Concrete Block Calculator
Enter the length and height of your wall and subtract any doors or windows. You get the number of concrete blocks (CMU) and bags of mortar.
Last updated November 13, 2026
How many concrete blocks do I need?
A standard block has a nominal face of 8 × 16 inches, which is 0.89 square feet. Divide your wall area by 0.89, or multiply it by 1.125, to get the number of blocks.
Example: a wall 20 ft long and 6 ft high is 120 sq ft. 120 × 1.125 = 135 blocks. With 5% for cuts and breakage, buy 142.
Blocks by wall area
| Wall area | Blocks (exact) | Blocks + 5% | 80 lb mortar bags |
|---|---|---|---|
| 50 sq ft | 56.2 | 60 | 5 |
| 100 sq ft | 112.5 | 119 | 10 |
| 150 sq ft | 168.8 | 178 | 14 |
| 200 sq ft | 225 | 237 | 19 |
| 300 sq ft | 337.5 | 355 | 28 |
| 500 sq ft | 562.5 | 591 | 46 |
Blocks for common walls
| Wall (length × height) | Area | Blocks + 5% |
|---|---|---|
| 10 ft × 4 ft | 40 sq ft | 48 |
| 20 ft × 4 ft | 80 sq ft | 95 |
| 20 ft × 6 ft | 120 sq ft | 142 |
| 30 ft × 6 ft | 180 sq ft | 213 |
| 40 ft × 8 ft | 320 sq ft | 378 |
Mortar for block walls
The QUIKRETE Mortar Mix data sheet gives up to 13 standard 8 × 8 × 16 inch blocks per 80 lb bag. “Up to” is a best case, so round up and add a bag. See how much mortar per block? for bricks and water.
Getting the count right
- Subtract openings for doors and windows, but keep a few extra blocks for the cuts around them.
- Corners and ends need half blocks or cut blocks. A 5% to 10% allowance covers these.
- Check the block you are buying. Half-high and specialty blocks cover a different area. Pick the face size in the calculator.
- Footing first. A block wall sits on a concrete footing. Size it with the concrete footing calculator.
How many cinder blocks do I need?
Cinder block, concrete block and CMU are three names people use for the same wall unit, so the count works the same way. A standard block has a face of 8 × 16 inches including the mortar joint, which means 1.125 blocks cover one square foot of wall.
Worked example. A wall 20 ft long and 4 ft high is 80 square feet. 80 × 1.125 = 90 blocks. You can check it by courses: 4 ft is 6 courses, and 20 ft is 15 blocks per course, so 6 × 15 = 90. Add 5% for cuts and breakage and buy 95.
- Subtract door and window openings from the wall area before you count.
- Corners and wall ends need half blocks or corner blocks. Ask your supplier which they stock.
- The wall needs a footing under it. Size the concrete with the footing calculator.
- If the plans call for filled cores, rebar or bond beams, those quantities come from the engineer’s drawings, not from a block count.
Frequently asked questions
How many concrete blocks do I need?
Divide the wall area in square feet by 0.89, the face area of a standard 8 × 16 inch block. That is 1.125 blocks per square foot. A 20 ft × 6 ft wall is 120 sq ft and needs 135 blocks before waste.
How many blocks are in 100 square feet of wall?
112.5 standard 8 × 8 × 16 inch blocks, so buy 113 plus your waste allowance.
What is the difference between a CMU, a cinder block and a concrete block?
CMU stands for concrete masonry unit, the trade name for a concrete block. Cinder block is an older name that many people still use for the same thing. All three are counted the same way.
How much mortar do I need for concrete blocks?
The manufacturer’s data sheet says one 80 lb bag of mortar mix lays up to 13 standard blocks. For 135 blocks that is 11 bags, and it is wise to buy one extra.
What size is a standard concrete block?
The nominal size is 8 × 8 × 16 inches. The block itself is about 3/8 inch smaller each way, which leaves room for the mortar joint.
How many cinder blocks do I need for a 20 ft wall?
90 blocks for a wall 20 ft long and 4 ft high using standard 8 × 16 inch blocks. Add 5% for cuts and breakage.