Bricks, blocks and the mortar joint
A standard US modular brick is 7 5/8 by 2 1/4 inches. Add the three-eighths-inch mortar joint that goes round it and every brick occupies eight by two and five eighths — which is exactly a third of a foot by just under a quarter.
That is not a coincidence. The sizes were chosen so that brick plus joint lands on a round module, and it is why the familiar rules of thumb come out as near-whole numbers. Once you see it, the whole of masonry estimating is one multiplication.
The joint is part of the unit
Count anything that is laid in mortar by its size plus one joint, in both directions. A brick is not 7 5/8 by 2 1/4 for counting purposes — it is 8 by 2 5/8, because there is a bed joint under it and a perpend beside it, and each unit only carries one of each.
Run that through and a standard modular brick works out at 6.86 bricks per square foot of wall face. The rule of thumb everybody quotes is seven, which is that number rounded, and it is close enough to be useful and wrong enough to be worth doing properly on a large wall.
Concrete block is the same idea at a larger scale, and the sizes are even more obviously deliberate. A standard block is 15 5/8 by 7 5/8 inches so that block plus joint is exactly sixteen by eight — a third of a foot by two thirds. That gives 1.125 blocks per square foot, which is nine blocks for every eight square feet of wall.
Brick 7 5/8 x 2 1/4 in, joint 3/8 in
Occupied 8 in x 2 5/8 in = 21 sq in
144 / 21 = 6.86 bricks per sq ft
Block 15 5/8 x 7 5/8 + joint = 16 x 8 in = 1.125 per sq ft
Work it out: Brick Calculator →
A worked wall
Take a twenty foot wall, eight feet high, single wythe, with two windows in it. The gross face is 160 square feet and the windows take off twenty-four, leaving 136. At 6.86 bricks per square foot with ten per cent waste that is 1,026 bricks.
The same wall in block is 189 blocks, twelve courses high. The difference in unit count — over five times as many units — is most of why brickwork costs what it does. Each unit is a separate handling, and the mortar joint round each one is a separate line to strike.
Openings come off before the waste allowance, not after. Waste is proportional to the work you are actually doing, and a window is work you are not doing.
Work it out: Concrete Block Calculator →
Mortar, and why the figure is a bag count
Mortar is estimated from the unit count rather than the wall area, because the mortar goes round units rather than across surfaces. The trade figures are about 125 bricks to an eighty-pound bag of mix, and about 33 blocks — so the wall above wants nine bags in brick and six in block.
Those are figures for a standard joint on a single-wythe wall, and they assume ordinary waste. Thicker joints, hollow units being buttered on more faces, and cold weather all push it up.
Reinforced blockwork needs grout in the cores on top of that, and rebar to whatever governs the job locally. That is a structural quantity rather than an estimating one, and it is not something a rule of thumb should be deciding.
Ten per cent, and why breakage is real here
Masonry waste is not mostly offcuts. It is breakage — units chipped in transit, dropped on site, or broken when a cut goes wrong — plus the closers and cut units at every opening and corner.
Ten per cent is the usual figure for brick and five for block, and the difference is exactly the handling: a brick is small enough to be dropped and there are five times as many of them.
The other reason to over-order is colour. Brick colour varies between production runs in a way that is obvious on a wall and invisible in a yard. Ordering short and topping up later can leave a visible patch that no amount of pointing fixes.
Questions
How many bricks per square foot?
About 6.86 for a standard modular brick at a three-eighths-inch joint, which is where the "seven bricks a square foot" rule comes from. The exact figure depends on the brick size and the joint.
How many blocks per square foot?
1.125 — nine blocks for every eight square feet. A standard block plus its joint measures exactly sixteen by eight inches, which is why the number is so tidy.
Does the mortar joint change the count?
Yes, and it is the main thing that does. Each unit occupies its own size plus one joint in each direction, so a wall built at a half-inch joint needs measurably fewer units than the same wall at three eighths.
How much mortar does a wall need?
Roughly one eighty-pound bag per 125 bricks or per 33 blocks. That is a single-wythe wall at a standard joint — thicker joints and hollow units buttered on more faces both increase it.
How much waste should I allow?
Ten per cent for brick, five for block. Most of it is breakage and cut closers rather than offcuts, and brick loses more simply because there are five times as many units being handled.
Why should I over-order rather than top up later?
Brick colour shifts between production runs. A batch bought two months later can be visibly different on the wall, and there is no way to blend it in afterwards.