What this calculator does
Filling block cores takes more grout than most people expect. Ninety standard blocks hold 33.75 cubic feet of core volume, or 37.13 with a 10% allowance, which is nearly one and a half cubic yards.
The volume is easy to underestimate because the cores are hidden. Each block holds 648 cubic inches, which does not sound like much, but ninety of them is over a cubic yard of grout. Running short mid-lift is a real problem, since a partly filled wall has a cold joint where the grout stopped.
The formula
The core volume per block is the inside width multiplied by the inside thickness and the block height, giving cubic inches, which is converted to cubic feet. Multiplying by the block count and adding the waste allowance gives the total. The inside dimensions exclude the face shells and webs, so they are smaller than the nominal block size.
| Term | Meaning |
|---|---|
| Core | The hollow void through a masonry block, which grout fills. |
| Grout | A high-slump concrete mix that flows into the cores. Not the same as mortar. |
| Face shell | The outer wall of the block. Its thickness is excluded from the core dimensions. |
| Lift | The height of wall grouted in one pour, typically limited to around 5 feet. |
The inputs explained
| Field | What to enter |
|---|---|
| Inside core width (block width minus face shells) (in) | Inside core width in inches: the block length less the face shells and webs. |
| Inside core thickness (block thickness minus webs) (in) | Inside core thickness: the block thickness less the two face shells. |
| Block height (in) | Block height, usually 8 inches nominal. |
| Number of blocks to fill | Number of blocks to fill. Only fill the cores the design requires, not every block. |
| Waste allowance (%) | Waste allowance. 10% covers spillage and the grout left in the mixer and pump. |
When to use it
Grouting a reinforced block wall
Cores containing reinforcement must be filled, and the volume determines the grout order.
Filling a retaining wall
Block retaining walls are commonly core-filled for mass and strength.
Estimating a partial fill
Many designs fill only the cores containing vertical bars, which is far less than every block.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How much grout for each block count?
A range of block counts with the grout needed.
| Blocks to fill | Grout volume needed | In cubic yards | Core volume per block |
|---|---|---|---|
| 30 blocks | 12.38 ft³ | 0.458 yd³ | 648 in³ |
| 60 blocks | 24.75 ft³ | 0.917 yd³ | 648 in³ |
| 90 blocks | 37.13 ft³ | 1.375 yd³ | 648 in³ |
| 120 blocks | 49.50 ft³ | 1.833 yd³ | 648 in³ |
Questions
Do I need to fill every block?
Rarely. Most designs fill only the cores containing vertical reinforcement, plus bond beam courses. Filling every core adds considerable weight and cost for little benefit unless the design calls for a solid wall. Follow the structural drawings.
What is the difference between grout and mortar?
Mortar bonds the blocks together and is stiff. Grout fills the cores and is deliberately high-slump so it flows into the voids and around reinforcement without leaving gaps. Using mortar to fill cores leaves voids and does not achieve the intended strength.
How high can I grout in one lift?
Commonly limited to around 5 feet unless high-lift grouting procedures are used, because the fluid pressure can blow out the mortar joints below. The limit varies by code and by whether the wall has cured sufficiently, so check the local requirement.
Why allow for waste?
Grout is lost to spillage, to the mixer and pump, and to overfilling at the top of each core. A 10% allowance is normal. Running short mid-lift leaves a cold joint in the grout column, which compromises exactly the continuity the fill was meant to provide.
For the blocks themselves, see the brick and block calculator. For general concrete volumes, see the concrete calculator.