How to Calculate Concrete Blocks
Concrete masonry units (CMU) are sold as individual blocks. Calculate quantity by dividing wall surface area by the face area of one block plus mortar joints. Add 5 to 10 percent for cuts, breakage, and end-of-course waste.
Block Count Formula
One standard 8x8x16 CMU (actual 7-5/8 x 7-5/8 x 15-5/8 in) plus a 3/8 in mortar joint covers 8 x 16 = 128 sq in, or 0.889 sq ft.
Blocks=Wall Areasq ft0.889
For every 100 sq ft of wall, plan on about 113 standard blocks (1.125 blocks per sq ft).
Mortar Volume Formula
Mortarcu ft=Wall Areasq ft×0.08
A 3/8 in mortar joint consumes roughly 0.08 cu ft per sq ft of wall, which works out to 8 to 9 cu ft per 100 sq ft (about 10 bags of 80 lb premix).
Common Block Sizes
| Nominal | Actual | Per 100 sq ft |
|---|---|---|
| 4 x 8 x 16 | 3-5/8 x 7-5/8 x 15-5/8 | 113 |
| 6 x 8 x 16 | 5-5/8 x 7-5/8 x 15-5/8 | 113 |
| 8 x 8 x 16 | 7-5/8 x 7-5/8 x 15-5/8 | 113 |
| 12 x 8 x 16 | 11-5/8 x 7-5/8 x 15-5/8 | 113 |
All Widths Cover the Same Area
Block width changes (4, 6, 8, 12 in) but face size stays 8 x 16 in. Blocks per sq ft is the same for any width. Wider blocks are used for taller walls, foundations, and fire ratings.Mortar Required
- 3/8 in joint (standard) - 7 to 9 cu ft per 100 sq ft
- 1/2 in joint - 9 to 11 cu ft per 100 sq ft
- Type S premix (80 lb) - ~0.6 cu ft, covers ~27 blocks
Wall Reinforcement
| Element | Size / Spacing |
|---|---|
| Vertical rebar | #4 @ 48 in OC, grout-filled cells |
| Horizontal rebar | #4 every 4 courses, or bond beams at 4 ft |
| Truss/ladder wire | Every 2 courses |
| Bond beam | Top of wall, at openings (2 #4 bars) |
Seismic and High-Wind Zones
In SDC D, E, or F (California, Alaska, parts of Pacific Northwest) or high-wind Gulf Coast zones, reinforcement requirements increase significantly. Consult local code or a structural engineer.Lightweight vs Normal Weight CMU
Lightweight block weighs 22 to 28 lb (vs 38 lb for normal) and is easier to lift but costs 15 to 30 percent more. Both have the same structural capacity when properly reinforced and grouted.