Reinforced block walls have vertical rebar in some or all cores, and those cores are filled with grout or pea-gravel concrete. Basement and retaining walls are often solid-grouted, while above-grade walls may only be filled at rebar locations (for example every 32 or 48 in). The volume of the hollow cores depends on the block width.
How to use the Concrete Block Fill Calculator
- Wall length
- Wall height
- Block width
- Cores filled
- Waste
- Premix bag size
- Price per bag
- Grout price (if ordered as ready-mix)
Results update as you type, and every field accepts the units shown in its menu.
How it works
Enter the wall length and height, choose the block width, the percentage of cores to fill and waste, and the bag size. The calculator counts blocks, multiplies by the core volume of each block and the fill fraction, and converts to bags.
Premixed bag yields
| Bag | Yield | Bags per yd³ |
|---|---|---|
| 40 lb | 0.30 ft³ | 90 |
| 50 lb | 0.375 ft³ | 72 |
| 60 lb | 0.45 ft³ | 60 |
| 80 lb | 0.60 ft³ | 45 |
| 25 kg | ≈ 0.0117 m³ | ≈ 85 per m³ |
Tips for an accurate estimate
- Measure the formwork, not the drawing – forms bulge and subgrades are rarely perfectly flat, so add 5–10%.
- Below about 1 cubic yard, premixed bags are usually cheaper; above that, ready-mix saves time and gives a more consistent pour.
- Order ready-mix in quarter- or half-yard steps and ask about short-load fees for small deliveries.
- Keep fresh concrete damp for at least a week – curing matters as much as the mix for final strength.
Common mistakes to avoid
- Running short mid-pour, which leaves a weak cold joint.
- Adding extra water on site to make concrete easier to place – it lowers strength and increases cracking.
- Ignoring the volume of rebar, posts or pipes in the pour, or the gravel base under it.
Inputs explained
| Field | What to enter |
|---|---|
| Wall length | Units: ft, m, in, yd, cm |
| Wall height | Units: ft, m, in, yd, cm |
| Block width | Choose one: 6 in (≈ 0.17 ft³ core per block), 8 in (≈ 0.25 ft³), 10 in (≈ 0.35 ft³), 12 in (≈ 0.43 ft³) |
| Cores filled | In % |
| Waste | In % |
| Premix bag size | Choose one: 40 lb bag (0.30 ft³), 50 lb bag (0.375 ft³), 60 lb bag (0.45 ft³), 80 lb bag (0.60 ft³) … |
| Price per bag | Units: /piece |
| Grout price (if ordered as ready-mix) | Units: /yd³, /m³, /ft³ |
What each result means
- Blocks in wall = ceil([Wall length]×[Wall height]÷0.08258 – 0.000001)
- Fill volume = [Blocks in wall]×[Block width]×[Cores filled]÷100×(1 + [Waste]÷100)
- Bags of concrete = ceil([Fill volume]÷[Premix bag size])
- Cost with bags = [Bags of concrete]×[Price per bag]
- Cost with ready-mix grout = [Fill volume]×[Grout price (if ordered as ready-mix)]
Formula
Blocks = wall area ÷ 0.889 ft²Fill = blocks × core volume per block × fill fraction × (1 + waste)Bags = fill ÷ bag yieldWorked example
Inputs — Wall length: 40 ft; Wall height: 8 ft; Block width: 8 in (≈ 0.25 ft³); Cores filled: 100 %; Waste: 10 %; Premix bag size: 80 lb bag (0.60 ft³); Price per bag: $6.5 /piece; Grout price (if ordered as ready-mix): $180 /yd³.
Results — Blocks in wall = 361; Fill volume = 3.677 yd³; Bags of concrete = 166; Cost with bags = $1,079.00; Cost with ready-mix grout = $661.83.
Frequently asked questions
How much concrete fills an 8 in block?
About 0.25 ft³ per block, so 1 yd³ fills roughly 105–110 blocks.
Do I have to fill every core?
Only where the design calls for it – typically cores with rebar, bond beams and below-grade walls.
Should I use grout or concrete?
Use coarse grout (ASTM C476) or a pea-gravel mix with high slump so it flows into every void.
What formula does the Concrete Block Fill Calculator use?
Blocks = wall area ÷ 0.889 ft².