acalculator

How many blocks do I need for a wall?

Type the wall’s length and height, any openings, and the block size. The concrete block calculator counts the blocks to buy, with extra for cuts and breakage, and the bags of mortar to lay them.

Your numbers

Units
Block and mortar
Blocks to buy
99

You need 99 blocks for 80 ft² of wall, and 8 bags of mortar.

Blocks in the wall
90
Bags of mortar
8
Wall area (ft²)
80
Wall area (m²)
7.432
Blocks per ft²
1.125

Blocks to buy: 99. You need 99 blocks for 80 ft² of wall, and 8 bags of mortar.

How to calculate

Counts the concrete blocks (CMU or cinder blocks) for a wall from its length, height and openings, with extra for cuts, plus the bags of mortar to lay them.

Example with the default inputs (Wall length 20 ft, Wall height 4 ft, Openings 0 ft², Block length (nominal) 16 in, Block height (nominal) 8 in, Extra for cuts and breakage 10%, Blocks per bag of mortar 12): You need 99 blocks for 80 ft² of wall, and 8 bags of mortar.

Method: Blocks = (length × height − openings) ÷ (nominal block length × nominal block height) × (1 + extra ÷ 100), rounded up; bags of mortar = blocks in the wall ÷ blocks per bag, rounded up.

  • Block sizes are nominal: the block plus one 3/8 in mortar joint. A standard 8 × 8 × 16 in block is 7 5/8 × 7 5/8 × 15 5/8 in and covers 16 × 8 = 128 in² of wall, so 1.125 blocks per ft².
  • Extra for cuts and breakage: 10% by default (Nicolock orders 10% extra for its pavers); straight walls with few cuts need less.
  • Mortar: 12 blocks per 80 lb bag by default (Sakrete Type S); type the number on your bag. Mortar is counted for the blocks in the wall, not the extra.
  • Corners, bond beams, lintels, core fill and footings are not counted.

Machine-readable copies: Markdown, JSON.

Worked examples

Each example is checked against the calculator on every build.

  1. Wall length 20 ft, Wall height 4 ft, Openings 0 ft², Block length (nominal) 16 in, Block height (nominal) 8 in, Extra for cuts and breakage 10%, Blocks per bag of mortar 12 gives Blocks to buy 99, Blocks in the wall 90, Bags of mortar 8, Wall area (ft²) 80, Blocks per ft² 1.125.Source: Archtoolbox, Concrete Block (CMU) Sizes, Shapes, and Finishes (nominal size = actual size + the 3/8 in mortar joint), https://www.archtoolbox.com/cmu-sizes-shapes-finishes/; Sakrete, Mortar Mix Type S (an 80 lb bag lays 12 standard 8 in blocks), https://www.sakrete.com/product/mortar-mix-type-s/; Nicolock, Simplify Your Project: How to Use a Paver Calculator (order 10 percent extra for waste), https://nicolock.com/2024/06/04/simplify-your-project-how-to-use-a-paver-calculator-for-accurate-estimates/
  2. Wall length 30 ft, Wall height 8 ft, Openings 21 ft², Block length (nominal) 16 in, Block height (nominal) 8 in, Extra for cuts and breakage 5%, Blocks per bag of mortar 12, Price per block $2.50 gives Blocks to buy 259, Blocks in the wall 246.375, Bags of mortar 21, Wall area (ft²) 219, Cost of blocks $647.50.Source: Archtoolbox, Concrete Block (CMU) Sizes, Shapes, and Finishes (nominal size = actual size + the 3/8 in mortar joint), https://www.archtoolbox.com/cmu-sizes-shapes-finishes/; Sakrete, Mortar Mix Type S (an 80 lb bag lays 12 standard 8 in blocks), https://www.sakrete.com/product/mortar-mix-type-s/
  3. Wall length 16.4 ft, Wall height 6.562 ft, Openings 0 ft², Block length (nominal) 15.75 in, Block height (nominal) 7.874 in, Extra for cuts and breakage 0%, Blocks per bag of mortar 12 gives Blocks to buy 125, Blocks in the wall 125, Bags of mortar 11, Wall area (m²) 10.Source: Archtoolbox, Concrete Block (CMU) Sizes, Shapes, and Finishes (nominal size = actual size + the 3/8 in mortar joint), https://www.archtoolbox.com/cmu-sizes-shapes-finishes/

How it works

  • Wall area = length × height − openings.
  • One block’s face = nominal length × nominal height (the block plus one mortar joint).
  • Blocks in the wall = wall area ÷ one block’s face.
  • Blocks to buy = blocks in the wall × (1 + extra ÷ 100), rounded up to a whole block.
  • Bags of mortar = blocks in the wall ÷ blocks per bag, rounded up to a whole bag.
  • Blocks per ft² = 1 ft² ÷ one block’s face.
  • Cost (when you type a price) = blocks to buy × price per block.

