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Concrete block calculator: CMU count, mortar and grout
A standard 8 × 16 block covers 128 square inches of wall including its mortar joint, so a wall takes 1.125 blocks per square foot. Enter the wall, pick the unit and the grout pattern, and get blocks, mortar bags and grout yardage.
The wall
What to order
Blocks
378
1.125 per sq ft, waste included
Mortar (80-lb bags)
30
At 13 blocks per bag
Grout
0.59 CY
14.4 cu ft net, +10% waste
Net wall area
320 sq ft
Length × height − openings
Grout volumes are approximate — core geometry varies by manufacturer. For structural walls, the engineer’s drawings govern which cells are grouted and the supplier’s table governs the volume.
Quantities are half a bid. Send these to the free estimate builder to add labor, burden and markup, then hand the customer a proposal.
Every field takes arithmetic — type 24+18+12 and it adds. Nothing is sent anywhere; the math runs in your browser.
How to calculate concrete blocks for a wall
Blocks are sold by nominal size — 8 × 16 means the actual 7⅝ × 15⅝ inch face plus a ⅜-inch joint. That face is 128 square inches, and 144 ÷ 128 = 1.125 blocks per square foot. Width (6, 8, 10 or 12 inches) does not change the count; it changes weight, grout and cost.
Worked example: a 40 × 8 ft wall is 320 sq ft. 320 × 1.125 = 360 blocks, plus 5% waste = 378. At 13 blocks per 80-lb bag that is 30 bags of premixed mortar. Solid-grouted in 8-inch block, the wall takes about 86 cubic feet of grout — 3.2 yards net, about 3.5 with waste.
Blocks per 100 square feet
| Face (nominal) | Blocks per sq ft | Blocks per 100 sq ft |
|---|---|---|
| 8 × 16 (standard) | 1.125 | 112.5 |
| 4 × 16 (half-high) | 2.25 | 225 |
Grout to fill every cell
| Unit width | Cu ft per sq ft of wall | Cu yd per 100 sq ft |
|---|---|---|
| 6" | 0.19 | 0.7 |
| 8" | 0.27 | 1 |
| 10" | 0.37 | 1.37 |
| 12" | 0.46 | 1.7 |
Approximate, solid-grouted, before waste. Core geometry varies by manufacturer — verify with the supplier’s table for structural work, and the engineer’s drawings govern grout spacing. Partially grouted walls scale by the share of cells filled: cells at 48 inches on center are about one in six.
Mortar: bags versus masonry cement
The 13-blocks-per-bag figure is a manufacturer rule of thumb for 80-lb premixed mortar and small jobs. Production crews buy masonry cement and sand and mix at the wall, which changes the unit you order but not the block count. The field in the calculator is editable — set it to whatever your mix actually lays.
Block walls sit on footings: size those in the concrete calculator. Brick veneer over block comes from the brick calculator, and masonry estimating software turns the count into a priced bid with labor per block. Send the quantities to the free estimate builder to price them now.
Frequently asked questions
How many concrete blocks do I need per square foot?
1.125 standard 8 × 16 blocks per square foot of wall, or 112.5 per 100 sq ft. The nominal 8 × 16 face already includes the 3/8-inch mortar joint, so each block covers 128 square inches. Half-high 4 × 16 units double that to 2.25 per square foot.
How many blocks does a bag of mortar lay?
A common manufacturer rule of thumb is about 13 standard 8 × 8 × 16 blocks per 80-lb bag of premixed mortar. Production crews usually buy masonry cement and sand instead and mix on site, which is cheaper per block — check the coverage printed on whatever you buy.
How much grout does it take to fill a block wall?
Roughly 0.27 cubic feet per square foot for a solid-grouted 8-inch wall — about one cubic yard per 100 sq ft. Grouting only the reinforced cells at 48 inches on center takes about a sixth of that. Core volumes vary by manufacturer, so use the supplier's table for ordering and the engineer's drawings for which cells get grout.
Should I subtract doors and windows when counting block?
Yes — deduct the opening area, then keep the 5% waste factor. Openings create cut units and require lintels or bond beams, and the cuts consume most of what a small opening saves. For walls with many small openings, some estimators skip the deduction entirely.
What is the difference between CMU and cinder block?
Cinder block originally used coal cinders as aggregate; modern concrete masonry units use sand and gravel or lightweight aggregate. The sizes are the same, so the count per square foot, the mortar and the grout math do not change.
Quantities are half the bid.
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