Concrete Block & Masonry Calculator
Estimate the number of blocks, mortar, and total cost for your wall project.
How to Use the Block Calculator
Planning a masonry project requires precision to ensure you don't run out of materials mid-build or overspend on unnecessary surplus. This block calculator is designed to help DIYers and contractors alike determine the exact number of concrete masonry units (CMUs) needed for any rectangular wall.
Standard Block Dimensions
In North America, a "standard" concrete block is referred to as an 8x8x16. However, its actual dimensions are usually 7 5/8″ x 7 5/8″ x 15 5/8″. The remaining 3/8″ is accounted for by the mortar joint. Our calculator automatically factors in the standard 3/8″ (0.375 inch) mortar joint to give you a real-world estimate.
The Math Behind the Calculation
To calculate how many blocks you need, we use the following formula:
- Calculate Wall Area: Length (ft) × Height (ft) = Total Square Footage.
- Calculate Block Coverage: (Block Length + Joint) × (Block Height + Joint) / 144 = Square Feet per Block.
- Calculate Quantity: Wall Area / Square Feet per Block = Total Blocks.
- Add Waste: Most masons recommend adding 5-10% for cuts and breakage.
Practical Example
Imagine you are building a retaining wall that is 20 feet long and 4 feet high using standard 8×16 blocks.
- Wall Area: 20 × 4 = 80 sq. ft.
- Standard Block Coverage: One 8×16 block covers roughly 0.89 sq. ft. (including mortar).
- Base Requirement: 80 / 0.89 ≈ 90 blocks.
- With 10% Waste: 90 + 9 = 99 blocks total.
Mortar Estimation
A general rule of thumb for mortar is that 3 bags of pre-mixed mortar (80lb each) are required for every 100 standard blocks. Our calculator provides a rough estimate based on this ratio to help you start your shopping list.
Professional Tips for Masonry Projects
- Footings Matter: Ensure your wall sits on a concrete footing that is twice as wide as the block itself.
- Leveling: The first course of blocks is the most critical. If the first row is not perfectly level, the error will amplify as the wall gets higher.
- Reinforcement: For walls over 3 feet high, consider using rebar and filling the block cores with grout for structural integrity.