Concrete block calculator
How many standard 8 × 8 × 16 concrete (cinder) blocks and bags of mortar a wall takes, less doors and windows.
Blocks to buy
189 blocks, 8 × 8 × 16
- Mortar mix
- 15 × 80 lb bags
- Wall area
- 160 sq ft
- Courses
- 6 × 30 blocks
- Full wall
- 180 blocks
- Openings
- − 0 blocks
- Estimated cost
Includes 5% extra. Mortar uses the bag's "up to 13 blocks" yield, so buy a bag or two more for thick joints. For a structural or retaining wall, follow local code and the engineer's plans for reinforcement and grout.
How to calculate concrete blocks
A standard block is sized so that, with its mortar joint, it takes up 8 × 16 in of wall. So count the courses (rows) up the wall and the blocks along each course.
- Courses = wall height (in) ÷ 8, rounded up
- Blocks per course = wall length (in) ÷ 16, rounded up
- Blocks = courses × blocks per course, less whole blocks for openings (1.125 per sq ft), plus extra
- Mortar = blocks ÷ 13, rounded up, in 80 lb bags
Worked example
Your 40 ft wall is 480 in long, or 30 blocks per course, and 48 in high, or 6 courses: 6 × 30 = 180 blocks. With 5% extra, buy 189 blocks and 189 ÷ 13 = 15 bags of mortar mix.
Blocks for a wall by length and height
Standard 8 × 16 in blocks in whole 8 in courses, no openings or extra. Mortar at up to 13 blocks per 80 lb bag (QUIKRETE Mortar Mix (No. 1102) data sheet (revised November 2020)).
| Wall length | 2 ft high | 4 ft high | 6 ft high | 8 ft high |
|---|---|---|---|---|
| 8 ft | 18 (2 bags) | 36 (3 bags) | 54 (5 bags) | 72 (6 bags) |
| 16 ft | 36 (3 bags) | 72 (6 bags) | 108 (9 bags) | 144 (12 bags) |
| 24 ft | 54 (5 bags) | 108 (9 bags) | 162 (13 bags) | 216 (17 bags) |
| 40 ft | 90 (7 bags) | 180 (14 bags) | 270 (21 bags) | 360 (28 bags) |
| 100 ft | 225 (18 bags) | 450 (35 bags) | 675 (52 bags) | 900 (70 bags) |
Concrete block questions
How many concrete blocks are in a square foot?
1.125. Each standard block covers 8 × 16 in of wall including its mortar joint, which is 0.889 sq ft, so 100 sq ft of wall takes 112.5, or 113 whole blocks.
How many bags of mortar for 100 blocks?
About 8 bags of 80 lb mortar mix: the maker's figure is up to 13 standard blocks per bag. "Up to" assumes neat joints, so a beginner or a wall with thick joints will use more.
Is a cinder block the same as a concrete block?
For buying purposes, yes. "Cinder block" is the old name from when blocks used cinders (coal ash) as the aggregate; today's gray blocks are concrete masonry units (CMU). The standard 8 × 8 × 16 block is counted the same way either way.
How many blocks do I need for a 4 ft high wall?
A 4 ft wall is 6 courses, so each foot of wall length takes 4.5 blocks: 48 blocks for a 10 ft run and 180 for 40 ft, before extra. The table above has other sizes.
Do I fill the cores?
Freestanding garden walls often go unfilled; foundation, retaining and load-bearing walls usually need rebar and grout in some or all cores, as the plans and local code require. This calculator doesn't count grout or rebar, so add those from your plans.