Concrete Block Fill
Calculate concrete or mortar volume needed to fill concrete block cores for any wall. Enter wall dimensions and block size for fill volume, bags, and cost with charts.
About This Calculator
This Concrete Block Fill Calculator helps contractors, builders, and DIY enthusiasts estimate the exact volume of concrete or mortar needed to fill the hollow cores of concrete masonry units (CMUs) when constructing a block wall. Whether you are building a retaining wall, foundation, load-bearing wall, or decorative block fence, knowing the fill volume ensures you order the right amount of materials and avoid costly shortages or waste.
The calculation works by first determining the total wall area (width x height) and dividing it by the face area of one block to find the number of blocks needed. Each block size has a standard core volume — for example, 8-inch blocks have approximately 550 cubic inches of core space per block. The total fill volume is core volume per block multiplied by the number of blocks. The result is then converted to cubic feet and adjusted for wastage (default 10%). Finally, the calculator divides the total fill volume by the volume per bag of concrete mix (0.45 cubic feet for a 60-lb bag) to give you the exact number of bags needed, along with the total material cost.
Regional Notes
United States and Canada: Standard CMU blocks are 8×8×16 inches nominal (actual 7-5/8×7-5/8×15-5/8 inches). Concrete is typically sold in 60-lb or 80-lb bags yielding 0.45 and 0.60 cubic feet respectively. Ready-mix concrete is ordered by the cubic yard (1 yard = 27 cubic feet).
United Kingdom: Standard concrete blocks are 440×215×100 mm (length × height × thickness). Blocks are often referred to as 100 mm, 140 mm, or 215 mm wide for different wall thicknesses. Concrete is ordered by the cubic metre. Our calculator supports inches and feet for US/imperial measure; for metric, convert wall dimensions to feet first (1 m = 3.28 ft) and use the closest standard block thickness.
India: Concrete hollow blocks (CHB) come in 400×200×100 mm, 400×200×150 mm, and 400×200×200 mm sizes. The fill volume calculations follow the same principles. Concrete is typically ordered as ready-mix in cubic metres or as bagged cement for site mixing. Use the custom block dimensions feature to match your local block sizes.
Frequently Asked Questions
What is concrete block fill?
Concrete block fill refers to the concrete or mortar used to fill the hollow cores (cells) inside concrete masonry units (CMUs) when constructing a block wall. Filling the cores adds structural strength, improves fire resistance, and creates a solid wall system. When steel reinforcement bars (rebars) are placed in the cores before filling, the concrete bonds with the rebar to form a reinforced concrete column-like structure throughout the wall.
How do I calculate concrete block fill volume?
To calculate concrete block fill volume: 1) Multiply wall width by wall height to get wall area. 2) Divide wall area by the face area of one block (length x height) to find the number of blocks needed. 3) Multiply the core volume per block (which depends on block thickness) by the number of blocks. For standard 8-inch blocks, each block core has approximately 550 cubic inches. 4) Convert to cubic feet by dividing by 1,728. 5) Add 5-10% wastage for spillage and overfill.
How much concrete to fill an 8x8x16 inch block?
You need approximately 550 cubic inches of concrete to fill a standard 8x8x16 inch concrete block. This is calculated as: inside width (16 - 2 x 1.25 shell - 1 web = 12.5 in) x inside thickness (8 - 2 x 1.25 shell = 5.5 in) x height (8 in) = 550 cu in. This equals about 0.318 cubic feet per block. For a wall with 90 blocks, you would need approximately 28.6 cubic feet or about 64 bags of 60-lb concrete mix.
Should concrete blocks be filled?
Concrete blocks should be filled with concrete or mortar when maximum structural strength is required, especially for load-bearing walls, retaining walls, and foundations. Filling the cores creates a solid wall that resists cracking and provides better fire resistance. When steel rebars are placed vertically in the cores before filling, the concrete bonds with the steel to create reinforced concrete columns within the wall, significantly improving seismic and wind resistance. Filling is also recommended for walls taller than 8 feet or in high-wind zones.
How many bags of concrete do I need for block fill?
The number of bags depends on the total fill volume and bag size. A standard 60-lb bag of concrete mix yields approximately 0.45 cubic feet. An 80-lb bag yields about 0.6 cubic feet. For example, a 30 ft x 10 ft wall using 8-inch blocks needs approximately 90 blocks with a fill volume of about 28.6 cubic feet. With 60-lb bags: 28.6 / 0.45 = 64 bags. Always round up and add 5-10% extra for wastage.
What is the difference between concrete block fill and mortar?
Concrete block fill (also called grout) is a fluid concrete mix designed to flow into the hollow cores of concrete blocks and around rebar. It has a higher slump (wetter consistency) than standard concrete. Mortar is a thicker mixture used to bond blocks together in the horizontal and vertical joints. Mortar contains more lime and sand and less coarse aggregate, making it workable for trowel application but unsuitable for filling cores. For core fill, always use a grout or concrete mix designed to flow freely into narrow cavities.
How do I account for wastage in block fill calculation?
A standard wastage factor of 5% to 10% should be added to the calculated fill volume to account for spillage, overfill of cores, leftover mix in the mixer, and material lost during transport. For smaller projects or complex walls with many corners, use 10% wastage. For larger, straightforward walls, 5% may suffice. Our calculator defaults to 10% wastage but you can adjust this based on your project complexity and experience.
What block sizes are available for fill calculation?
Standard concrete block thicknesses include 4 inches (100 mm), 6 inches (150 mm), 8 inches (200 mm), 10 inches (250 mm), and 12 inches (300 mm). The core volume varies by thickness: 4-inch blocks have approximately 225 cu in per core, 6-inch blocks 392 cu in, 8-inch blocks 550 cu in, 10-inch blocks 706 cu in, and 12-inch blocks 858 cu in. Standard block face dimensions are 16 inches long by 8 inches high, but you can enter custom block dimensions in our calculator.