Concrete Block Calculator

How many concrete blocks you need: 8 × 8 × 16 in CMU, UK 440 × 215 mm or Australian 390 × 190 mm blocks, with mortar, cement and sand.

Concrete blocks (cinder blocks or CMU in the US) are sized so that one block plus its mortar joint makes a neat module. A US 8 × 8 × 16 in block is actually 7⅝ × 7⅝ × 15⅝ in, so with a ⅜ in joint it covers 8 × 16 in: 1.125 blocks per ft². A UK 440 × 215 mm block with a 10 mm joint covers 450 × 225 mm, about 9.9 per m², and an Australian 390 × 190 mm block covers 400 × 200 mm, exactly 12.5 per m². A wall 20 ft long and 4 ft high takes 90 blocks, 95 with 5% extra. Enter your walls and block size and the calculator also works out the mortar, cement and sand, laying hollow blocks on their face shells.

Units
Bag sizes
Wall
ft
ft
Feet and inches work too: type 12'6 or 12.5.
Doors and windows to leave out
ft
ft
ft
ft
%
in
%
Blocks
95

1.13 per ft²

Mortar
1.38 ft³

39 L

Cement
1 bag

94 lb bag (1 ft³) · 33 lb

Sand
147 lb

0.074 tons

Hollow blocks are laid on their face shells, so they take far less mortar than solid blocks. Cores you fill with concrete or grout for reinforcement are extra.
How it was worked out
  1. Wall: 20 ft × 4 ft = 80 ft²
  2. Area to cover: 80 ft²
  3. Each 15.625 × 7.625 in block with a 0.375 in joint takes 16 × 8 in: 1.13 per ft²
  4. 80 ft² × 1.13 = 90; plus 5% = 94.5, rounded up to 95 blocks
  5. Mortar on the face shells only (1.25 in each): 1.15 ft³, plus 20% = 1.38 ft³
  6. 1 : 4 mortar: 33 lb of cement (1 bag of 94 lb) and 147 lb of sand

Mortar is worked out from the joints alone. Frogs laid upwards, droppings and mixing waste use more, which is what the extra mortar allowance is for. If you buy premixed mortar, use the mortar volume and the yield printed on the bag.

Common use cases

  • A retaining or garden wall in concrete block
  • Foundation walls and footings (with the sizes from your plans)
  • A garage, shed or workshop wall
  • Pricing blocks and mortar before getting quotes

How to use

  1. Choose your units and country (for block sizes and cement bags).
  2. Enter each wall's length and height.
  3. Choose the block size. For hollow blocks, check the face shell thickness.
  4. Pick the mortar mix and read the blocks, mortar, cement and sand.

FAQ

How many concrete blocks per square foot or square metre?

1.125 per ft² for standard US 8 × 16 in blocks (so 112.5 per 100 ft²), about 10 per m² for UK 440 × 215 mm blocks, and 12.5 per m² for Australian 390 × 190 mm blocks, all with 10 mm (⅜ in) joints.

What's the difference between cinder blocks and concrete blocks?

Cinder blocks were originally made with coal cinders as the aggregate; most blocks sold today, including those still called cinder blocks, are concrete masonry units made with sand and gravel or crushed stone. The sizes and counts are the same.

How much mortar do hollow blocks need?

Hollow blocks are usually laid on face-shell bedding: mortar runs only along the two outer shells, not across the webs, so they take far less mortar than solid units. Cores you fill with grout or concrete for reinforcement are extra.

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