Block, Brick & Mortar Quantity
Block and brick counts, courses and mortar bags — and why the nominal size already contains the joint, which is what makes it all divide neatly.
Openings to deduct
Nominal, actual, and per square foot
Every one of these densities falls straight out of the nominal face — and it only does so because the nominal size already contains the joint.
The nominal size includes the joint
An "8 × 16" block actually measures 7⅝ × 15⅝. The missing ⅜″ in each direction is the mortar joint — and that's the entire reason masonry arithmetic is clean. Laid at ⅜″, every unit occupies exactly 8 × 16 in the wall, so courses land on 8″ multiples and lengths on 16″ multiples without anybody measuring. It's why 1.125 blocks per square foot and 6.75 modular bricks per square foot are exact numbers rather than estimates.
Which means the coursing only works at the design joint. Lay ⅜-nominal block with a ½″ joint and every course gains ⅛″ — ten courses gain an inch and a quarter, and the wall stops landing where the drawing says. It's invisible on the first course and unmissable at the top. A mason working to a course rod rather than a tape is compensating for exactly this, by adjusting the joint rather than trusting it.
And you do deduct the openings — the opposite convention to sheathing. Masonry simply isn't laid where the window goes, whereas sheathing is fitted over the opening and cut out afterwards. Getting the two the wrong way round is a common habit and an expensive one in both directions.
- 6″, 8″ and 12″ block share a face, so they need the same count per square foot — but a 12″ block drinks noticeably more mortar and is a different day's work to lay.
- Bond pattern changes nothing about quantity and everything about cutting. Running bond needs a half unit at each end of every other course; stack bond needs none but is weaker and wants reinforcement.
- Stronger mortar is not better mortar. The joint should be weaker than the unit, so that movement cracks the joint — which can be repointed — rather than the block, which can't.
- Dry units drink mortar. A thirsty block pulls water out of the mortar and takes more of it; the bag counts move with how absorptive the units are and how wide the joints run.
- A wall retaining soil is an engineered structure, not a stack of block. Reinforcement, grouted cores and bond beams are design questions, not quantity ones.
How to use
- Pick the unit and enter the wall size.
- Deduct the openings — masonry is not laid where the window goes.
- Keep the joint at ⅜″ unless you have a reason, or the coursing drifts.
- Order a little over: running out mid-wall costs more than the surplus.
Frequently asked questions
How many blocks per square foot?
Exactly 1.125 for standard 8 by 16 nominal block, or 112.5 per hundred square feet. It is an exact figure rather than a rule of thumb because the nominal face is exactly 8 by 16 inches — 128 square inches, which divides into a square foot 1.125 times. Modular brick works out at exactly 6.75 the same way.
Why is a block called 8 by 16 when it measures less?
Because the nominal size includes the mortar joint. A standard block actually measures 7⅝ by 7⅝ by 15⅝, and the missing three eighths of an inch in each direction is the joint it will be laid with. Laid at ⅜″, every unit occupies exactly 8 by 16 in the finished wall — which is the entire reason masonry arithmetic comes out in whole numbers.
What happens if I use a different joint thickness?
The coursing stops working. Lay ⅜-nominal block with a half inch joint and every course gains an eighth of an inch, so ten courses gain an inch and a quarter and the wall no longer lands where the drawing says. It is invisible on the first course and unmissable at the top, which is why masons work to a course rod rather than a tape — the rod holds the module and the joint gets adjusted to suit.
Do I deduct window and door openings?
Yes — and this is the opposite of the convention for sheathing, which is fitted over an opening and cut out afterwards. Masonry is simply not laid where the window goes, so the opening area comes off the order. Getting the two conventions the wrong way round is a common habit and expensive in both directions: too much block, or not enough plywood.
How many bricks per square foot?
Exactly 6.75 for modular brick, which is the commonest size. That comes from its nominal face of 8 inches by two and two thirds — three courses to eight inches. Queen brick is slightly taller and works out around 6.5, and utility brick at 4 by 12 nominal is exactly 3, which is why it lays so much faster.
How much mortar do I need?
Roughly one 80 lb bag of mortar mix per twelve or thirteen standard blocks, or per twenty-eight to thirty modular bricks. The figure moves with joint thickness and with how absorptive the units are — a dry, thirsty block pulls water out of the mortar and takes noticeably more of it. Buy a little over, because running out mid-wall means a cold joint in the mortar as well as a trip to the merchant.
Do 6, 8 and 12 inch blocks need different counts?
No — they share the same 8 by 16 face, so they need the same number per square foot. What changes is the wall thickness, the weight, and how much mortar each takes: a 12 inch block has far more bedding area and drinks appreciably more. It is also a genuinely different day’s work to lay, which the quantity calculation says nothing about.
Does the bond pattern change how many I need?
Not the quantity, only the cutting. Running bond offsets each course by half a unit, so every second course needs a half at each end — and those halves come out of whole units. Stack bond needs no cuts at all but is structurally weaker, because the joints line up vertically, and it normally needs reinforcement to compensate.
Should I use the strongest mortar available?
No, and this surprises people. The joint should be weaker than the unit, so that movement cracks the mortar rather than the block. A cracked joint can be raked out and repointed; a cracked block cannot. Over-strong mortar in a wall of soft units is a classic way to destroy the units, and it shows up decades later in old brickwork repointed with modern cement.
How much waste should I allow?
Around five per cent for a plain wall, more where there are many openings or corners, because both generate cut units and cuts come out of whole ones. Breakage in handling accounts for the rest. Block is cheap relative to the labour of a second delivery, so the surplus is nearly always the right side to err on.
Is a block wall a structure I can just build?
A low free-standing garden wall, often. Anything retaining soil is an engineered structure and not a stack of block — lateral earth pressure is substantial and increases with the square of the height, and the reinforcement, grouted cores and footing that resist it are design questions. So is anything carrying a floor or roof above it.
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