Calculator
Concrete Block (CMU) Calculator
Concrete block does not come in fractions, and neither does a course. A wall built from standard 8 x 8 x 16 CMU is laid to a fixed module the moment you pick the block: 16 inches along a course, 8 inches per course up. Whatever your wall's actual length is, the mason lays whole blocks to that module and cuts what's left over at a corner or a jamb — so the number this page produces is not an area divided by a coverage rate, it is a grid: so many blocks across, times so many courses up.
That is a genuinely different calculation from converting square feet, and it matters because a doorway does not remove a smooth slice of wall the way it would from a poured surface — it removes whole blocks, and only the ones sitting fully inside the opening. This calculator builds the course grid first, then subtracts openings in whole modules and folds in corners as a separate count, which is the order a mason actually works in rather than the order an area sum would suggest.
Calculate your quantity
15-5/8 in actual for a nominal 16 in block.
Block breaks less than brick — 5% is usually enough.
Blocks needed
189 blocks
- Blocks per course
- 15 blocks
- Courses high
- 12 courses
- Deducted for openings
- 0 blocks
- Corner blocks
- 0 blocks
- Stretcher blocks
- 189 blocks
- Wall height 8 ft as laid — 12 courses at 8 in per course.
Shopping summary
- • 189 stretcher blocks
This is an estimate — confirm structural work with a professional.
How this calculation works
A standard load-bearing block measures 15 5/8 x 7 5/8 inches on its face and is laid with a 3/8 inch head and bed joint, which brings it up to exactly 16 x 8 inches — the same undersizing trick brick uses, for the same reason: round module dimensions from odd-sized units. Divide the wall length in inches by 16 and round up, and you have blocks per course. Divide the wall height in inches by 8 and round up, and you have courses. Multiply the two and you have the gross block count before anything is deducted.
Openings come out next, and only in whole blocks. The opening's square footage is divided by one block's module face area (16 x 8 inches, or 0.89 square feet) and the result is rounded DOWN, never up — a hatch or a vent that covers a block-and-a-half only earns a deduction of one block, because the arithmetic can't hand back a fraction of a unit you didn't buy. Corners are handled separately again: a corner block sits in the same position a stretcher would have occupied, so it replaces one stretcher per course at every outside corner rather than adding to the count.
- Blocks per course and courses are each rounded up independently, then multiplied — a course 17.4 blocks long still buys 18, and a wall 11.6 courses tall still buys 12, and those two roundings compound rather than cancel.
- Opening deductions round down, on purpose, so a small opening near the edge of a module can't accidentally over-deduct a block you'll still need to cut.
- Corner blocks are a different-shaped unit that replaces a stretcher position, not an addition to the wall total — order them as their own line item.
- Wall height in anything other than a multiple of 8 inches still costs a full extra course; picking a target height on the module avoids paying for a course you only need a sliver of.
The formula
blocks = (blocksPerCourse × courses − openingBlocks) × (1 + waste); blocksPerCourse and courses are each rounded up before the multiplication
- blocksPerCourse
- How many blocks span the wall length: wall length in inches divided by the 16 inch module, rounded up. A wall a half-block short of a full module still needs that whole block.
- courses
- How many rows stack to the wall height: wall height in inches divided by the 8 inch module, rounded up the same way.
- openingBlocks
- Whole modules removed for doors and windows — the opening's square footage divided by one block's 0.89 sq ft module face, rounded down so the deduction never claims more than the opening actually clears.
- corner blocks
- One block per course at every outside corner, occupying a stretcher's position rather than adding to it — the total block count doesn't change, only how it's split between stretcher and corner units on the shopping list.
- waste
- Percentage held back for breakage and site cuts at returns and openings. It runs lower than brick's typical figure because CMU is a tougher, rectangular unit that survives handling better than fired clay.
Rounding blocks-per-course and courses up separately, then multiplying, double-counts the partial block in both directions on purpose — that is what actually happens on a jobsite. It is not the same number you'd get by rounding the wall's raw square footage up once, and on a short or oddly-proportioned wall the two methods can disagree by several blocks.
