Concrete Block Calculator
Find how many concrete blocks, also called cinder blocks or CMU, you need for a block wall. Enter the wall, subtract doors and windows, and choose your block size to see the blocks and bags of mortar mix to buy.
Add more walls, or subtract openings such as doors and windows.
- Choose US or Metric. You can still change the unit of any single field.
- Enter the wall. For a single wall, choose Rectangle and enter the wall's Length and Height. For the four walls of a garage, a shed, or a foundation, choose Room walls. For a gable or a wall with a sloped top, add a Triangle section to the rectangle below it.
- Use Add section for more walls. To leave out doors and windows, add a section for each one and set it to Subtract. Use Quantity for several of the same size.
- Choose the Block size. For another size, choose Custom size and enter the actual length and height of the block.
- Check the Waste allowance. It starts at 10% and applies to both the blocks and the mortar.
- Optionally, enter the Blocks per mortar bag printed on another mortar product, and a Block price and Mortar price to estimate the cost.
Dimension fields accept whole numbers, decimals, fractions, and mixed
numbers, such as 20, 7.5, 3/8, or
15 5/8.
How many concrete blocks you need
The calculator finds the area of the wall, subtracts the openings, adds the waste allowance, and divides by the area of one block plus its share of the mortar joints:
Total area = net wall area × (1 + waste % ÷ 100)
Blocks = total area ÷ ((block length + joint) × (block height + joint)), rounded up
Bags of mortar = total area ÷ area of one block with joints ÷ blocks per bag, rounded up
This is the method in the Concrete Masonry & Hardscapes Association (CMHA) guide TEK 04-02A, Estimating Concrete Masonry Materials: divide the wall area, less the windows, doors, and other openings, by the face area of one block. The bags of mortar are counted from the blocks you lay before rounding up, because mortar is used as you lay the blocks. With several sections, the calculator adds the sections set to Add and subtracts those set to Subtract. The area of each shape is calculated as follows:
| Shape | Area |
|---|---|
| Rectangle | length × height |
| Room walls | 2 × (length + width) × height |
| Triangle (base and height) | base × height ÷ 2 |
| Known area | the area you enter |
For Room walls, measure the outside of the building. The four corners are then counted twice, so the block count comes out slightly high.
Concrete block sizes
The calculator lists the standard block for each region. The block count depends only on the face of the block, so blocks of any width (wall thickness) with the same face take the same number.
| Block | Face with joints |
|---|---|
| 8 × 8 × 16 in (US) | 16 × 8 in, or 1.125 blocks per ft² |
| 440 × 215 mm (UK and Ireland) | 450 × 225 mm, or about 9.88 blocks per m² |
| 390 × 190 mm (Australia and New Zealand) | 400 × 200 mm, or 12.5 blocks per m² |
US blocks are named by their nominal size, which includes a 3/8 in mortar joint, and are written as width × height × length. The actual block is 3/8 in smaller, such as 7 5/8 × 7 5/8 × 15 5/8 in. CMHA lists nominal widths of 4, 6, 8, 10, 12, 14, and 16 in, all with the same 8 × 16 in face. UK and Australian blocks are named by their actual face size and laid with 10 mm joints.
For half-high blocks (8 × 4 × 16 in) or any other size, choose Custom size and enter the actual length and height, such as 15 5/8 × 3 5/8 in. If you enter the nominal size, set the mortar joint to 0, or the count will come out low.
Corner, end, lintel, and bond beam blocks depend on the layout of the wall. CMHA's guide says to count these separately and subtract them from the total.
How many bags of mortar you need
The calculator lists bags of mortar mix, the premixed sand and cement that only needs water. A bag lays this many blocks:
| Block | Blocks per bag | Source |
|---|---|---|
| 8 × 8 × 16 in | 80 lb: 12, 60 lb: 9, 40 lb: 6 | Sakrete Mortar Mix Type N technical data sheet |
| 440 × 215 mm | 20 kg: 20 | Tarmac Blue Circle Quality Assured Mortar data sheet |
| 390 × 190 mm | 20 kg: 10 | Cement Australia Trade Mortar |
These are guides, and they vary by product and how the blocks are laid. Sakrete's figures are for a full bed of mortar and allow nothing for waste. Quikrete says an 80 lb bag of its Mortar Mix lays up to 13 blocks, and CMHA's guide gives about 16 per 80 lb bag when mortar goes only on the face shells. Check the bag you buy, and if it differs, enter its number as the Blocks per mortar bag.
Tarmac's figure is for 100 mm thick blocks. The same data sheet says 1 m³ of mortar, or 75 bags, lays 2,000 blocks 75 mm thick or 1,000 blocks 150 mm thick. That's about 26.7 and 13.3 per bag, so enter 26 or 13 as the Blocks per mortar bag for those.
For masonry cement that you mix with sand, enter the blocks per bag printed on the cement. CMHA's guide says a 70 lb bag of masonry cement lays about 30 hollow blocks with face shell bedding, and uses about 1 ton of sand for every 8 bags. Buy the sand separately.
Extra blocks and mortar for waste
The waste allowance starts at 10%, following Cement Australia's mortar calculator, which adds 10% to both the blocks and the mortar. It covers cut blocks at the ends of the wall, breakage, and spilled mortar.
CMHA's guide allows 5% for blocks. For a straight wall with few corners and openings, you can lower the allowance toward 5%. For a wall with many corners, returns, or openings, raise it.
Examples
A garden wall
A wall 20 ft long and 6 ft high is 120 ft², or 132 ft² with 10% added. An 8 × 8 × 16 in block covers 16 × 8 in with its joints, or 8/9 ft², so 132 ÷ 8/9 = 148.5, and you need 149 blocks. For mortar, 148.5 ÷ 12 = 12.4, so buy 13 bags of 80 lb mortar mix.
A wall with two windows
Subtract two windows 4 ft × 3 ft from the same wall: 120 − 24 = 96 ft², or 105.6 ft² with 10% added. That's 105.6 ÷ 8/9 = 118.8, so you need 119 blocks and 10 bags of 80 lb mortar mix.
Metric dimensions
A wall 10 m long and 1.8 m high is 18 m², or 19.8 m² with 10% added. With 440 × 215 mm blocks, each covering 0.10125 m², you need 196 blocks and 10 bags of 20 kg mortar. With 390 × 190 mm blocks, each covering 0.08 m², you need 248 blocks and 25 bags.
FAQ
About 1.125 for standard 8 × 8 × 16 in blocks, before waste. Each block with its mortar joint covers 16 × 8 in, or 8/9 ft², so 100 ft² of wall takes 112.5 blocks.
About 9.88 for 440 × 215 mm blocks and 12.5 for 390 × 190 mm blocks, with 10 mm joints and before waste.
About 12 standard 8 in blocks per 80 lb bag of mortar mix, according to Sakrete, and about 20 blocks of 440 × 215 × 100 mm or 10 blocks of 390 × 190 × 190 mm per 20 kg bag. Check the yield printed on the bag you buy.
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