Concrete block wall calculator

How many 8×8×16 blocks a wall takes, the half-blocks the ends need, and the bags of mortar to lay them. The block count is the easy part — the mortar and the half-blocks are what send people back to the yard.

01 How the math works

A block wall is three counts stacked on one wall. Work out the face area, and the block count follows from coverage: a nominal 8×8×16 block claims a 16 in by 8 in patch of wall, so 144 ÷ (16 × 8) = 1.125 block per ft² (12.11 per m²) at a 3/8 in (9.5 mm) joint. Multiply area by that, add waste, round up. That is the number every calculator gives you.

The other two counts are the ones that matter on delivery day. Mortar is figured from a lay-rate, not a guess: an 80 lb (36.3 kg) bag of Type S mason mix lays about 13 of these blocks, so bags are block count ÷ 13, rounded up. And half-blocks come from the bond. Laid in running bond, every other course starts and ends with a half so the wall ends stay flush — which means each exposed end needs a half block on half its courses. Those halves substitute for full block at the ends, so they come out of the full count, not on top of it.

That last line is the part flat calculators drop. A half block fills half a coursing position, so full-plus-half-over-two always lands back on the by-area count. Count the halves as extra and you over-order; ignore them and you're cutting full blocks on site, which is slower and wastes the offcut.

02 Worked example

Take a free-standing single-wythe wall, 20 ft long and 8 ft tall (6.10 m × 2.44 m), in standard 8×8×16 block. The face area is 160 ft² (14.86 m²).

By area, that's 160 × 1.125 = 180 block. Add the 5% standard masonry allowance and you order 189. The wall stands 12 courses high (96 in ÷ 8) and runs 15 block per course (240 in ÷ 16). With two exposed ends, the half-block count is 2 × floor(12 ÷ 2) = 12 halves, and the full block drops to 180 − 12 ÷ 2 = 174. So the wall is 174 full + 12 half — still 180 by area, just split the way you'll actually lay it.

Mortar rides on the block you lay, not the waste on the pallet: 180 ÷ 13 = 13.85, so 14 bags of 80 lb (36.3 kg) Type S. That figure assumes face-shell bedding, the standard for a running-bond wall above grade. Bed the block full and you'll want more, which is the next section.

03 When this calculator is wrong

The mortar count is the softest number here, because the bag's lay-rate assumes face-shell bedding — mortar on the block's outer webs only. That's right for a standard above-grade wall. The first course on the footing, any course below grade, and any wall the structural drawings call to bed full all take mortar on the full block, which uses more per block and drops the yield below 13. On a full-bedded wall, buy extra mortar over the face-shell count, and confirm the rate against your mix's own coverage chart.

• The half-block math is running bond on a straight wall. Corners don't take half block — they take full block woven in an overlap, and some layouts use a sash or corner unit instead. If your wall turns corners, the half count for those ends drops to zero (set exposed ends to 1 or 0), and the corner block is a separate line the calculator doesn't size.
• Control joints, bond beams, and lintels aren't counted. A long wall needs vertical control joints, and openings need a bond-beam or lintel block over them. These are specialty units, ordered by the piece from the opening and joint layout — not from wall area.
• The default 5% waste is for a plain, straight wall in full block. The standard 10% waste factor is calibrated for production-scale work on regular geometry; on small or cut-heavy walls the real rate runs 15–20%, because short runs have more breakage and less room to use an offcut. The exception is a long, straight, opening-free wall — there 5% is right and 10% is generous. Scale the factor to the job, not the other way around.
• It's block and mortar, not the reinforcement or the footing. The count doesn't size rebar, the grout for filled cells, ladder or truss wire, or the concrete under the wall. Vertical steel and grouted cells in particular add a separate concrete volume — figure those from the cell spacing your engineer specs.

04 What to do with the result

Order the rounded-up block count in one delivery, and split it into full and half at the counter — the yard stocks the 8×8×8 half as its own unit, and buying it beats cutting it. Add the mortar as whole bags with one spare on the pallet; a bag costs less than a second trip mid-course, and mason mix keeps dry if you don't open it.

Confirm the block module before you finalize. Standard here is the nominal 8×8×16 (200×200×400 mm), actual 7-5/8 × 7-5/8 × 15-5/8 in (194×194×397 mm) — the missing 3/8 in on each face is the mortar joint's share showing up on your tape measure. If your supplier stocks a different module, the 1.125 coverage figure moves, so take them your wall dimensions and let them confirm the count against the block they'll actually deliver.

05 Common questions

How many concrete blocks do I need per square foot?

1.125 standard 8×8×16 block per ft² (12.11 per m²) at a 3/8 in joint, per the Portland Cement Association. Multiply your wall's face area by that figure, then add a waste factor.

How much mortar do I need for a block wall?

About one 80 lb (36.3 kg) bag of Type S mason mix per 13 blocks in face-shell bedding (Quikrete No. 1136). The 180-block wall above takes 180 ÷ 13 = 14 bags. Full mortar bedding — the first course, below grade — uses more, so buy extra over the face-shell count there.

How many blocks are in a square metre?

12.11 standard 8×8×16 block per m², the imperial 1.125 per ft² converted with the exact square-foot-to-square-metre factor.

What are the actual dimensions of a standard concrete block?

Nominal 8×8×16 in (200×200×400 mm); actual 7-5/8 × 7-5/8 × 15-5/8 in (194×194×397 mm) per ASTM C90. The nominal size includes the 3/8 in mortar joint, so the block itself is 3/8 in short on each face.

How many bags of mortar do I need per 100 blocks?

100 ÷ 13 = 7.7, so 8 bags of 80 lb Type S per 100 block in face-shell bedding. Round up, and add a bag if any of the run is bedded full.