# How many blocks do I need for a wall?

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.

- Page: https://www.acalculator.org/construction/concrete-block-calculator
- JSON spec: https://www.acalculator.org/construction/concrete-block-calculator.json
- Version: 179e50df3bb6

## Default answer

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.

## Inputs

| Key | Label | Description |
| --- | --- | --- |
| l | Wall length | The length of the wall. |
| h | Wall height | The height of the wall. |
| o | Openings | The total area of doors and windows in the wall, if any. |
| bl | Block length (nominal) | The block’s length plus one mortar joint: 16 in for a standard block. |
| bh | Block height (nominal) | The block’s height plus one mortar joint: 8 in for a standard block. |
| waste | Extra for cuts and breakage | Blocks to add for cuts and breakage, in percent. |
| bag | Blocks per bag of mortar | How many blocks one bag of mortar lays, from the bag: 12 for an 80 lb bag of Sakrete Type S. |
| price | Price per block | What one block costs. Leave empty to skip the cost. |

## Outputs

| Key | Label | Description |
| --- | --- | --- |
| blocks | Blocks to buy | Net wall area ÷ one block’s nominal face × (1 + extra ÷ 100), rounded up. |
| laid | Blocks in the wall | Net wall area ÷ one block’s nominal face, before the extra. |
| bags | Bags of mortar | Blocks in the wall ÷ blocks per bag, rounded up. |
| squareFeet | Wall area (ft²) | Length × height − openings, in square feet. |
| squareMeters | Wall area (m²) | Length × height − openings, in square metres. |
| perSquareFoot | Blocks per ft² | 1 ÷ one block’s nominal face in square feet. |
| cost | Cost of blocks | Blocks to buy × the price per block. |

## 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.

## Assumptions

- 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.

## Worked examples

1. l = 6.096, h = 1.2192, o = 0, bl = 0.4064, bh = 0.2032, waste = 10%, bag = 12 gives blocks = 99, laid = 90, bags = 8, squareFeet = 80, perSquareFoot = 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. l = 9.144, h = 2.4384, o = 1.950964, bl = 0.4064, bh = 0.2032, waste = 5%, bag = 12, price = $2.50 gives blocks = 259, laid = 246.375, bags = 21, squareFeet = 219, cost = $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. l = 5, h = 2, o = 0, bl = 0.4, bh = 0.2, waste = 0%, bag = 12 gives blocks = 125, laid = 125, bags = 11, squareMeters = 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/.

## FAQ

### 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.

## Sources

- Archtoolbox, Concrete Block (CMU) Sizes, Shapes, and Finishes (the nominal dimension is the actual dimension plus the mortar joint; typical CMU mortar joints are 3/8 in). https://www.archtoolbox.com/cmu-sizes-shapes-finishes/ (retrieved 2026-10-02)
- Sakrete, Mortar Mix Type S (an 80 lb bag lays 12 standard 8 in block). https://www.sakrete.com/product/mortar-mix-type-s/ (retrieved 2026-10-02)
- Nicolock, Simplify Your Project: How to Use a Paver Calculator for Accurate Estimates (order 10 percent extra for waste). https://nicolock.com/2024/06/04/simplify-your-project-how-to-use-a-paver-calculator-for-accurate-estimates/ (retrieved 2026-10-01)
- NIST Special Publication 811, Appendix B (1 ft = 12 in = 0.3048 m exactly). https://www.nist.gov/pml/special-publication-811 (retrieved 2026-10-02)