A block wall takes three orders, not one: the blocks, the mortar between them, and the grout poured into the cores. Nail the face count, then layer mortar and grout on top. This guide runs from block sizes to rebar, code, and cost — distilling the 100 questions masons and DIYers ask most into one readable pass.
CMU stands for Concrete Masonry Unit — the engineering term for a standard concrete block. A full-size block has actual dry dimensions of 7-5/8″ × 7-5/8″ × 15-5/8″. Add a 3/8″ mortar joint and it rounds to a clean nominal 8″ × 8″ × 16″, which is what calculators use because it tiles into a simple grid.
That nominal 8″ × 16″ face is 128 square inches, or 0.888 square feet — so a wall needs exactly 1.125 blocks per square foot (about 12.5 per square meter). The one quirk worth memorizing: blocks come in several widths (4, 6, 8, 10, 12″), but the face stays 8″ × 16″, so a wider wall uses the same number of blocks — only the mortar and grout volumes change.
Area = 40 × 8 = 320 sq ft. Blocks = 320 × 1.125 = 360 blocks. Add 10% waste → 396, then round up to whole pallets (~72–90 each) → order 5 pallets.
For any rectangular wall, multiply linear length by height for the area, then by 1.125. Real walls then need a few tweaks. Openings: subtract each door and window (width × height) from the gross area — but skip anything under about 4 square feet, since those extra blocks cover half-cuts and corner bonding. L-shaped corners: total both runs, subtract the overlapping corner block so you don't double-count, then multiply by height. T-junctions: figure the main run first, then measure the intersecting wall from the face of the main wall.
A few specific layouts to keep straight:
Mortar is the paste that bonds the block faces; it scales with block count, not wall area. Type S (minimum 1,800 PSI) is standard for structural blockwork, mixed at the usual 1 part cement to 3 parts sand. Wider blocks have thicker webs and edges, so they need a bit more bed-joint mortar.
| Material | Per 100 blocks | Per 1,000 blocks |
|---|---|---|
| Mixed wet mortar | 4–4.5 cu ft | 40–45 cu ft |
| Masonry cement | 1–1.25 bags | 11–13 bags |
| Masonry sand | 0.15 cu yd (~400 lb) | 1.5 cu yd (~4,000 lb) |
| Pre-mixed 80 lb bag | lays 10–12 blocks | — |
Grout is different from mortar — it's a fluid concrete mix poured into the hollow cores to encase rebar and turn the wall into a solid structural unit. The two are not interchangeable. Grout is where wall thickness finally matters, because the cavities get bigger:
| Fill | Per block | Per 100 blocks |
|---|---|---|
| 8″ block, all cores | 0.33–0.40 cu ft | 1.2–1.5 cu yd |
| 12″ block, all cores | 0.55–0.60 cu ft | ~2.2 cu yd |
| 8″ bond beam | 0.40–0.45 cu ft / linear ft | — |
You rarely fill every core. Partial grouting fills only the vertical cores that hold rebar (say every 32 or 48″), which slashes the grout volume: count the grouted core lines, multiply by the volume per core, then by wall height. Use fine grout (sand only) for tight cavities, or coarse grout with 3/8–1/2″ pea gravel where cores are roomy. Plan a 10–15% grout waste factor — fluid grout seeps into surface variations and through small joint gaps.
Two kinds of steel go into a block wall. Vertical rebar (typically #4 or #5 for residential) sits in grouted cores: divide wall length by the spacing interval (32, 40, or 48″) and add one for the starting bar, then multiply the bar count by wall height — and add 15–20% for foundation dowel lap splices.
Horizontal joint reinforcement (ladder/truss wire, "Laddertrack") is laid in the bed joints to control shrinkage cracking and add lateral strength — code typically wants it every 16″ vertically (every second course). Multiply wall length by the number of reinforced courses for total linear footage. For the math behind the bars themselves, see our rebar estimating guide.
A waste factor covers blocks broken in shipping, cut into halves, or lost to onsite errors. Running short halts the job and triggers an expensive small redelivery, so always round up to the nearest full pallet.
| Situation | Waste factor |
|---|---|
| Straight, uniform wall | 5–10% |
| Many corners, arches, custom cuts | 15% |
| Core-fill grout | 10–15% |
Plan wall lengths in multiples of 8 inches (the half-block module) so courses land on whole and half blocks. It all but eliminates custom cuts — the single biggest source of block waste and labor.
Most variants share the standard 8″×16″ nominal face and estimate normally — the difference is finish, weight, or function:
Finally, mind load-bearing vs. non-load-bearing: load-bearing units have thicker walls and webs for structural weight, while thinner non-load-bearing blocks are for light interior partitions.
A standard 8″ block weighs about 35–38 lb, which adds up fast for footing and lintel design. Load-bearing CMU must meet ASTM C90 (minimum 1,900 PSI compressive strength). Grout roughly doubles a wall's weight:
| Item | Weight |
|---|---|
| Single 8″ block | 35–38 lb |
| Empty 8″ wall | ~30 lb / sq ft |
| Fully grouted 8″ wall | 75–80 lb / sq ft |
Unreinforced block walls can buckle or tip under wind and soil pressure, which is why code caps their height and rebar + grout supply the missing tensile strength. Other items in a full takeoff: wall ties (one per 2.67 sq ft) to anchor a veneer to a block backup, control joints (sealant-filled vertical gaps every 20–25 ft for thermal movement), pilasters (integrated columns — count their special blocks and add their deep cores to grout), weep holes for drainage, a damp-proof membrane at the frost line, and lintels sized to the opening width plus 8″ of bearing each side. Seismic hook details need engineered designs, not a basic calculator.
Total cost is the block, mortar, sand, and grout quantities times local prices, plus delivery and tax. Blocks ship on pallets of roughly 72–90 units, so order in whole-pallet increments.
| Item | Typical figure |
|---|---|
| Standard 8″ block (material) | $1.50–$3.00 each |
| Bricklaying labor | $2.00–$4.50 per block (or $10–$25 / sq ft) |
| Mason productivity | 100–150 blocks per 8-hour day |
| Pallet (cube) | ~72–90 blocks |
Essential DIY tools: a block trowel, a line and line blocks for level courses, a jointer to finish the mortar, a rubber mallet, and a wet masonry saw for clean cuts. On grouted walls over about 5 feet, leave cleanout openings in the bottom course so you can clear mortar droppings from the cores before the pour.
Let the laid wall's mortar cure 24–48 hours before pouring core grout, so the joints can resist the grout's hydraulic pressure. Like concrete, the assembly reaches ~70% strength in 7 days and full strength at 28 days. Protect a fresh wall: cover it with plastic if rain threatens, since a downpour washes cement out of unset joints and streaks the faces. Common issues and their estimate impact:
Manual counts and online calculators can differ because calculators fold in the 3/8″ joint automatically. The fast gut-check is the same shortcut as the core formula:
On site, confirm wall measurements with a steel tape, re-run the shortcut, and cross-check against the supplier's delivery invoice before construction starts — the last chance to catch an ordering error cheaply.