Home & Construction Calculators
Estimate concrete masonry units, courses, pallets, mortar and a detailed project cost. Enter actual block dimensions plus mortar joint thickness for accurate modular coverage.
Concrete block formula
Net area = gross area − openings
Modular block face = (actual length + joint) × (actual height + joint)
Base blocks = net area ÷ modular face area
Order quantity = round up(base blocks × (1 + waste %))
The modular face includes one mortar joint in each direction. Standard US blocks commonly have actual dimensions about 3/8 inch smaller than their nominal dimensions, allowing for a 3/8-inch mortar joint.
Layout check
The calculator also shows full courses and blocks per full course. Openings make real layouts more complex, so use the net-area count for purchasing and the layout count to plan bond, cuts and corners.
Worked example
A 20 ft × 8 ft wall has 160 ft² gross area. Subtracting 20 ft² of openings leaves 140 ft². A 15 5/8 in × 7 5/8 in block with 3/8-in joints has a 16 in × 8 in modular face, covering 0.889 ft². The wall needs about 158 blocks before waste and 166 blocks with 5% waste.
What else to include
- Half blocks, bond beams, corners and specialty units.
- Lintels above doors and windows.
- Vertical and horizontal reinforcement.
- Core grout where specified.
- Flashing, anchors, control joints and caps.
Measurement checklist
- Separate walls with different block sizes.
- Measure doors and windows and total their areas.
- Confirm whether supplier dimensions are nominal or actual.
- Check pallet quantities for the exact block style.
- Use the mortar product yield for your block and joint profile.
- Round purchasing quantities upward only after adding waste.
Walls with many corners, cuts, bond beams or architectural units may require a larger allowance than a simple straight wall.
Frequently asked questions
How many 8 × 8 × 16 blocks cover one square foot?
With the nominal 16 in × 8 in face, each block covers about 0.889 ft², so about 1.125 blocks cover one square foot before waste.
Should openings be subtracted?
Yes for net-area estimating, but small openings may not reduce purchasing quantity much because cuts and waste remain.
Does this calculate grout fill?
No. Use the related block-fill calculator because grout volume depends on block core geometry and the number of filled cells.
Are mortar bags exact?
No. Joint size, block shape, waste and installer technique affect yield. Replace the default with the manufacturer coverage.
Common mistakes
- Mixing nominal and actual block dimensions.
- Forgetting mortar-joint dimensions.
- Subtracting openings twice.
- Ignoring corner and specialty units.