Should Concrete Blocks Be Filled?
Constructing walls with concrete blocks leads to void spaces within the walls because of the concrete blocks' hollow structure. To achieve the maximum strength that a concrete block wall can potentially have, it is best to fill these void spaces with concrete or mortar.
Like in most concrete structures, incorporating steel reinforcing bars (rebars) will also strengthen the concrete block wall, especially against cracking. Filling in the cores where the rebars are placed ensures proper bonding between the concrete block and the rebars, creating a column-like structure throughout the wall's length.
Although concrete blocks come under various names — concrete masonry units (CMU), concrete hollow blocks, or cinder blocks — the steps for determining the volume of filling material apply to all of them, as long as their shapes are generally the same.
How Much Concrete to Fill a Concrete Block?
Since we can buy concrete blocks in different sizes, there are differences in how much concrete is needed to fill each type. Concrete blocks are made by casting concrete into molds forming two shells connected by a series of webs. Between the webs are void spaces called cores or cells — these are the spaces we fill with concrete or mortar.
By finding the total volume of the cores per block and multiplying it by the number of blocks needed to build the wall, we can determine how much concrete is needed to fill the entire wall.
Standard Block Fill Volumes
| Block Size (Imperial) | Fill Volume per Block |
|---|---|
| 6" × 8" × 16" | ~0.070 ft³ per block |
| 8" × 8" × 16" (standard) | ~0.102 ft³ per block |
| 10" × 8" × 16" | ~0.128 ft³ per block |
| 12" × 8" × 16" | ~0.158 ft³ per block |
| Block Size (Metric) | Fill Volume per Block |
|---|---|
| 150 × 200 × 400 mm | ~2.0 L per block |
| 200 × 200 × 400 mm (standard) | ~2.8 L per block |
| 250 × 200 × 400 mm | ~3.7 L per block |
| 300 × 200 × 400 mm | ~4.5 L per block |
How to Use Our Concrete Block Fill Calculator
- Choose your unit system — Imperial (feet) or Metric (meters).
- Enter wall dimensions — input the length and height of the wall you are constructing.
- Select the block size — choose the standard block size you are using.
- Set fill percentage — 100% for full fill (all cores), 50% for partial fill (every other core or half-filled).
- Adjust wastage — add 10% (default) to account for spillage and over-ordering.
- Optional: enter price — add your local concrete price per cubic yard or m³ to get a cost estimate.
- Click "Calculate" — get the total fill volume in multiple units.
Sample Calculation
Let's say you want to build a concrete block wall that is 20 ft long and 8 ft high, using standard 8" × 8" × 16" blocks, fully filled (100%), with 10% wastage.
- Wall area: 20 ft × 8 ft = 160 ft²
- Block face area: 8" × 16" = 128 in² = 0.889 ft²
- Number of blocks: ⌈160 ÷ 0.889⌉ = 180 blocks
- Fill per block: 0.102 ft³
- Total fill: 180 × 0.102 = 18.36 ft³
- With 10% wastage: 18.36 × 1.10 = 20.20 ft³ ≈ 0.748 yd³
FAQs
Do I have to fill all concrete block cores?
Not necessarily. Building codes and structural requirements vary. For non-load-bearing partition walls, partial fill or no fill may be acceptable. For load-bearing walls, retaining walls, or seismic zones, full fill with rebar is typically required.
What mix should I use to fill concrete blocks?
A standard grout mix (cement, sand, water) is typically used. For higher strength or when placing rebars, a coarser grout or fine concrete mix is recommended. Consult your local building code for specific requirements.
How many bags of concrete do I need?
A standard 80 lb bag of concrete mix yields approximately 0.60 ft³ of concrete. Divide your total fill volume (with wastage) by 0.60 to get the number of bags. For example, 20.20 ft³ ÷ 0.60 ≈ 34 bags.
What is the difference between partial and full fill?
Full fill (100%) means every core in every block is filled with concrete. Partial fill (50%) typically means alternating cores are filled, or the cores in reinforced cells only. Always verify with your engineer or local code.
Does this calculator work for cinder blocks?
Yes. Cinder blocks (which contain ash or cinders as aggregate) and standard concrete masonry units (CMU) have the same nominal dimensions, so the same fill volumes apply.