A standard brick is 230 by 76 millimetres. Laid with a 10 millimetre mortar joint it occupies 240 by 86 millimetres of wall, because the joint belongs to the brick as far as the wall is concerned.
Ignore that and you get 57.2 bricks per square metre. Account for it and you get 48.4. Over a modest 16 square metre wall that is the difference between ordering 962 bricks and ordering 814, an 18 per cent overshoot on the largest line in the order.
Mortar is about 15 per cent of the face
The joint adds about 4 per cent to the length of each unit and 13 per cent to the height, and those compound. The result is that roughly 15 per cent of what you see when you look at standard brickwork is mortar rather than brick.
It is easy to miss because the joint looks thin. Ten millimetres against a 76 millimetre brick height does not feel like an eighth, but it is, and the brick and block calculator takes the joint as an input specifically because leaving it out is the default mistake.
Overlaps run the same way, in reverse
Board-on-board fencing has the same structure with the sign flipped. Each board covers less than its own width because it overlaps its neighbour, so you need more boards, not fewer.
A 20 metre fence in 140 millimetre boards takes 143 boards butted edge to edge. With a 25 millimetre overlap the effective spacing drops to 115 millimetres and the count rises to 174. At a 70 millimetre overlap it reaches 286, exactly double the butted count, because each board is now advancing the fence by only half its width.
The board-on-board calculator works from the effective spacing rather than the board width for this reason.
Grout, where the intuition overshoots instead
Tiling reverses the psychology. Here people massively overestimate, because grout lines look like a lot of surface and are not.
Twenty square metres of 300 millimetre tiles with a 3 millimetre joint needs 5.0 kilograms of grout, because the joints occupy only 1.97 per cent of the area. Even a wide 10 millimetre joint only reaches 6.35 per cent of the surface and 16.2 kilograms.
The reason is that a grout line is thin in one direction only, while a mortar joint in brickwork is thick relative to a small unit. The grout calculator reports the area share alongside the weight, which makes the scale obvious.
The general rule
Whenever a material is installed with a deliberate gap or overlap, the installed module is not the product dimension. Work out what one unit actually occupies on the finished surface, then divide.
That single step handles bricks, blocks, boards, battens, pavers and tiles. It is also the reason a waste allowance is not a substitute: 10 per cent of waste does not cover an 18 per cent systematic error in the module, and the two are answering different questions.
For flooring and tiles by the box, see the flooring calculator. For the related trap of turning an area into an order, see the piece on ordering materials.