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A standard concrete masonry unit — the concrete block most people still call a cinder block — is 7⅝ × 7⅝ × 15⅝ in and lays up to 8 × 8 × 16 in with its 3/8 in mortar joints. Its face covers 128 sq in, so a square foot of wall takes 144 ÷ 128 = 1.125 blocks, or 112.5 per 100 sq ft, and the count is the same for 4, 6, 8, 10 and 12 in wide block. The Concrete Block Calculator multiplies the wall length by its height, subtracts typical window and door openings, multiplies by 1.125 and adds a waste allowance, usually 5%. It shows the layout — 8 in courses and 1.5 blocks per lineal foot — and the mortar, from QUIKRETE's published yield of up to 13 standard blocks per 80 lb bag of Mortar Mix plus 15% waste. A 40 × 8 ft wall is 320 sq ft: 12 courses of 30 blocks, 360 blocks or 378 with 5%, and 32 bags of mortar.
A standard concrete block (CMU, often called a cinder block) is 7⅝ × 7⅝ × 15⅝ in and lays up to 8 × 16 in on the face with its 3/8 in joints, so a square foot of wall takes 144 ÷ 128 = 1.125 blocks — 112.5 per 100 sq ft, whatever the width of the block. A course is 8 in tall and takes 1.5 blocks per lineal foot. QUIKRETE Mortar Mix lays up to 13 standard blocks per 80 lb bag; add 15–25% for mortar waste (BIA Technical Note 10).
| Wall | Area | Courses | Blocks | 80 lb mortar bags |
|---|---|---|---|---|
| 10 × 4 ft | 40 sq ft | 6 | 45 | 4 |
| 20 × 4 ft | 80 sq ft | 6 | 90 | 8 |
| 20 × 8 ft | 160 sq ft | 12 | 180 | 16 |
| 24 × 8 ft | 192 sq ft | 12 | 216 | 20 |
| 30 × 8 ft | 240 sq ft | 12 | 270 | 24 |
| 40 × 8 ft | 320 sq ft | 12 | 360 | 32 |
| 40 × 10 ft | 400 sq ft | 15 | 450 | 40 |
| 100 × 6 ft | 600 sq ft | 9 | 675 | 60 |
Multiply the wall length by its height in feet, subtract any openings, and multiply by 1.125 — a standard 8 × 16 in block face (7⅝ × 15⅝ in plus 3/8 in joints) covers 128 sq in, so there are 1.125 blocks per square foot or 112.5 per 100 sq ft. Add about 5% for breakage and cuts. A 40 × 8 ft wall takes 360 blocks, 378 with 5%.
1.125 standard 8 × 16 in blocks per square foot. The width of the block — 4, 6, 8, 10 or 12 in — does not change the count, because every size has the same 8 × 16 in nominal face.
1.5 blocks per lineal foot, because each block is 16 in long including its joint. A 40 ft wall takes 30 blocks per course, and each course is 8 in tall, so an 8 ft wall is 12 courses — 360 blocks.
QUIKRETE's Mortar Mix data sheet gives an 80 lb bag as enough for up to 13 standard 8 × 8 × 16 in blocks. Add 15–25% for mortar waste: 360 blocks take 32 bags with 15%.
Today, yes in practice. True cinder block used coal cinders as aggregate and is rarely made now; what stores sell as cinder block is concrete masonry unit (CMU) made to ASTM C90 with sand, gravel or lightweight aggregate. The calculator works for both because the face size is the same.
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