CMU Block Calculator
Enter wall dimensions and block size to get the CMU count with joints and breakage included, plus mortar bags, sand volume and the wall's weight for footing checks.
Last updated: 2026-09-22
How the calculation works
- Nominal block dimensions include the 3/8 in mortar joint — an 8×8×16 CMU actually measures 7⅝ × 7⅝ × 15⅝ in.
- Face area per block is computed from the nominal size, so joint spacing is automatically included.
- A 5% allowance covers breakage, cuts at corners and openings, and the occasional chipped unit.
- Mortar and sand follow the masonry working averages: 3 bags of Type N per 130 block, ~3 ft³ of sand per 100 block.
Formula
blocks = wall_ft² × blocks_per_ft² × 1.05 blocks_per_ft² = 1 / (nominal_W_ft × nominal_H_ft) mortar bags ≈ 3 per 130 block sand ≈ 3 ft³ per 100 block
| Symbol | Meaning | Unit |
|---|---|---|
A | Wall face area | ft² |
W×H | Block nominal face dimensions (joint included) | in |
1.05 | Breakage / cutting allowance | — |
Worked example
Interpreting the result
The count includes joints and 5% breakage — order the whole figure. Bond beams, lintel block and open-bottom units for reinforcing cells come from the same pallet but add cost; a typical residential foundation wall adds 5–10% special units. Half-high block (8×4×16) doubles the block count per area — courses are 4 in, not 8 in, and the labor doubles with them.
Assumptions
- Running bond, single wythe, standard 3/8 in joints.
- 8×8×16 CMU at ~35 lb each; other sizes scale by actual weight (6 in ≈ 28 lb, 12 in ≈ 50 lb).
- Wall area taken at full length × height — large openings (>20 ft²) should be deducted manually.
Limitations
- No lintels, control joints or bond beams itemized.
- Reinforced cells (vertical rebar + grout) are separate material not counted here.
- Mortar quantities are working averages — severe weather or absorptive block can raise them.
Why nominal dimensions include the joint
Masonry is the one building material specified by its laid size, not its manufactured size. A '16 inch' block plus one 3/8 in joint is 16⅜ in of wall — which means three courses of block plus three joints measure 24 in exactly, and 12 courses of 8-in nominal block build a wall precisely 96 in tall. Every layout in masonry works backward from this: architects dimension walls in course counts because the mortar joint is part of the module. Buying by actual size would leave you 5% short and a course misaligned with the opening heights on the drawing.
Frequently asked questions
How many concrete blocks do I need per square foot?
For standard 8×8×16 block: 1.125 blocks per ft² of wall face (112.5 per 100 ft²), joints included. Half-high 8×4×16 doubles that to 2.25 per ft².
How many bags of mortar per 100 blocks?
About 2.3 bags — the working figure is 3 bags per 130 block. Heavily absorptive block in hot weather can push that up 10–15%.
Is cinder block the same as CMU?
Colloquially yes. True cinder block (ash aggregate) is largely historical; modern CMU uses sand and gravel aggregate and is stronger. The dimensions and counts here apply to both.