Concrete Block (CMU) Calculator
Estimate a CMU wall from its size: block count, mortar (cement + sand), core-fill grout, and rebar for 6, 8, and 12 in units per NCMA TEK 14-13. Subtract openings and add a waste allowance. Free, no sign-up.
What to calculate next
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Explanation
A concrete block wall is bought as a take-off: how many concrete masonry units (CMU), how much mortar to lay them, how much grout to fill the cells, and how much rebar to reinforce them. This calculator starts from the wall length and height, subtracts the openings, and returns every quantity at once — blocks, masonry cement and sand, core-fill grout, and the vertical and horizontal reinforcement — so you can order a full wall from one screen.
How block, mortar, grout, and rebar are calculated
Every quantity is driven by the net wall area. Standard CMU all share a nominal 16 in long by 8 in high face — about 0.89 ft² — so one square foot of wall takes 1.125 blocks regardless of whether the unit is 6, 8, or 12 in wide:
Mortar follows the block count. The industry rule of thumb is three bags of masonry cement for every 100 blocks, mixed with about one cubic yard of sand per seven bags of cement:
Grout fills the hollow cells. A fully grouted wall fills every cell, while a reinforced wall usually grouts only the cells that hold a vertical bar. The volume is the number of filled cells times the grout each one needs, and rebar is counted from the spacing you set. The grout you pour is the same material covered by the concrete calculator, so you can size a footing pour on the same trip.
| Symbol | Meaning |
|---|---|
| Net area | wall length × height, minus door and window openings (ft²) |
| 1.125 | blocks per net ft² for any 16×8 in face CMU (NCMA TEK 14-13) |
| 33.3 | blocks laid per bag of masonry cement (≈ 3 bags per 100 blocks) |
| 7 | bags of masonry cement per cubic yard of sand |
| Grout/block | core-fill volume to solidly grout one block (ft³) |
CMU sizes and core-fill grout
Block width does not change the block count, but it changes the grout. Wider units have larger cells, so a fully grouted 12 in wall takes roughly three times the grout of a 6 in wall. The table below lists the solid core-fill volume per block and the resulting grout per 100 ft² of fully grouted wall:
| Nominal size | Grout per block (ft³) | Grout per 100 ft² (yd³) |
|---|---|---|
| 6 in (6×8×16) | 0.15 | ≈ 0.62 |
| 8 in (8×8×16) | 0.27 | ≈ 1.12 |
| 12 in (12×8×16) | 0.48 | ≈ 2.00 |
Reinforced walls are far lighter on grout because only the bar cells are filled. The vertical bars also set the grout, so the spacing you pick for rebar weight feeds straight into the core-fill quantity.
Notes and assumptions
Block counts use 3⁄8 in mortar joints and nominal dimensions. Grout volumes are typical core-fill figures; actual void space varies by manufacturer and unit configuration, so confirm against the producer’s data sheet for a large pour.
Rebar lengths are the in-wall lengths only — add laps, dowels, and bends from your structural drawings. Reinforcement spacing, grouting, and minimum cover for a load-bearing or retaining wall are set by the engineer of record under ASTM C90 and the applicable masonry code, not by a rule of thumb. Treat the take-off as an ordering estimate and round up at the supplier.