Reference formulas and worked examples below keep their stated units. Monetary examples use USD; dimensional examples use US customary units unless labeled otherwise. Metric calculator inputs are converted to these reference units before calculation. Choosing another currency does not convert example amounts or exchange money.
How to use this concrete block calculator
Enter matching wall dimensions and the total opening area across all walls. Nominal block dimensions already include a joint; with actual dimensions, enter the joint separately.
How the calculation works
Net face area = wall length × wall height × matching walls − total opening area. Divide by the nominal face area of one block. For actual dimensions, add one joint to length and height before computing the module area.
Blocks to order = area-equivalent blocks × (1 + waste percentage ÷ 100), rounded up. Blocks before allowance are rounded up separately; the difference is the additional whole-block allowance.
Mortar bags = whole blocks before allowance ÷ supplier coverage × (1 + mortar allowance ÷ 100), rounded up. Extra ordered blocks do not automatically require mortar. Material subtotal adds ordered blocks and estimated mortar at your prices.
Worked example
A 10 × 8 ft wall without openings has 80 sq ft of face area. A nominal 16 × 8 in block covers 0.8889 sq ft, giving 90 blocks before allowance. With 5% extra, order 95. At 13 blocks per mortar bag and no extra mortar allowance, the estimate is 7 mortar bags.
Limits and assumptions
An area estimate, not an exact course-by-course layout or structural design. It does not optimize cut pieces, bond patterns, corners, specialty blocks, reinforcement or grout. The 13-block mortar coverage is a product-specific example, not a universal yield. Delivery, labor and tax are excluded.
Frequently asked questions
What is the difference between nominal and actual dimensions?
Nominal face dimensions include the intended mortar joint. Actual dimensions describe the block itself; the tool adds your entered joint once.
Can I subtract doors and windows?
Yes. Enter their total combined area across all matching walls. The total cannot exceed the gross wall area.
Will every mortar bag lay 13 blocks?
No. That is an editable product example. Coverage depends on block size, joint, mortar and workmanship; use the manufacturer’s figure.
Is the result an exact block layout?
No. It estimates material from net area. A layout is still needed to account for bonds, cuts, corners and special units.