Units: 1 ft = 12 in = 0.3048 m exactly, 1 ft² = 144 in².

Exact arithmetic. Each size is read as the exact decimal you typed, in the unit you typed it in, and every step is an exact fraction: 80 ft² ÷ 128 in² is exactly 90 blocks, never 90.0001. When you type in a unit, the page stores the size in metres and reads it back as the decimal with the fewest significant digits, in one of the box’s units, that gives the same stored number (a feet-and-inches pair counts by its value in inches; on a tie the first unit in the menu wins).

Assumptions

  • Sizes are nominal. A standard 8 × 8 × 16 in block (actual 7 5/8 × 7 5/8 × 15 5/8 in, with a 3/8 in joint) covers 16 × 8 in, so 1.125 blocks per ft². The block’s width (depth) does not change the count.
  • Extra for cuts and breakage: 10% by default (Nicolock’s allowance for its pavers); from 0% to 100%.
  • Mortar: 12 blocks per 80 lb bag by default (Sakrete Type S). It is counted for the blocks in the wall only, not for the extra.
  • Corners, bond beams, lintels, core fill and footings are not counted.

Rules

  • Wall length and height: more than 0, at most 1,000 m. Openings: 0 to 1,000,000 m². Nominal block length and height: 1 cm to 2 m. Blocks per bag: 1 to 1,000.
  • When the openings are as large as the wall or larger, the page gives no answer, with a message.

Output format. Blocks to buy and bags are whole numbers, rounded up. Blocks in the wall shows with at most 1 decimal; square feet with at most 2; square metres and blocks per ft² with at most 3; the cost in dollars and cents. Each is rounded half up from its exact value.

Worked examples by hand

A 20 × 4 ft wall of standard blocks, 10% extra. Area 80 ft² = 11,520 in². One block 16 × 8 = 128 in². 11,520 ÷ 128 = 90 blocks in the wall; × 1.1 = 99 to buy. Mortar: 90 ÷ 12 = 7.5 → 8 bags. 144 ÷ 128 = 1.125 blocks per ft².

A 30 × 8 ft wall with a 3 × 7 ft door, 5% extra, $2.50 a block. Area 240 − 21 = 219 ft². 219 × 1.125 = 246.375 blocks in the wall; × 1.05 = 258.69 → 259 to buy, $647.50. Mortar: 246.375 ÷ 12 = 20.53 → 21 bags.

A 5 × 2 m wall of 40 × 20 cm blocks, no extra. 10 m² ÷ 0.08 m² = 125 blocks. Mortar: 125 ÷ 12 = 10.4 → 11 bags.

Other questions people ask

How many concrete blocks do I need per square foot?

A standard 8 × 8 × 16 in block covers 16 × 8 = 128 in² of wall with its mortar joint, so a square foot (144 in²) takes 144 ÷ 128 = 1.125 blocks, or 112.5 blocks per 100 ft².

How do I calculate blocks for a wall?

Multiply length by height, subtract doors and windows, and divide by the face of one block with its joint. A 20 × 4 ft wall is 80 ft², so 80 × 1.125 = 90 blocks; with 10% extra, buy 99.

Why use the nominal block size?

Blocks are made 3/8 in smaller than their nominal size (a 16 in block is 15 5/8 in long) so that the 3/8 in mortar joint brings each block back to 16 × 8 in on the wall. The nominal size is what one block covers.

How many bags of mortar do I need?

It depends on the mortar and your joints. Sakrete says an 80 lb bag of its Type S mortar mix lays 12 standard 8 in blocks, so 90 blocks need 90 ÷ 12 = 7.5, or 8 bags. Type the number printed on your bag.

How much extra should I buy?

This page adds 10% by default, the waste allowance Nicolock suggests for its masonry pavers. A straight wall with few cuts needs less; many corners, cuts or openings need more.

Does this count cores, rebar or footings?

No. It counts the blocks and the mortar to lay them. Grout or concrete to fill cores, rebar, bond beams, lintels and the footing are separate.