What Is a Concrete Block?
Concrete block walls — the same product people call cinder blocks or CMUs (concrete masonry units) — are estimated off one convenient fact: the standard 8×8×16 nominal block — actual size 7⅝ × 7⅝ × 15⅝, with the ⅜-inch mortar joint making up the difference — covers exactly 0.89 square feet of wall face. That turns any wall into simple multiplication: area × 1.125 = blocks.
This calculator handles the two most common block sizes, both unit systems, and the waste allowance that separates a clean estimate from a mid-job supply run. What it deliberately doesn't guess: rebar, grout fill for cores, and footings — those depend on your structural plan and local code.
Estimating the mortar's raw material or a poured foundation instead? See our Cement Calculator.
How to Calculate Concrete Blocks for a Wall
Face-area math, three steps:
Blocks = wall length × wall height × 1.125 × (1 + waste%) — for 8×8×16 blocks
Worked example: a 20 ft × 8 ft wall = 160 sq ft × 1.125 = 180 blocks; with 5% waste, order 189. Openings (doors/windows) can be subtracted the same way — a 3×7 ft door removes 21 sq ft ≈ 24 blocks — though many masons deliberately don't subtract small openings and let it serve as extra waste allowance.
Concrete Block Chart: Blocks by Wall Size
Pre-computed with the same math the calculator runs:
| Wall (L × H) | Area | Blocks to order | Mortar (80-lb bags) |
|---|---|---|---|
| 10 × 6 ft | 60 sq ft | 71 | 3 |
| 20 × 8 ft | 160 sq ft | 189 | 7 |
| 30 × 8 ft | 240 sq ft | 284 | 10 |
| 40 × 10 ft | 400 sq ft | 473 | 16 |
Sanity anchor: about 113 standard blocks per 100 sq ft of wall before waste. If a quote's block count is far off that line for the wall area, ask why — special blocks (corners, bond beams, caps) are legitimate reasons; padding is not.
How Much Mortar per 30 Blocks?
Mortar consumption runs roughly one 80-lb bag per 30 laid blocks at standard ⅜-inch joints — order by that rule and round up. Waste guidance: 5% covers a straight wall with experienced hands; take 10% for first builds, walls with many cuts, or fragile delivery conditions.
The usual forgotten line items: corner blocks (cut-block corners look poor and waste material), a bond-beam course if your design calls for one, cap blocks for the top course, and ladder-wire reinforcement every couple of courses in many designs. None change the face-area math — all change the shopping list.