Where these numbers come from
- 15 5/8 x 7 5/8 in actual size for a nominal 16 x 8 in block
- Manufacturer-published nominal sizing for standard load-bearing concrete masonry units. Units are cast undersized by 3/8 inch in both directions so that a standard head and bed joint brings a course back to the round 16 x 8 in module — the same convention modular brick uses for the same reason.
- 16 x 8 in course module
- Trade convention, near-universal across CMU manufacturers in North America, because it lets a mason lay out a wall in whole and half blocks from a tape measure without a running calculation. Lightweight and specialty units vary, which is why block dimensions are inputs here rather than constants.
- 0.89 sq ft module face area
- Derived on this page from the module: (16 x 8) ÷ 144. Used only to floor-divide opening square footage into whole blocks, not quoted as a supplier figure.
- 5% waste
- Trade convention for a plain wall with few openings. Lower than brick's working default because concrete block is heavier, ships strapped on a pallet, and does not chip the way fired clay does — raise it toward 10% for a wall with several corners or cut returns.
Worked examples
A plain 24 by 8 foot block wall, no corners or openings
| Wall length | 24 ft |
|---|---|
| Wall height | 8 ft |
| Block length | 15.625 in |
| Block height | 7.625 in |
| Mortar joint | 0.375 in |
| Outside corners | 0 |
| Opening area | 0 sq ft |
| Waste factor | 5 % |
| Blocks needed | 227 blocks |
|---|---|
| Blocks per course | 18 blocks |
| Courses high | 12 courses |
| Deducted for openings | 0 blocks |
| Corner blocks | 0 blocks |
| Stretcher blocks | 227 blocks |
Eighteen blocks per course times twelve courses is 216 blocks before waste, and 5% waste takes that to 227. Notice that with no corners and no openings this number is very close to what a naive area-and-coverage sum would have produced — 192 square feet at roughly 1.125 blocks per square foot is about the same figure. That agreement is the tell: the grid method and the area method only diverge once corners and openings enter the picture, which is exactly what the second example does.
Twelve courses at 8 inches is 96 inches, or 8 feet exactly — this wall's height is already a clean multiple of the module, so nothing is being paid for and unused at the top. That is worth checking before you pour a footing: a wall height picked without the 8 inch module in mind almost never lands this evenly.
An L-shaped crawlspace wall with two corners and a hatch
| Wall length | 40 ft |
|---|---|
| Wall height | 4 ft |
| Block length | 15.625 in |
| Block height | 7.625 in |
| Mortar joint | 0.375 in |
| Outside corners | 2 |
| Opening area | 12 sq ft |
| Waste factor | 5 % |
| Blocks needed | 176 blocks |
|---|---|
| Blocks per course | 30 blocks |
| Courses high | 6 courses |
| Deducted for openings | 13 blocks |
| Corner blocks | 12 blocks |
| Stretcher blocks | 164 blocks |
This wall covers 160 square feet against the first example's 192, but the block count of 176 doesn't scale down the way that ratio suggests, because two things are pulling against each other. The 12 square foot hatch only earns a 13-block deduction — the exact division is 13.5 modules, and the calculator floors it rather than rounds it, so you're covered even if the hatch doesn't land on a clean block boundary. Meanwhile two corners add nothing to the total but do reshape it: 12 of the 176 blocks are corner units, standing in for 12 stretcher positions rather than adding to them.
The practical read: order 164 stretcher blocks and 12 corner blocks as separate line items, because they are genuinely different-shaped units and a supplier who only hears '176 blocks' will hand you 176 stretchers. If the hatch ends up centered on a block rather than split across two, you might save one more block on site than the calculator claims — but ordering the floored number is what keeps you from coming up short instead.
Common mistakes
- Reaching for a brick-style bricks-per-square-foot rate and dividing the wall area by it. Block is laid to a fixed module and rounds up per course AND per row, which produces a different — usually higher — number than dividing a continuous area ever would.
