CALCULATION METHOD
How many concrete blocks do I need?
Find the gross wall area, subtract the total area of doors and windows, and divide by the nominal face area of one block. The calculator rounds the layout count up, adds the selected waste allowance, and rounds the purchase quantity up again to a whole block.
Net wall area = gross area − openings
Layout blocks = net area ÷ nominal block face area
Blocks to order = layout blocks × (1 + waste), rounded up
Mortar bags = order blocks ÷ supplier yield, rounded up
Concrete block wall example
A 20 ft × 8 ft wall has 160 sq ft of face area. A nominal 16 × 8 in block covers 0.889 sq ft, so the layout requires 180 blocks. Adding 5% waste gives an order of 189 blocks. At 13 blocks per 80 lb mortar bag, the estimated mortar order is 15 bags.
Common concrete block face sizes
Block thickness affects the wall assembly but not the visible face count when length and height stay the same. Select the nominal face module shown by your plan or supplier.
| Nominal face | Face area | Blocks per sq ft |
|---|---|---|
| 16 × 8 in standard CMU | 0.889 sq ft | 1.125 |
| 8 × 8 in half block | 0.444 sq ft | 2.25 |
| 16 × 4 in | 0.444 sq ft | 2.25 |
| 12 × 8 in | 0.667 sq ft | 1.50 |
Nominal size, actual size, and mortar joints
For standard CMU layout, nominal dimensions represent the repeating wall module. A nominal 8 × 8 × 16 in unit is commonly manufactured smaller so a 3/8 in mortar joint brings the installed module to its nominal size. Do not add another joint to a nominal face dimension.
Why mortar yield is editable
Bag yield changes with product, bag weight, block geometry, joint thickness, workmanship, and waste. The default follows the published yield for an 80 lb QUIKRETE Mason Mix bag and a standard 8 × 8 × 16 in block. Replace it when your selected product lists a different yield.
QUIKRETE Mason Mix data sheet, QUIKRETE block-wall guidance, and Sakrete Type S Mortar yield. Accessed September 11, 2026.