How this calculator works
A standard concrete block is nominally 8 in tall by 16 in long (the actual block is 3/8 in smaller each way, to leave room for the mortar joint). One block therefore covers 128 in², or about 0.89 ft² of wall. We divide your net wall area by that, add waste, and round up.
Worked example: a 20 ft wall, 4 ft high: 80 ft² ÷ 0.889 = 90 blocks, or 95 with 5% waste. Four feet is six courses of 8 in blocks, with 15 blocks in each course.
Mortar varies with joint thickness and how well it is applied. We use about 35 standard blocks per 80 lb bag as a planning figure.
Tips before you buy
- Buy corner or end blocks (and bond-beam or cap blocks where needed) separately. They are not interchangeable with standard blocks in a count.
- Retaining walls and anything holding back soil need engineering and reinforcement. Do not rely on a quantity calculator for the design.
- Wall width (4, 6, 8 or 12 in) does not change the block count, only the weight and cost.
Tools & materials for this project
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Frequently asked questions
How many cinder blocks do I need for a wall?
Divide the wall area in square feet by 0.89, since one 8×16 block covers about 0.89 ft², then add 5% for waste. Our calculator does exactly that.
How many blocks are in a course?
Divide the wall length in inches by 16. A 20 ft wall is 240 in, so 15 blocks per course.
What is the difference between cinder block and concrete block?
True cinder blocks used coal cinders and are rarely made now. Modern "cinder blocks" are concrete masonry units (CMU), which is what this calculator counts.
Related calculators & guides
Estimates only. Results assume standard materials and simple shapes. Supplier products vary, and the final call on quantity, structure and safety is yours. Check your local code and your supplier's guidance before ordering.