.9 m² / 20 ft²
%

Block count formula

Net wall area

A = (L × H) − openings

Measure the wall face you will build, then subtract every door and window opening.

Blocks

Blocks = ⌈A ÷ (block length + joint) ÷ (block height + joint) × (1 + waste%)⌉

The block’s face size plus one mortar joint gives the area each block covers. With 10 mm (3/8 in) joints a 440 × 215 mm block covers 0.45 × 0.225 = 0.10125 m², and a US 8 × 16 CMU covers 0.889 ft².

Blocks per square metre and square foot

Block (actual face)With jointBlocks per m²Blocks per ft²
440 × 215 mm (UK)450 × 225 mm9.880.92
390 × 190 mm (metric modular)400 × 200 mm12.51.16
15⅝ × 7⅝ in (US 8 × 16 CMU)16 × 8 in12.111.125

Thickness does not change the count, only the weight and the amount of mortar. US 4 in, 6 in, 8 in and 12 in CMU all share the 8 × 16 in face.

Mortar for blockwork

Metric blocks: the calculator works out the mortar bed geometrically (a full 10 mm bed and perpend joint around every block, through the full thickness) and adds 20% for droppings. Mortar is a 1 : 5 cement : sand mix, using a 1.33 dry-volume factor and 1,440 kg/m³ cement.

US CMU: it uses the common estimating rule of about 3 bags of masonry cement per 100 blocks and 1 cubic yard of mortar sand for every 7 bags, which assumes 3/8 in joints with face-shell bedding.

Worked examples

10 m × 2.4 m wall, 440 × 215 × 100 mm blocks, no openings
  1. Area = 10 × 2.4 = 24 m²
  2. Blocks = 24 × 9.88 = 237.0, +5% = 249 blocks
  3. Mortar ≈ 0.20 m³ wet → 1:5 mix needs about 63 kg cement (3 × 25 kg bags) and 0.22 m³ building sand
30 ft × 8 ft CMU wall with one 3 × 7 ft door
  1. Net area = 240 − 21 = 219 ft²
  2. Blocks = 219 × 1.125 = 246.4, +5% = 259 blocks
  3. Masonry cement = 259 × 3 ÷ 100 = 7.8 → 8 bags; sand = 8 ÷ 7 ≈ 1.1 yd³

How to use the calculator

  1. Choose metric or US CMU. The block list changes to match.
  2. Enter the wall length and height. For several walls, add their lengths if they share the same height.
  3. Deduct openings by adding up the area of doors and windows.
  4. Pick the block size and waste. 5% covers normal cutting; use 10% for walls with many corners or openings.

Frequently asked questions

How many blocks are in 1 square metre?

About 9.88 (round up to 10) standard UK 440 × 215 mm blocks per m², or 12.5 modular 390 × 190 mm blocks, with 10 mm mortar joints.

How many cinder blocks do I need for a 10 × 10 ft wall?

100 ft² × 1.125 = 112.5, so 113 standard 8 × 16 in blocks, or about 119 with 5% waste.

How much mortar do I need for 100 blocks?

For US 8 × 8 × 16 CMU, a common rule is 3 bags of masonry cement and about 0.43 yd³ of sand per 100 blocks.

Does block thickness change the number of blocks?

No. The count depends only on the face size. Thicker blocks need more mortar and are heavier to handle.

Brick Wall CalculatorBricks for walls Mortar Mix CalculatorCement and sand by ratio Foundation CalculatorConcrete under the wall Plaster CalculatorRender the finished wall

Results are estimates for ordering. Check block dimensions with your supplier, as sizes vary slightly between manufacturers.

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