Concrete masonry units — blocks, in plain English — are sold by the count, not the square footage. Most homeowners and even a fair number of contractors miscount their first job because they confuse nominal size (the name on the pallet) with the actual face dimension. Here's the fix.

The 1.125 rule

Standard 8×8×16 CMU has a face dimension of 16 in × 8 in with a 3/8 in mortar joint baked in. That works out to 0.889 ft² per block face, which means 1.125 blocks per square foot of wall. So a wall 30 ft long × 8 ft tall = 240 ft² × 1.125 = 270 blocks. Add 5% waste for breakage and cuts and you order 284.

The block depth (4, 6, 8, or 12 in) doesn't change the face math — only the wall thickness, the rebar spacing, and the block weight. A 4×8×16 partition block has the same 1.125 ratio as an 8×8×16 structural block.

Mortar — 3 bags per 100 blocks

Standard estimate is three 80-lb bags of Type S mortar per 100 blocks at a 3/8 in joint. Type S is the spec for load-bearing CMU walls (1,800 psi compressive strength). For below-grade foundations or high-seismic zones, an engineer may upgrade to Type M (2,500 psi). Type N (750 psi) is too weak for structural CMU and should not be used.

The CMU block calculator automatically returns mortar bags alongside block count.

Rebar — when and how much

Most jurisdictions require rebar in any CMU wall over 8 ft tall, every retaining wall, every basement wall, and every wall in seismic zones D1 or D2 (most of California, Oregon, Washington, parts of Utah and Nevada, and pockets of the East). Per IRC R606 and IBC Chapter 21, the typical non-engineered residential pattern is:

  • #4 (½ in) vertical rebar at 4 ft on center grouted into the cells, plus one extra at each corner and opening jamb
  • #4 horizontal in every fourth course bond beam (every 32 in vertically), tied to the verticals

Cells around rebar must be grouted solid with masonry grout (not mortar — different mix, higher slump, pumpable). Plan on roughly 1 ft³ of grout per 8 vertical bars for an 8-ft wall.

Bond pattern affects the count

Running bond — the half-block offset every course — is the standard pattern and what the calculator assumes. Stack bond (every block aligned vertically) needs more horizontal reinforcement because there's no interlocking shear path between courses. Soldier courses, decorative caps, and sailor pieces usually take their counts in pairs and burn extra cuts.

Common errors

Forgetting half-block at the ends. Every wall in running bond ends with half-blocks at every other course. If you don't count them as full blocks (which is what suppliers sell — you cut them on site), you'll come up short. The calculator's 5% waste covers this.

Skipping the bond beam. A wall over 8 ft tall without horizontal reinforcement at the top course is a hazard during framing — the top can rack out of plumb under wind load before the slab or roof ties it down. Run a bond beam regardless of code minimums.

Mixing mortar and grout. Both are cement products but they're not the same. Mortar bonds blocks together at the bed and head joints; masonry grout fills the cells around rebar. Grout has higher water content and a finer aggregate so it pumps and self-consolidates.

Quick FAQ

How many CMU blocks per pallet? 90-108 for 8×8×16 standard, depending on supplier. Pallets weigh ~3,500 lb each — order curb delivery with the truck's lift gate, not in your pickup.

Can I dry-stack CMU? Surface bonding cement (parged on both faces) lets you dry-stack a non- structural wall, but every load-bearing or code-regulated wall needs traditional mortar joints.

What's the difference between 8x8x16 and 8x16x16? 8x8x16 is the standard size; 8x16x16 (sometimes called "double-height") doubles the face height to 16 in, cuts your block count in half, and is used in bond beams and lintels. They're not interchangeable in pattern.

Estimate only. The CMU block calculator uses standard non-engineered residential rules. Load-bearing walls, retaining walls over 4 ft, basement walls, and any design in a seismic zone need a structural engineer's stamp — verify before ordering or pouring.