Formula
Blocks are the wall area times 1.125, then rounded up after waste. The 1.125 is 144 square inches divided by a 16 × 8 inch module. The actual unit is about 15-5/8 by 7-5/8, and the rest of the module is the 3/8 inch joint.
full bed, in³ per block = 15.625 × 7.625 × 0.375 + 7.625 × 7.625 × 0.375
face shell = 2 × 1.25 × 0.375 × (15.625 + 7.625)
grout = blocks × share × cubic feet per block
Full bed is the entire bed joint plus one head joint, 66.48 in³, about 0.0385 ft³. Face shell uses two 1.25 inch shells, the ASTM C90 minimum face-shell thickness for an 8 inch unit, and lands near 21.8 in³. Mortar waste is applied after that, and bags round up. Grout defaults to 0.195 ft³ for both cores of one block. Every other cell uses half of that. Vertical bars are the cell columns, not a weight. Weigh them on the rebar calculator. Footings are the concrete calculator.
Worked examples
10 × 8 ft, no block waste. 80 ft² × 1.125 = 90 blocks exactly.
Those 90 blocks, full bed, no mortar waste. 90 × 66.48 / 1,728 = 3.46 ft³ of mortar.
100 ft². 112.5 blocks before rounding, so the order is 113 with no waste. Grout at 0.195 for every cell is 112.5 × 0.195 ft³, using the exact count rather than the rounded order.
Reference
Joint geometry for one nominal 8 × 16 module. The bag yield is editable because 0.66 ft³ was not confirmed against a sack.
| Bed | in³ per block |
|---|---|
| Full bed and head | 66.48 |
| Two 1.25 in face shells | 21.80 |
Questions
Why 1.125 blocks per square foot?
A nominal module is 16 by 8 inches, including the 3/8 inch joint. That is 128 square inches, and 144 / 128 is 1.125. A 10 by 8 foot wall is exactly 90 blocks.
Is the mortar yield from a bag I can buy?
The default 0.66 cubic feet per bag is a planning figure, not a data sheet this page confirmed. Type the yield printed on the bag you are holding.
What does the grout number assume?
0.195 cubic feet fills both cores of one 8 inch two-core block, for one course. That is a planning volume, about 37% of the gross module, not an ASTM C90 net area. Face-shell bedding is a separate toggle and uses two 1.25 inch shells.
Sources
- Nominal CMU module 16 × 8 in, including a 3/8 in mortar joint. Actual unit about 15-5/8 × 7-5/8 in.
- ASTM C90 minimum face-shell thickness of 1.25 in for an 8 in unit, used only for the face-shell mortar toggle.
- Grout at 0.195 ft³ for both cores, and mortar at 0.66 ft³ per bag, are planning defaults. The bag chart wins.
- Brick calculator for a clay brick wythe. This page is CMU.