Concrete & Masonry
Concrete Block Calculator
CMU block count for a wall, with mortar bags and core-fill concrete included.
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Adjust the inputs to see your result.
About this calculator
A standard 8×8×16 in block with a 3/8 in mortar joint covers about 1.125 blocks per square foot of wall. This calculator counts blocks for your wall dimensions, deducts openings, and adds the mortar and optional grout fill that complete the materials list.
What this number means
A block wall order has three lines that must agree: the blocks, the mortar that joins them, and — for reinforced work — the grout that fills the cores. This calculator builds all three from the same wall dimensions so the counts cannot drift apart: mortar follows the block count at the trade rate of three 80 lb bags per hundred blocks, and grout follows the wall's core volume.
Block counts use nominal dimensions — the size including one mortar joint — which is why a standard block "covers" 128 sq in of wall even though the block itself is 7.625 × 15.625 in. That is also why the magic number for standard block is 1.125 per square foot rather than a tidy 1.
The formula
- Blocks per sq ft = 144 ÷ (nominal length × nominal height) — 1.125 for 8×16 block
- Blocks = (Wall area − openings) × blocks per sq ft × (1 + waste %), rounded up
- Mortar = Blocks × 3 bags per 100 blocks (80 lb pre-mix), rounded up
- Grout (full) ≈ Net wall area × 0.0094 cu yd per sq ft for 8 in block; vertical-cores-only ≈ 30% of that
Worked example
A 20 × 8 ft foundation wall in standard block.
- Wall area = 160 sq ft
- Blocks = 160 × 1.125 = 180 blocks before waste; with 5% for cuts and breakage: 189 blocks
- Mortar = 189 × 3 ÷ 100 → 6 × 80 lb bags
- Fully grouted, the same wall would take about 1.5 cu yd of grout — set core fill to see it
Assumptions and limitations
- Mortar rate assumes 3/8 in joints and pre-mixed 80 lb bags. Site-mixed mortar (cement, lime, sand) follows different proportions — the Mortar Calculator covers bag counts by type.
- Grout estimates assume standard 8 in (nominal) hollow block with roughly 40% core voids. Lightweight block and 6 in or 12 in block have different void ratios; treat grout as a ±20% planning figure and confirm with your supplier.
- Half blocks, bond-beam blocks and lintel blocks are specials within the count, not additions to it — a typical wall needs a bond-beam course at the top, swapped from the standard count.
- Rebar itself is not estimated here; vertical cores at 48 in on centre is the common residential pattern the grout option assumes.
Frequently asked questions
With standard 8×8×16 in block, 112.5 — so 113 blocks before waste, about 119 with a 5% allowance. The 1.125 blocks-per-sq-ft figure comes from the nominal face size (16 × 8 in including the joint = 128 sq in; 144 ÷ 128 = 1.125).
About three 80 lb bags of pre-mixed mortar per 100 standard blocks at 3/8 in joints. The calculator applies that rate to your block count and rounds up to whole bags.
Yes — enter their total square footage in the openings field. Blocks are not laid where the opening is, though you will still use some half blocks and lintel blocks around it, which the waste allowance covers.
For standard 8 in block, fully grouted walls take roughly 0.0094 cu yd per sq ft of wall — about 1.5 cu yd for a 160 sq ft wall. Walls with only the rebar cores filled (typically every 48 in) take about a third of that. Both are estimates; void ratios vary by block manufacturer.
An "8×8×16" block actually measures 7.625 × 7.625 × 15.625 in. The nominal size adds one 3/8 in mortar joint, which is why wall maths uses nominal dimensions: a wall built of them comes out to the planned length and height exactly.