Block Wall Calculator
A block wall takes 1.125 standard 8×8×16 blocks per square foot, one 80 lb mortar bag per 16 blocks (a rule of thumb) and grout in filled cells. A 40 × 8 ft wall grouted at 32 in o.c. needs 360 blocks (378 with 5%), 22.5 mortar bags (buy 23) and 1.04 yd³ of grout. See the Concrete Block Calculator.
How it's calculated: Blocks = net area × 1.125; mortar = blocks ÷ 16 bags of 80 lb preblended mortar (rule of thumb, waste included); grout = net area × 0.350 ft³/ft² (net) × share of cells grouted.
Result appears here
Fill in the fields and press Calculate.
Formula, calculation, assumptions and sources
Blocks = Net wall area (ft²) × 1.125
Mortar (80 lb preblended bags) = blocks ÷ 16 (rule of thumb, nominal waste included — no allowance added)
Grout = Net area × 0.350 ft³/ft² × (cell spacing fraction) [8-in CMU, net]
Run the calculation to see each step with your own numbers.
A 40 × 8 ft wall, no openings, vertical cells 32 in o.c., 5% waste.
Blocks = 320 × 1.125 = 360 → order 378
Mortar = 360 ÷ 16 = 22.5 bags of 80 lb preblended mortar → buy 23 (nominal waste included, no allowance added)
Grout = 320 × 0.350 × 0.25 = 28 ft³ = 1.04 yd³ → order 1.09 yd³ (5%)
- Standard 8 × 8 × 16 in CMU, running bond, 3/8-in joints, face-shell bedding.
- Mortar is a rule of thumb, not an exact takeoff: about 16 conventional blocks per 80 lb bag of preblended mortar (CMHA TEK 04-02A, Table 2), nominal waste included, so the waste allowance is not added to it.
- Grout uses CMHA TEK 04-02A Table 3's 8-in rate with the table's 3% waste removed, so the waste allowance is the only waste added; a bond beam course adds grout not counted here.
- Rebar, bond beams, control joints, ties, flashing and any veneer are separate.
- Does a block wall need grout?
- An unreinforced garden wall doesn't. A structural or below-grade wall has vertical rebar in grouted cells (commonly 32 or 48 in o.c.) plus grouted bond beams — follow the structural drawings.
- How many blocks per square foot of wall?
- About 1.125 standard 8×8×16 in blocks per square foot of wall face — a nominal block covers 8/9 of a square foot. A 40 × 8 ft wall is around 360 blocks before waste.
- Which cells get grouted?
- Follow the structural drawings. An unreinforced garden wall needs none; a structural or below-grade wall grouts the vertical cells with rebar, commonly at 32 or 48 in o.c., plus the bond-beam courses.
- How much mortar do I need for a block wall?
- As a rule of thumb, about one 80 lb bag of preblended mortar per 16 conventional blocks with face-shell bedding — about 6.25 bags per 100, nominal waste included. A 40 × 8 ft wall of 360 blocks needs about 22.5 bags, so buy 23. It is an estimate, not an exact takeoff.
Blocks / mortar / grout
Geometry: a standard 16×8 in nominal CMU face (ASTM C90; a 15-5/8 × 7-5/8 in unit plus a 3/8-in mortar joint) covers 128 in², so 144 ÷ 128 = 1.125 blocks per ft² of wall. Mortar (rule of thumb): Rule of thumb for conventional CMU (8 × 16 in face) laid with face-shell mortar bedding: one 80 lb bag of preblended mortar lays about 16 units — about 6.25 bags per 100 blocks. The figure includes nominal amounts for waste, and the number of units can vary with the mason's skill, non-modular layouts, returns and corners. Concrete Masonry & Hardscapes Association TEK 04-02A, 'Estimating Concrete Masonry Materials', Table 2. Grout per area: Net grout to fill every cell of a hollow two-core CMU wall: 0.249 / 0.350 / 0.456 / 0.572 ft³ per ft² of wall for 6 / 8 / 10 / 12 in units. These are CMHA TEK 04-02A Table 3's 25.6 / 36.1 / 47.0 / 58.9 ft³ per 100 ft² with the table's included 3% waste removed (÷ 1.03), so waste is added only once, by the waste allowance. Concrete Masonry & Hardscapes Association TEK 04-02A, 'Estimating Concrete Masonry Materials', Table 3.
About the Block Wall Calculator
The Block Wall Calculator gives the blocks, mortar and cell grout for a concrete block wall in one calculation, from the wall size and how many cells are grouted. It assumes standard 8×8×16 in units in running bond. Rebar, bond beams, control joints and any veneer are separate. For grout only, use the Concrete Block Fill Calculator; for the block count alone, the Concrete Block Calculator.
What you enter
- Wall length
- Wall height
- Openings to deduct
- Cells grouted
- Waste allowance — Default 5%. Adjust to your own allowance — the exact figure is always shown too.
What you get back
- Blocks (8×8×16) — shown as the exact figure and, separately, a recommended figure with your waste allowance added.
- Mortar — 80 lb preblended bags (rule of thumb) — About 16 conventional blocks per 80 lb bag with face-shell bedding (CMHA TEK 04-02A, Table 2). Nominal waste is already included, so the waste allowance is not added — round up to whole bags. A rule of thumb, not an exact takeoff.
- Grout for filled cells — 0 if unreinforced. 1 yd³ ≈ 45 bags of 80 lb grout mix.
The formula, a worked example, the assumptions behind it and the sources are shown in full below the calculator.
Related calculators
- Concrete Block Calculator — Number of concrete masonry units (blocks) for a wall of a given area.
- Concrete Block Fill Calculator — Grout volume needed to fill the cores of a concrete block wall.
- Mortar Calculator — Mortar quantity for a brick or block wall.
- Rebar Calculator — Rebar quantity, spacing and lineal feet for a slab or footing grid.
Part of the Masonry calculators.