Concrete Block Calculator
Calculate exactly how many cinder blocks or concrete blocks you need for your wall. Enter your dimensions below to find block counts, square footage, and project costs.
Whether you are building a retaining wall, a sturdy garage foundation, or an entire house, accurately calculating the materials for a masonry project is vital. Concrete blocks (often referred to as CMU or cinder blocks) are heavy and expensive to transport; ordering too many wastes money, while ordering too few halts your construction crew.
Our free Concrete Block Calculator eliminates the complex math. By inputting your wall dimensions, it automatically factors in standard mortar joints to determine the exact number of blocks you need, along with the appropriate waste allowance and estimated cost.
How to Calculate Concrete Blocks Needed (The Formula)
To manually calculate how many concrete blocks you need to build a wall, you must divide the total surface area of your wall by the surface area of the face of a single block. Because masonry always involves mortar, you must use the nominal size of the block, which includes a standard 3/8-inch (10mm) mortar joint.
Block Calculation Formula
Number of Blocks = Wall Area ÷ Block Face Area
Step-by-Step Example
Let’s calculate the blocks needed for a wall that is 20 feet long and 6 feet high using standard 8″x8″x16″ blocks.
- Calculate the Wall Area: 20 ft (length) × 6 ft (height) = 120 square feet.
- Calculate the Block Face Area: A standard block face is nominally 8 inches high by 16 inches long. Multiply 8 by 16 to get 128 square inches. To convert this to square feet, divide by 144 (the number of square inches in a square foot). 128 ÷ 144 = 0.888 square feet per block.
- Divide the Areas: 120 sq ft (Wall) ÷ 0.888 sq ft (Block) = 135.1 base blocks.
- Add Waste Factor: Always add a 10% allowance for broken blocks and half-cuts at the ends of walls. 135.1 × 1.10 = 148.6. Round up to 149 total blocks.
If you are building a wall with standard 8x8x16 blocks, there is an easy mathematical shortcut. It takes exactly 1.125 blocks to cover one square foot of wall space. Simply multiply your total wall square footage by 1.125 to get your base block count!
Nominal vs. Actual Block Sizes
Like dimensional lumber (where a 2×4 is actually 1.5″ x 3.5″), concrete masonry units (CMU) are sold by their Nominal Size. The nominal size includes the standard 3/8-inch mortar joint.
A standard 8″x8″x16″ concrete block actually measures 7 5/8″ wide × 7 5/8″ high × 15 5/8″ long. When a mason lays the block with a 3/8″ layer of mortar on the bottom and one side, the block mathematically occupies an 8×16 grid space in the wall. You should always calculate quantities using the nominal dimensions.
Standard Concrete Block Sizes Chart
In the United States, block dimensions are typically listed as Width (depth into the wall) × Height × Length. Only the Height and Length affect how many blocks you need to cover a wall area.
| Nominal Size (W x H x L) | Actual Size | Face Area (Sq Ft) | Blocks per Sq Ft |
|---|---|---|---|
| 4″ × 8″ × 16″ (Partition Block) | 3 5/8″ × 7 5/8″ × 15 5/8″ | 0.888 sq ft | 1.125 blocks |
| 6″ × 8″ × 16″ | 5 5/8″ × 7 5/8″ × 15 5/8″ | 0.888 sq ft | 1.125 blocks |
| 8″ × 8″ × 16″ (Standard Block) | 7 5/8″ × 7 5/8″ × 15 5/8″ | 0.888 sq ft | 1.125 blocks |
| 12″ × 8″ × 16″ (Heavy Structural) | 11 5/8″ × 7 5/8″ × 15 5/8″ | 0.888 sq ft | 1.125 blocks |
| 8″ × 8″ × 8″ (Half Block) | 7 5/8″ × 7 5/8″ × 7 5/8″ | 0.444 sq ft | 2.25 blocks |
| 8″ × 4″ × 16″ (Half-Height Block) | 7 5/8″ × 3 5/8″ × 15 5/8″ | 0.444 sq ft | 2.25 blocks |
Concrete Blocks vs. Cinder Blocks
Many people use these terms interchangeably, but they are technically different materials, despite having the same shape and dimensions.
- Cinder Blocks: Historically, these were made using the ash (cinders) left over from burning coal. They are lighter, less dense, and generally not as strong. True cinder blocks are rarely manufactured today and are not approved for modern load-bearing construction.
- Concrete Blocks (CMU): Modern blocks are made using a mixture of Portland cement, water, and aggregates (sand and fine gravel). They are much heavier and significantly stronger, meeting strict building codes for foundations, retaining walls, and structural housing.
Why You Need a Waste Factor
Do not just order the exact base mathematical number of blocks. You must order extra material, commonly referred to as a “waste factor.” We recommend a 10% waste allowance for the following reasons:
- Breakage: Concrete blocks are brittle. Edges and corners frequently chip or crack during transport, unloading, and staging on the job site.
- Cuts: Unless your wall length is a perfect multiple of 16 inches, you will have to cut blocks to finish a row. Cutting blocks with a masonry saw often results in unusable scraps.
- Staggering: Masonry walls are built in a “running bond” pattern (staggered joints) for strength. This means every alternating row starts and ends with a half-block cut.
Concrete Block Calculator Frequently Asked Questions
To calculate the number of concrete blocks needed, calculate the square footage of your wall (Length × Height). Then, calculate the square footage of a single block face. Divide the wall area by the block area to get the total blocks. Finally, add a 5% to 10% waste factor for cuts and breakages.
Measure the nominal face of the block (which includes the mortar joint). A standard 8x8x16 block has a nominal face of 8 inches high by 16 inches long. Multiply these to get 128 square inches, then divide by 144 to find that one block covers exactly 0.888 square feet of wall space.
For a 100 square foot wall using standard 8x8x16 concrete blocks (which cover 0.888 sq ft each), you mathematically need 112.5 blocks. Including a standard 10% waste factor for breakages and cuts, you should order roughly 124 blocks.
It takes exactly 1.125 standard 8x8x16 concrete blocks to cover one square foot of wall area. You can easily find your total base blocks by multiplying your wall’s entire square footage by 1.125.
A standard hollow 8x8x16 Concrete Masonry Unit (CMU) typically holds between 1,700 and 1,900 pounds per square inch (psi) in compressive strength. When oriented correctly with the hollow cores vertical, a single block can theoretically support over 100,000 pounds of evenly distributed weight.
Poured concrete walls are typically more expensive upfront due to the specialized labor and materials required for building heavy wooden forms and hiring a concrete pump truck. Concrete block walls are generally cheaper on a materials basis and easier for DIYers, though they take significantly more time to physically stack.
While concrete block (CMU) is heavily used in commercial construction and regions prone to hurricanes (like Florida), it is less common for standard residential homes in the US because wood framing is faster, cheaper, allows for easier thermal insulation, and makes running plumbing and electrical lines much simpler inside the hollow walls.
Standard 8x8x16 concrete blocks typically cost between $1.50 and $3.00 each at a local hardware store. Therefore, 100 blocks will usually cost between $150 and $300, excluding the additional costs of mortar mix, rebar, and delivery fees.
Purchasing 1,000 standard 8x8x16 concrete blocks will typically cost between $1,500 and $3,000. Buying in bulk (by the pallet) directly from a local masonry supply yard or manufacturer rather than a big box retail store will typically secure you a lower price per unit.
Reference: https://en.wikipedia.org/wiki/Concrete_block