Concrete Block Calculator
Estimate 8 × 8 × 16 concrete blocks and mortar bags for a wall. Deduct openings and add waste before ordering.
Blocks needed
189
- Net wall area
- 160 sq ft
- Blocks (8 × 8 × 16 in)
- 189
- 80 lb mortar bags
- 7
- Estimated material cost
- $542.50
Uses nominal 8 × 8 × 16 in block (0.89 sq ft face), 30 blocks per 80 lb mortar bag and a 5% waste allowance. Core fill, rebar and lintels are not included.
Concrete block is sold by the piece, and the count depends on the nominal face size, not the actual block. This calculator uses the standard 8 × 8 × 16 inch module, deducts openings, and adds a waste allowance for cuts and corners.
It also estimates 80 lb mortar bags at 30 blocks per bag. Core fill, rebar, lintels and bond beams are separate items; a structural block wall needs an engineered design, not just a block count.
How the block calculator works
Net wall area = length × height − openings. A nominal 8 × 8 × 16 inch block covers 16/12 × 8/12 = 0.889 sq ft of wall including the mortar joint. Blocks = net area × (1 + waste) ÷ 0.889, rounded up. Mortar bags = blocks ÷ 30, rounded up.
Worked example
A 20 × 8 ft wall is 160 sq ft. With 5% waste, 160 × 1.05 ÷ 0.889 = 189 blocks. At 30 blocks per bag that is 7 bags of mortar.
Nominal vs actual block size
An 8 × 8 × 16 block is called that because the mortar joint brings it up to the nominal module. The actual block is about 7-5/8 × 7-5/8 × 15-5/8 inches. That is why dividing the wall area by the actual face size overcounts.
Use the nominal face of 16 × 8 inches for estimating, then confirm the coverage printed on the pallet or quoted by the supplier. Split-face, sash and corner blocks can change the count.
Mortar, grout and core fill
- A standard 80 lb bag of mortar lays about 30 blocks with a 3/8 inch joint.
- Hollow block walls may be partially or fully grouted for strength, fire rating or anchorage. Grout volume is separate from mortar.
- Vertical rebar in grouted cells needs to be specified by an engineer; the block count does not include it.
- Lintels over openings, bond beams and pilasters are additional materials.
Block types and typical uses
| Block | Typical use | Notes |
|---|---|---|
| Hollow CMU | Walls, foundations | Most common; cells can be grouted |
| Solid block | Piers, garden walls | Heavier; fewer cores |
| Split-face | Decorative walls | Textured face; count corners separately |
| Retaining wall block | Landscape walls | Interlocking; not standard CMU |
Frequently asked questions
How many 8 × 8 × 16 blocks are in a square foot?
About 1.125 blocks per square foot of wall, including the mortar joint. A 100 sq ft wall needs about 113 blocks before waste.
How many blocks do I need for a 10 × 10 wall?
A 10 × 10 ft wall is 100 sq ft. At 1.125 blocks per square foot and 5% waste, order about 118 blocks.
How many bags of mortar for 100 blocks?
About 3.3 bags of 80 lb mortar at 30 blocks per bag. Buy 4 bags to allow for waste and a little extra.
Do I need rebar in a concrete block wall?
Structural walls usually require vertical and horizontal reinforcement in grouted cells, designed by an engineer. A low garden wall may not, but local code still applies.
How thick is a standard block wall?
A standard hollow CMU is 8 inches nominal, so the finished wall is about 8 inches thick plus finish. Six-inch and 12-inch blocks are also available.
Can I build a block wall on an existing slab?
Not without checking the slab thickness, reinforcement and edge support. A block wall needs a proper footing below frost depth or an engineered connection to the slab.
Assumptions and accuracy
- Dimensions are converted to feet before the calculation; 1 ft = 12 in = 0.3048 m.
- Waste allowances are estimates. Add more for irregular shapes, many cuts and uneven subgrade.
- Bag yields, coverage rates and material densities vary by product. Check the label or ask your supplier.
- Local building codes and site conditions always take precedence over a general-purpose calculator.
- Read our full methodology