The 1.125 figure and where it comes from
A standard concrete masonry unit is nominally 8 by 8 by 16 inches, which means the block itself is 7-5/8 by 15-5/8 and the missing 3/8 in each direction is the mortar joint. Laid up, one block plus its joints occupies exactly 8 by 16 inches of wall face, which is 128 square inches. Divide 144 by 128 and you get 1.125 blocks per square foot. That is not an approximation, it is what the module is designed to produce, and it is why block walls are so much easier to estimate than almost anything else on a site.
It only holds while the joint is 3/8. Run a 1/2 inch joint on rough units and the module grows to 8.125 by 16.125, which drops the count to about 1.10 per square foot. That is a two percent difference, comfortably inside the waste allowance, so it matters for tidiness rather than for the order. What does matter is the course height, because a 1/2 inch joint puts your top course 1.5 inches higher over twelve courses, and that shows up when the wall has to meet a plate or a lintel at a fixed height.
Grout is the number people miss
Mortar is what holds the blocks together and there is surprisingly little of it: face shell bedding on an 8 inch block uses about 0.013 cubic feet per block, so an 80 pound bag lays somewhere around 50 blocks before waste and around 35 to 40 with realistic waste. Grout is what fills the cells, and it is a different order of magnitude. One cell of an 8 inch block holds about an eighth of a cubic foot per course, so a cell grouted through twelve courses takes 1.5 cubic feet, and a wall with cells at 32 inches on centre has eight of them per twenty feet.
| Block | Void per block | Solid grouted, per 100 sq ft |
|---|---|---|
| 6 in | about 0.17 cu ft | about 0.7 cu yd |
| 8 in | about 0.25 cu ft | about 1.0 cu yd |
| 10 in | about 0.35 cu ft | about 1.5 cu yd |
| 12 in | about 0.44 cu ft | about 1.8 cu yd |
Those void figures vary between manufacturers because web thickness and shell thickness are not identical across plants, and lightweight units differ from normal weight ones. Treat the table as the right order of magnitude and the waste allowance as the buffer. If the wall is large enough that grout is the main cost, ask the block supplier for the actual net volume of the unit they are selling you.
Bond beams and the double-counting trap
A bond beam is a course of blocks with the webs knocked out or moulded away so that grout runs continuously along the wall, usually with horizontal bar in it. It is most commonly at the top of the wall, sometimes also at a floor line or over openings. Filling one takes the full cell volume for every block in that course, not half of it, because both cells and the space where the web was all fill.
Where people get it wrong is adding the bond beam volume on top of the vertical cell volume without noticing that the grouted cells pass through the bond beam course, so that intersection has already been counted once. This calculator subtracts it. On a twenty foot wall with one bond beam and cells at 32 inches, the correction is only about a cubic foot, but on a long wall with three bond beams it is worth having.
Practical things that do not come out of arithmetic
Grout has to be able to get where it is going. Cells that are going to be filled need to be clean, which means keeping mortar droppings out of them as you lay, and on a tall wall it means cleanouts at the bottom course so you can actually see and clear the cell before filling. Grout also wants a much wetter consistency than the concrete you would put in a slab, because it has to flow into a two by four inch space around a bar without segregating, and then it loses that water into the surrounding block and stiffens up.
Lift height is the other one. Filling a full eight foot wall in a single pour puts a lot of hydraulic pressure on green mortar joints, and walls have been blown out that way. Doing it in lifts with time between them is the usual answer, and how tall a lift is acceptable depends on the grout, the wall and who is inspecting it. Ask.
If the wall you are counting is a retaining wall built from dry-stacked segmental units rather than mortared CMU, that is a different product and the retaining wall block calculator is the right page. For the footing under this wall, the concrete footing calculator handles the strip; for mortar mixed from scratch rather than bought premixed, the mortar mix calculator gives the portland, lime and sand proportions by type.
Questions people ask
How many blocks are in 100 square feet of wall?
One hundred and thirteen, at 1.125 blocks per square foot with a 3/8 inch joint. That is the net figure before waste and before deducting openings. Add five percent for breakage and cuts on a plain run and you are at 119, and ten percent on a wall with several openings puts you at 124. The count does not change with the block thickness, because a 6, 8, 10 and 12 inch block all have the same 8 by 16 face. Only the weight, the price and the grout volume change.
Do I have to grout the cells at all?
That depends entirely on what the wall is doing and it is not a question a calculator can answer. Plenty of low garden walls and non-loadbearing partitions are laid hollow. Walls that retain soil, carry a roof, resist wind on a tall span or sit in a seismic region are usually reinforced and grouted, at a spacing and with a bar size that somebody worked out for the conditions. Set the spacing to none if you know the wall is hollow, and otherwise use the figure from your drawing. Where you are, the building department decides what is acceptable.
How many blocks will one bag of mortar lay?
The geometry says an 80 lb bag yielding about 0.66 cubic feet covers roughly 50 blocks laid with face shell bedding and a 3/8 joint. In practice the widely used rule of thumb is closer to 3 bags per 100 blocks, or about 33 blocks a bag, and the difference is entirely waste: mortar that drops off the board, mortar that stiffens before it is used, and over-buttering by anyone who is not laying block every day. The calculator starts from the geometry and applies the waste percentage you set, so if you are new to it, push the waste above the 25 percent default rather than trusting the tidy number.
Is core fill grout the same as concrete?
No, and using the wrong one causes real problems. Grout for masonry cells is a fluid mix with fine aggregate, designed to flow into a narrow cell around a reinforcing bar and to bond to the block and the steel. Structural concrete at a normal slump will bridge in a cell and leave voids you cannot see. Suppliers sell it as core fill grout or masonry grout, in bags and by the truck. If you are filling cells by hand from bags, buy the product intended for it rather than mixing standard concrete mix wetter and hoping.
My wall height does not come out in whole courses. What now?
Very few do on the first attempt, and the answer is almost always to adjust the footing rather than to cut every block. A course is 8 inches with a standard joint, so wall heights land naturally at 4 feet, 5 feet 4 inches, 6 feet 8 inches, 8 feet and so on. If you need a specific height for a plate or a gate, either set the top of the footing so the courses arrive where you want, or plan a course of half-height block, which is sold for exactly this. Stretching the bed joints to make up the difference works over an inch or two and looks wrong beyond that.