Many block walls are not left hollow. Foundation walls, retaining walls and walls that carry heavy loads or resist wind and earthquake loads have vertical steel bars inside some of the cores, and those cores are then filled with grout or concrete to lock the steel and the blocks together. This calculator estimates how much fill you need from the size of the wall, the block thickness and how many cells are filled.
How to use the concrete block fill calculator
- Enter the wall length and height.
- Select the block thickness (4, 6, 8, 10 or 12 inch nominal).
- Pick the fill pattern: every cell for a solid-grouted wall, or the spacing of reinforced cells such as 32 or 48 inches on center.
- Check the hollow share of the block (default 45%) and add a waste allowance.
Block fill formula
Fraction of cells filled = 8 in ÷ cell spacing (8 in = every cell)
Hollow concrete block has a cell about every 8 inches along the wall (two cells per 16-inch unit), so a spacing of 16, 24, 32, 40 or 48 inches fills 50%, 33%, 25%, 20% or 17% of the cells. The calculator multiplies the fraction by the cell volume and adds the allowance for spillage and the extra grout that flows into voids and cracks.
Worked example: a 20 × 8 ft wall, every cell filled
- Wall area: 20 × 8 = 160 sq ft. Block thickness 7.625 in = 0.635 ft.
- Volume of the wall: 160 × 0.635 = 101.7 cu ft.
- Hollow share 45%: 101.7 × 0.45 = 45.8 cu ft = 1.69 cu yd to fill every cell.
- With cells filled only every 32 in and 10% waste: 45.8 × 0.25 × 1.10 = 12.6 cu ft = 0.47 cu yd.
Effect of cell spacing (10 × 8 ft wall, 8-inch block)
| Filled cells | Share of cells | Core volume (cu ft) | With 10% (cu ft) | Cubic yards | 80 lb bags |
|---|---|---|---|---|---|
| 8 in (every cell) | 100.0% | 22.88 | 25.16 | 0.93 | 42 |
| 16 in | 50.0% | 11.44 | 12.58 | 0.47 | 21 |
| 24 in | 33.3% | 7.62 | 8.39 | 0.31 | 14 |
| 32 in | 25.0% | 5.72 | 6.29 | 0.23 | 11 |
| 40 in | 20.0% | 4.58 | 5.03 | 0.19 | 9 |
| 48 in | 16.7% | 3.81 | 4.19 | 0.16 | 7 |
Choosing fill
- Grout. Flowable material designed for masonry cores. Pump or pour it in lifts as specified by the structural plans; high lifts can blow out fresh walls.
- Concrete. Bagged concrete mix is sometimes used to fill a few cores in small DIY projects, such as the post cores of a garden wall.
- Insulation. Not all cores are filled with grout; some are filled with loose or foamed insulation, which is estimated by volume in the same way.
Reinforcement
Where cores are grouted, vertical bars are placed in them before filling and bond-beam courses carry horizontal bars. Use the Rebar Material Calculator for bar quantities. Always follow the schedule on the plans, because grout spacing is an engineering decision tied to loads and local code.
Count the blocks with the Concrete Block Calculator, estimate the mortar with the Block Mortar Calculator and calculate larger pours with the Concrete Calculator.
Frequently asked questions
How much grout does it take to fill concrete block?
It depends on the block thickness and how many cells you fill. With the assumption that 45% of an 8-inch block is hollow, filling every cell takes about 0.286 cubic feet per square foot of wall, or 28.6 cubic feet per 100 square feet, before waste.
How are cells filled at 32 or 48 inches on center?
Blocks are 16 inches long with two cells, so a cell occurs every 8 inches. Filling cells at 32 inches on center fills one in four cells (25%), and 48 inches on center fills one in six (16.7%).
Where do I find the hollow share of my block?
Ask the supplier for the net area or the weight of grout per block, or check the manufacturer’s data. About 45% hollow is typical of an 8-inch two-core normal-weight block, but lightweight and other shapes differ.
Is grout the same as concrete?
Not exactly. Grout used in masonry has a high water content and contains small aggregate (fine grout) or pea gravel (coarse grout) so that it flows into the cores. Ready-mix suppliers sell masonry grout as a separate product, so ask for it by name.
Does this include bond beams and lintels?
No. Bond beam courses and lintel units are filled continuously, so calculate them separately as a long rectangular volume and add it.