The Block CalculatorWhat is concrete block fill?
Standard concrete masonry units have large vertical cavities called cores. To increase strength, shear resistance and load capacity, builders pour fluid concrete or grout into these cores, creating solid concrete columns inside the wall. Concrete block fill is the volume needed to pack those cavities, so you can order the right amount of ready-mix or bagged concrete.
Pros & cons of filling block cores
✓ Advantages
- Structural integrity: much higher load capacity with steel rebar
- Moisture barrier: less water gets through below-grade walls
- Sound insulation: dense cores damp noise
✕ Disadvantages
- Extra expense: more concrete and labour
- Thermal transfer: solid cores conduct heat more than air-filled cores
- Wall weight: much higher dead load
How does this calculator work?
It counts the blocks from the wall face area, works out the volume of one core (core width × core length × block height), multiplies by cores per block and block count, then converts to your chosen unit. The bag estimate assumes a bag yields 0.6 ft³ (typical 80 lb bags yield about 0.6 ft³; 60 lb bags about 0.45 ft³), so check the yield printed on your bag.
Formula used
Blocks Required = CEILING((Wall Length × Wall Height) ÷ (Block Length × Block Height))
Core Volume = Core Width × Core Length × Block Height
Total Fill = Core Volume × Cores per Block × Blocks Required
Cubic Feet = Total Fill (cubic inches) ÷ 1,728
Concrete Bags = CEILING(Cubic Feet ÷ 0.6)Example
Scenario: A wall 20 ft long and 10 ft high using 16″ × 8″ × 8″ blocks, each with 2 cores of 4″ × 5″.
Wall area: 240″ × 120″ = 28,800 sq in
Blocks: 28,800 ÷ 128 = 225
One core: 4 × 5 × 8 = 160 cu in
All cores: 160 × 450 = 72,000 cu in
Final result: 72,000 ÷ 1,728 = 41.67 ft³ (1.54 yd³), or 70 bags at 0.6 ft³ per bag.
Frequently asked questions
How much concrete does it take to fill one 8″ block?
A typical 8 × 8 × 16 two-core block holds about 0.19–0.22 ft³ of fill, depending on the core size.
How many 60 lb bags make a cubic yard?
At 0.45 ft³ per bag you need about 60 bags per cubic yard. Always use the yield printed on the bag you buy.
Last updated: . Results are estimates — see our disclaimer.