Concrete Block Calculator
A 20 ft by 8 ft wall of 8×8×16 blocks with a 3/8 in joint is 180.00 blocks, 1.125 per square foot, or 189 with 5 percent waste.
Last updated . The face starts at the actual 15 5/8 × 7 5/8 in block. The joint starts at 0.375 in, so the module is 16 × 8 in. Waste starts at 5 percent.
Count cinder blocks and other concrete masonry units from the wall area. Feet and meters both work. The 4, 6, 8, 10, and 12 inch thicknesses share one face, so thickness is not an input.
Segmental retaining-wall blocks are on the retaining wall calculator. Joint mortar by percent of wall volume is on the mortar calculator. Clay bricks are on the brick calculator.
Concrete blocks for a wall
Concrete blocks = net area ÷ (face length + joint) ÷ (face height + joint). The order line adds the waste percent and rounds up. A running-bond half unit at the end of a course is why the waste line exists.
Results
How many concrete blocks do you need?
A US concrete block is sold as 8×8×16, and the unit you pick up is 15 5/8 in long, 7 5/8 in high, and 7 5/8 in thick. The 3/8 in mortar joint brings the module back to 16 by 8 inches. Area divided by that module is the count. Twenty feet by eight feet is 160 square feet, and 160 × 1.125 = 180.00 blocks.
Blocks = net area ÷ (face length + joint) ÷ (face height + joint). The order line adds the waste percent and rounds up, so 180.00 × 1.05 becomes 189. Hollow cores are not mortar. Do not subtract a “solid” volume from a nominal box or you will buy mortar for the holes.
Blocks per square foot
No waste. The US row uses a 3/8 in joint on the actual face. The metric row uses a 10 mm joint on a 390 × 190 mm block. The nominal row already includes its joint, so the joint input must be 0.
| Block | Module | Per ft² | Per m² | Example |
|---|---|---|---|---|
| 8×8×16 | 16 × 8 in | 1.125 | 12.11 | 20 × 8 ft = 180.00 |
| 390 × 190 mm | 400 × 200 mm | 1.16 | 12.50 | 10 m² = 125.00 |
| 100 ft² of 8×8×16 | 16 × 8 in | 1.125 | 12.11 | 112.50 before waste |
Worked example
Leave the wall at 20 ft by 8 ft, the size on the US 8×8×16 face, the joint at 0.375 in, openings at 0, and waste at 5 percent. Blocks in the area read 180.00. The order reads 189. Per square foot reads 1.13, because 1.125 rounds to 1.13 on the two-decimal line, and per square meter reads 12.11. Net area reads 14.86 m², which is 160 square feet.
Frequently asked questions
How many cinder blocks do I need?
Divide the wall area by the block face plus the mortar joint. A standard 8×8×16 block is 15 5/8 by 7 5/8 inches, and a 3/8 inch joint makes a 16 by 8 inch module. A 20 ft by 8 ft wall is 180.00 blocks. Five percent waste orders 189.
How many concrete blocks per square foot?
The 16 by 8 inch module is 1.125 blocks per square foot, which is 12.11 per square meter. A 100 square foot wall is 112.50 blocks before waste, and 119 once 5 percent is added. The 4 inch, 6 inch, 8 inch, 10 inch, and 12 inch block widths share that same face.
How many 8x8x16 blocks for a 20 by 8 foot wall?
That is the default result: 180.00 blocks in the wall and an order of 189 at 5 percent. The wall is 160 square feet. Twenty feet divided by 16 inches is 15 blocks per course, and 8 feet divided by 8 inches is 12 courses. 15 times 12 is 180.
How many blocks are in a metric wall?
A 390 × 190 mm block with a 10 mm joint lays out as 400 × 200 mm, which is 12.50 blocks per square meter. Ten square meters is 125.00 blocks before waste. If you type a 400 × 200 mm face and leave the joint at 10 mm you will double-count the joint, so set the joint to 0 for a nominal face.
How much mortar do concrete blocks need?
This page counts blocks. It does not fill the hollow cores with mortar, because that would order far too much. Block mortar is the bed and head joints, often about 10 to 15 percent of the wall volume on the mortar calculator. A retaining-wall block and its gravel base are a different unit, on the retaining wall calculator.
Do corners and half blocks change the count?
The 180.00 is a full grid. A corner that turns the wall needs a block on the return, and a bond beam or lintel is extra. Five percent, the step from 180.00 to 189, covers a few halves and a broken block. A wall with several openings should have those areas subtracted first.
Is a cinder block the same as a concrete block?
For counting, yes. Both names point at the same 8×8×16 module in the US. The face math does not change if the aggregate is cinder, pumice, or dense concrete. Weight and fire rating do change, and this page does not estimate either one.