How to use the concrete block calculator
- Enter the wall. Type the length and the height of the wall. If the wall has doors or windows, enter their total area as openings.
- Choose the block. Pick the face size of the block. The standard block is 16 inches long and 8 inches high. Set the waste allowance, which starts at 5%.
- Read the blocks and the mortar. The result shows the blocks to buy, the courses, the bags of masonry cement and the sand for the mortar.
What is a concrete block calculator?
A concrete block calculator is a tool that counts the blocks in a wall. It divides the area of the wall by the area of one block face, including the mortar joint, and adds an allowance for waste.
Concrete block, cinder block and CMU (concrete masonry unit) are names for the same kind of unit, so this one calculator covers all three. The standard block is called 8 × 8 × 16: 8 inches thick, 8 inches high and 16 inches long.
Features
- The number of blocks for a wall, with door and window openings taken out
- Three block face sizes: 16 × 8, 16 × 4 and 8 × 8 inches
- The number of courses and the blocks in each course
- Bags of masonry cement and tons of sand for the mortar
- A waste allowance that starts at the 5% used by the Concrete Masonry & Hardscapes Association
- A cost from your own price per block
- A live drawing of the wall with every block, a 3D view, a PDF of the estimate and a WhatsApp share button
What you can use it for
- A block wall for a garage, a shed or a basement
- A garden wall or a raised planter
- A foundation wall or a stem wall
- Ordering mortar materials for a block job
How the number of blocks is calculated
Blocks = (wall length × wall height − openings) × blocks per square foot × (1 + waste ÷ 100), rounded up.
A block is counted by its nominal size, which is the block plus one mortar joint. The Concrete Masonry & Hardscapes Association states that the specified size of a block is 3/8 inch less than the nominal size, to leave room for a 3/8 inch joint. A nominal 8 × 8 × 16 block measures 7 5/8 × 7 5/8 × 15 5/8 inches.
| Block face (length × height) | Face area | Blocks per square foot | Blocks per 100 sq ft | With 5% waste |
|---|---|---|---|---|
| 16 in × 8 in (standard) | 0.89 sq ft | 1.125 | 112.5 | 119 |
| 16 in × 4 in (half-high) | 0.44 sq ft | 2.25 | 225 | 237 |
| 8 in × 8 in (half-length) | 0.44 sq ft | 2.25 | 225 | 237 |
The figure of 119 standard blocks per 100 square feet, with 5% for waste and breakage, is the one given in the association's guide Estimating Concrete Masonry Materials (TEK 04-02A).
Worked example: a wall 20 ft long and 8 ft high
- Wall area: 20 × 8 = 160 square feet.
- Blocks before waste: 160 × 1.125 = 180 blocks. That is 12 courses of 15 blocks.
- With 5% waste: 180 × 1.05 = 189 blocks.
- Masonry cement: 189 ÷ 30 = 6.3, so 7 bags.
- Sand: 189 ÷ 240 = 0.79 tons.
These are the numbers the calculator shows when the page opens.
Mortar for a block wall
The mortar estimate uses two figures from Estimating Concrete Masonry Materials (TEK 04-02A): one 70 lb bag of masonry cement lays about 30 hollow blocks, and one ton of masonry sand is needed for about 240 blocks. Both are for mortar placed on the face shells of the block only.
The thickness of the block does not change the number of blocks. A wall of 6 inch block and a wall of 12 inch block of the same length and height use the same count.
What the calculator includes
- The calculator counts by area. It does not lay out corners, half blocks, lintels, bond beams or cap blocks.
- Openings are entered as one total area. The calculator does not check that they fit in the wall.
- The mortar figures are for hollow blocks with face shell bedding. They are approximate and do not include grout.
- The calculator does not estimate grout to fill the cores, reinforcement or a footing.
- The drawing shows the full wall without openings. Above 30 courses or 40 blocks in a course it shows the outline only.
- Each dimension can be up to 10,000 in its unit and waste can be 0 to 50%.
Is it private?
Yes. The calculation and the PDF are made in your browser. The numbers you type are not sent to our server and are not saved.
Tips for an accurate estimate
- Measure the wall length and height on the face where the blocks will be laid.
- Count corners and openings separately, since cut blocks and lintels change the count.
- Choose the block size you will buy; the count changes a lot between sizes.
- Keep the waste allowance for broken and cut blocks.
- Order mortar sand and cement with the blocks so the work does not stop halfway.
Related calculators
Frequently asked questions
- How many concrete blocks do I need?
- Multiply the wall length by the wall height in feet, subtract the openings, and multiply by 1.125 for standard 16 by 8 inch blocks. Then add waste. A wall 20 ft long and 8 ft high needs 180 blocks, or 189 with 5% waste.
- How many concrete blocks are in a square foot?
- 1.125 standard blocks. One block with its mortar joint covers 16 by 8 inches, which is 0.89 square feet.
- How many blocks do I need for 100 square feet of wall?
- 112.5 standard blocks before waste. The Concrete Masonry & Hardscapes Association gives 119 blocks per 100 square feet, which includes 5% for waste and breakage.
- Is a cinder block the same as a concrete block?
- For counting, yes. Cinder block, concrete block and CMU are names used for the same standard unit sizes, so the number of blocks is the same.
- How much mortar do I need for a block wall?
- About one 70 lb bag of masonry cement for every 30 hollow blocks, and one ton of masonry sand for about 240 blocks, according to the Concrete Masonry & Hardscapes Association. A wall of 189 blocks needs 7 bags and 0.79 tons of sand.
- Does the thickness of the block change how many I need?
- No. The count depends on the face of the block, which is its length and height. A thicker block makes a thicker wall with the same number of blocks.
Sources
Every figure this calculator uses comes from the published sources below. The formulas are on this page, so any result can be checked by hand.
- Concrete Masonry & Hardscapes Association, CMU-TEC-001-23. Block sizes and 119 blocks per 100 square feet including 5% waste.
- Concrete Masonry & Hardscapes Association, TEK 04-02A, Concrete Masonry Materials. One 70 lb bag of masonry cement for about 30 blocks; one ton of sand for about 240 blocks.