- Subtracting opening area as a fraction of a block rather than a whole one. You cannot buy 13.5 blocks; the deduction always floors to the last whole module the opening clears.
- Ordering the wall total plus the corner block count on top of it. Corner blocks replace a stretcher position in every course they occupy — add them separately and you'll over-order by exactly the corner count.
- Setting a wall height that isn't a multiple of 8 inches and being surprised the course count doesn't come out even. A 7 ft 11 in wall and an 8 ft 2 in wall both cost 12 courses on standard block; only the second one uses all of what it paid for.
- Running this calculator with the default block dimensions on a job that actually uses 4 inch partition block or 12 inch structural block. The module changes with the unit, and 4 inch block does not share a face area with 8 inch block even though both are laid to a 16 inch course length.
Shopping summary
Order stretcher and corner blocks as separate counts, not a single combined figure — a yard quoting one price for '176 block' will default to stretchers and leave you short at every corner. Mortar for block is its own calculation driven by the unit count rather than the wall area: set the mortar mix calculator to concrete block rather than brick, since block eats mortar differently per unit than brick does.
If any cores are being filled — a bond beam, a reinforced pier, or vertical steel at a set spacing — that concrete and rebar are a separate order this page doesn't touch. The rebar grid calculator covers vertical and horizontal bar including the lap splices a filled-cell wall needs.
Where the block wall is doing structural retaining work rather than just enclosing a crawlspace, check it against the retaining wall block calculator instead — that page's segmental units, base stone and drainage requirements are a different job from a foundation or partition wall, even though both are laid course by course.
FAQ
Why does the block count round up twice — once per course, once per row of courses — instead of once at the end?
Because that's what actually happens on the wall. A course that needs 17.4 blocks to close still needs an 18th whole block, and a wall that's 11.6 courses tall still needs a full 12th course. Rounding once at the end, on the total square footage, hides both of those partial-unit costs and under-orders on short or oddly-proportioned walls.
Do I need to add extra block for a bond beam or a lintel course?
This calculator counts standard stretcher and corner units only. A bond beam or lintel course uses a U-shaped or knock-out block that isn't the same product, and that course is worth pricing separately with your supplier rather than folding into this count — the coverage arithmetic is the same 16 x 8 module, but the unit price and the fill it needs are not.
What happens if my wall height isn't a multiple of 8 inches?
You still pay for the full next course. A 7 ft 11 in wall (95 in) needs ceil(95/8) = 12 courses, the same as an 8 ft wall — you're buying a 96 inch wall's worth of block either way, and the last inch is made up in the bond beam, the footing, or the cut at the top. Picking a target height that's already a multiple of 8 avoids paying for height you don't get.
Why does a corner block come out of the total instead of adding to it?
A corner block sits exactly where a stretcher would have gone in that position on that course — it's a different casting (closed on one end) doing the same job, not an extra unit. Adding corner count on top of the plain grid total over-orders by exactly the number of corners times the number of courses.
How is this different from the brick calculator's bricks-per-square-foot figure?
Brick is quoted and ordered per square foot because it's small enough that a continuous rate makes sense; the brick calculator works that way deliberately. Block is laid to a coarse, fixed 16 x 8 module and a mason counts it as a grid, not an area, so this page mirrors that: blocks per course times courses, not a rate multiplied by square footage. The two pages use different arithmetic because the trades order the two materials differently, not because one method is more correct.
Does this page work for non-standard block, like a 4 inch partition wall or a 12 inch structural unit?
Yes, but the block's own face height changes the module too, not just its length. Corner units are the part most likely to trip you up: a thin partition block commonly turns a corner with an L-shaped or bullnose unit rather than a true load-bearing corner block, and that unit doesn't necessarily replace a stretcher position the same one-for-one way this page assumes for 8 inch structural block. Check your specific corner product against your supplier's layout guide before trusting this page's corner math on a non-standard wall.
Where to go next
The projects this number is a step of, the guides that explain the method behind it, and the rest of its trade group.