A standard concrete masonry unit, the gray block most walls are built from, has a nominal face of sixteen by eight inches once you include the mortar joint. That face covers about point eight-nine square feet, which works out to roughly one and one-eighth blocks for every square foot of wall. This concrete block calculator uses that ratio. Enter the wall length and height in feet and a waste percent, and it returns the wall area, the block count, and an estimated number of mortar bags. It is built for the standard eight-inch-high, sixteen-inch-long unit; if you are laying a different size, the count will differ. Figure each wall on its own and subtract large openings before you order.
Concrete Block Calculator: CMU Count for a Wall
Calculate how many concrete blocks a wall needs from its length and height, plus a waste allowance and an estimated mortar bag count. Built on the standard 16x8 CMU face.
Concrete Block Calculator
Enter the wall length and height in feet and a waste percent. Returns wall area, block count on the standard 16x8 face, and an estimated mortar bag count.
Wall area320.0 sq ft
Blocks needed378 blocks
Mortar11 bags~1 bag per 33 block
Estimates for planning only. Check the result against your plans and a supplier quote before you order.
How to Use the Calculator
Enter the wall length in feet and the wall height in feet. The calculator multiplies them for wall area, then applies the standard ratio of one and one-eighth blocks per square foot. Add a waste percent for cuts, breakage, and the odd bad block; five percent is a sensible default. The result is the block count to order and an estimated mortar bag count. For a wall with big openings, like a garage door or a run of windows, figure the gross wall first, then subtract the opening area times the same ratio so you are not buying block for holes.
The Formula
Wall area is length times height. Block count is area times one and one-eighth, times one plus the waste percent. The one-and-one-eighth comes from the sixteen-by-eight face: sixteen times eight is one hundred twenty-eight square inches, which is point eight-nine square feet, and one divided by point eight-nine is about one and one-eighth. A forty-foot wall eight feet tall is three hundred twenty square feet, about three hundred sixty blocks before waste, or three hundred seventy-eight with five percent added.
Mortar and Other Materials
The mortar line is an estimate at roughly one bag of mortar per thirty-three blocks laid, which is a common rule for standard block and a three-eighths joint. Sand runs about one ton per seven to eight bags of mortar. Neither number is exact — joint thickness, mason speed, and waste all move it — so treat the mortar count as a starting order and keep the supplier close for a top-up. If the wall is reinforced, you will also need grout for the filled cores and rebar on the schedule the engineer specifies, which this calculator does not size.
Subtract Openings
The calculator figures a solid wall. Real walls have doors, windows, and vents. For a small opening, the cut block around it roughly cancels the block you save, so it is fine to ignore. For a large opening like an overhead door or a wide window band, subtract it: take the opening's area in square feet, multiply by the same one-and-one-eighth ratio, and remove that many block from the total. Skipping this on a wall with big openings overbuys block and mortar you will end up returning or stacking in the yard.
Field Notes
Order full pallets and keep the broken and chipped units for cuts rather than trimming good block. Account for the bond pattern: a running bond wastes little, but a stack bond or a lot of corners and returns increases cutting and breakage, so lean toward the higher end of the waste range. If the wall steps with the grade or changes height, figure each section at its own height and total them. And confirm the block size on the plans, because six-inch and twelve-inch units cover the same face area but cost and weigh differently and change the mortar and grout math.
Concrete Block Reference
| Reference | Planning value | Field note |
|---|---|---|
| Standard face | 16 x 8 in | Includes the mortar joint |
| Blocks per sq ft | ~1.125 | 1 / 0.89 sq ft face |
| 40 x 8 wall | ~360 blocks | ~378 with 5% waste |
| Mortar | ~1 bag per 33 block | Standard block, 3/8 in joint |
| Sand | ~1 ton per 7-8 bags | Rough companion estimate |
FAQ
How many concrete blocks do I need per square foot? About one and one-eighth standard blocks per square foot of wall. The sixteen-by-eight face covers roughly point eight-nine square feet, so a square foot of wall takes a little over one block.
How many blocks are in a 40-foot by 8-foot wall? About three hundred sixty blocks before waste, or roughly three hundred seventy-eight with a five percent allowance for cuts and breakage. That wall is three hundred twenty square feet at about one and one-eighth blocks per square foot.
How much mortar does a block wall need? Roughly one bag of mortar per thirty-three standard blocks laid with a three-eighths-inch joint. It is an estimate that moves with joint thickness and waste, so order to it and keep the supplier handy for a top-up.
Do I subtract doors and windows? Subtract large openings like overhead doors or wide window bands by removing their area times the same blocks-per-square-foot ratio. Small openings are usually a wash because the cut block around them offsets the block you save.