Concrete Block Calculator
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About the Concrete Block Calculator
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
The Block-Count Formula
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.
Blocks for Common Wall Sizes (8×8×16, 5% waste)
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.
Mortar, Waste, and What Estimators Forget
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.
Frequently Asked Questions
How many concrete blocks do I need per square foot of wall?
1.125 standard blocks per square foot. A nominal 8×8×16 block covers 8 × 16 inches = 0.89 sq ft of wall face (the ⅜-inch mortar joints are already inside the nominal size). So: wall area in square feet × 1.125, plus 5–10% waste, rounded up.
How many blocks are in a 20-foot wall?
Depends on height: a 20 ft × 8 ft wall is 160 sq ft × 1.125 = 180 blocks, or about 189 with 5% waste. Per course, a 20-ft wall lays 15 standard blocks (each is 16 inches long), and an 8-ft height is 12 courses of 8-inch blocks.
How much mortar do I need per concrete block?
Rule of thumb: one 80-lb bag of mortar mix per 28–32 laid blocks at standard ⅜-inch joints — this calculator uses 30. A 189-block wall therefore needs about 7 bags. Thicker joints, wasteful mixing, and hot weather all push consumption up, so round generously.
Do I subtract doors and windows from the block count?
For large openings, yes — a 3×7 ft doorway removes about 24 blocks, worth subtracting. For small openings many estimators skip the subtraction and treat it as built-in waste allowance. Either is fine as long as you're consistent and still add explicit waste on top.
What's the actual size of an 8x8x16 block?
7⅝ × 7⅝ × 15⅝ inches. The 'nominal' 8×8×16 includes the standard ⅜-inch mortar joint on two faces, which is exactly why coverage math works in clean 8- and 16-inch increments once laid.
Does this estimate include rebar, grout, or footings?
No — deliberately. Vertical rebar spacing, which cores get grout-filled, and footing dimensions are structural-design decisions set by your plans and local code, not by wall area. Get those from the drawings or an engineer; use this tool for the block and mortar order.
Sources & References
- [1]Concrete Masonry & Hardscapes Association (industry body) — CMHA / NCMA
Methodology. This calculator uses standard construction and material-estimation formulas. It is reviewed and maintained by the Vast Calculators editorial team.
Last updated · July 2026
Results are estimates for general use; verify critical figures independently.