Concrete block calculator
How many 8 × 8 × 16 concrete blocks (CMU) a wall needs, and how many bags of mortar to lay them.
- Wall area
- Per sq ft
- 80 lb mortar bags
How to use it
Enter the wall's length and height in feet. The block size is already set for a standard 8 × 8 × 16 in block, which actually measures 7⅝ × 7⅝ × 15⅝ in so that it comes out to an even 8 × 16 with a ⅜ in mortar joint. Change the size for half-high blocks or other units.
How it's calculated
- Courses (rows) = wall height in inches ÷ (block height + joint), rounded up.
- Blocks per course = wall length in inches ÷ (block length + joint), rounded up.
- Blocks = courses × blocks per course, plus the breakage allowance.
- Mortar bags = blocks ÷ 13, rounded up.
A standard block with its joint covers 8 × 16 in, so a wall takes 1.125 blocks per square foot.
| Wall | Blocks (no extra) | 80 lb mortar bags |
|---|---|---|
| 20 × 4 ft | 6 courses × 15 = 90 | 7 |
| 40 × 4 ft | 6 × 30 = 180 | 14 |
| 20 × 8 ft | 12 × 15 = 180 | 14 |
About the mortar estimate
The bag count uses Quikrete's own label figure: an 80 lb bag of Mortar Mix lays up to 13 standard blocks. Masons' rules of thumb are much leaner, often about 3 bags per 100 blocks, partly because many mix their own mortar from cement and sand. The label figure is the safe side: you may have a bag or two left over, but you won't run out mid-course.
Tips
- Openings: the count doesn't subtract doors or windows. For a large opening, subtract its area ÷ 0.89 sq ft per block, then add back blocks for the cut pieces around it.
- Corners and ends: many walls need corner blocks or half blocks at the ends. Count them separately when you order.
- Structural walls need footings, reinforcing steel and grout as the local code requires. This calculator only counts blocks and mortar.