Concrete calculator
Concrete Block Calculator
Enter wall dimensions, total opening area, nominal block face dimensions, and a cuts allowance to estimate full-block equivalents, courses, and blocks to buy.
Planning estimate, not a contractor quote. Review the assumptions and replace presets with local prices when available.
Method
How this calculator works
Concrete masonry is laid out by nominal module dimensions, which include the mortar-joint allowance. A common unit has a nominal face of 8 in by 16 in even though the unit itself is typically 3/8 in smaller in each jointed direction.
Net wall area = wall length × wall height − opening area. Exact block equivalents = net wall area ÷ nominal block face area. Blocks to buy = exact equivalents × (1 + allowance percentage), rounded up to a whole unit.
Worked example
A realistic starting point
A 20 ft by 8 ft wall has 160 ft². Subtracting a 3 ft by 7 ft opening leaves 139 ft². Nominal 8 in by 16 in modules require 156.375 block equivalents; with 5% allowance, plan on 165 blocks.
What changes the answer
Factors worth checking
Use nominal face dimensions
Nominal dimensions coordinate the unit and mortar joint. A typical 8 × 8 × 16 in CMU is usually 3/8 in smaller than nominal in each jointed direction.
Subtract openings carefully
Use combined rough-opening area for doors and windows. The final layout may still need cut, jamb, lintel, and bond-beam units around them.
Plan special units separately
Corners, ends, bond beams, lintels, control joints, and partial-height courses can change the mix of units without changing net wall area.
Allowance is project-specific
Cuts, breakage, returns, bond pattern, and supplier packaging affect how many units to order. Adjust the default rather than treating it as universal.
Common questions
Concrete blocks FAQ
How many nominal 8 × 8 × 16 blocks cover 100 square feet?
The face module is 128 in², so the exact area calculation gives 112.5 units; rounding gives about 113 full-block equivalents before a project allowance.
Should I use actual or nominal block dimensions?
Use nominal face dimensions for modular wall area. CMHA notes that actual CMU dimensions are typically 3/8 in smaller to accommodate mortar joints.
Should door and window openings be subtracted?
Yes, enter their combined rough-opening area. Keep an allowance because units around openings often require cuts or specialty shapes.
Does the result include mortar or grout?
No. Mortar and grout depend on unit geometry, jointing, bedding, and structural layout, so estimate them from the selected product and project documents.
Evidence
Sources & review
- CMHA — Modular Layout of Concrete MasonryIndustry reference for nominal modules, typical dimensions, and 3/8 in joints.
- CMHA — Concrete Masonry Basement Wall ConstructionIndustry rule of thumb of 113 standard CMUs per 100 ft² and mortar context.
- QUIKRETE Concrete CalculatorManufacturer reference for standard block and mortar-joint calculations.
Published by Calcstead. Report a correction.
Method and source links reviewed September 4, 2026. Price presets are editable planning inputs and should be replaced with current local information.