Concrete Block Calculator
Get an accurate block count, mortar estimate, and grout fill quantity for any CMU wall — including openings for windows and doors.
Concrete Block Estimator
// Your Results
How Concrete Block Counts Are Calculated
All standard concrete blocks share the same 8"×16" face dimensions regardless of width (depth). With a 3/8" mortar joint, each block covers 0.889 square feet — or 1.125 blocks per square foot of wall. This ratio is constant whether you're using 4", 6", 8", or 12" wide blocks.
To calculate: (wall length × wall height) − opening area = net sq ft. Multiply by 1.125, then add your waste factor. Subtract openings before multiplying or you'll dramatically overorder.
Don't forget openings. A standard 36"×80" door = 20 sq ft. A 36"×48" window = 12 sq ft. Missing these is the #1 block estimation error.
Block Size Reference
| Block (nominal) | Blocks/Sq Ft | Best Use |
|---|---|---|
| 4×8×16 | 1.125 | Interior non-load-bearing partitions |
| 6×8×16 | 1.125 | Light load-bearing, fences |
| 8×8×16 | 1.125 | Most walls, garages, foundations |
| 12×8×16 | 1.125 | Heavy retaining, high-load bearing |
Grout Fill Requirements
Building codes typically require fully grouted cores for: walls over 4 feet supporting loads, retaining walls, seismic zones, and any wall with rebar. An 8" block fully grouted uses approximately 0.05 cubic yards of grout per block. Partial fill (every other core) uses about half. No fill is only acceptable for decorative garden walls and non-structural applications.
Frequently Asked Questions
- How many concrete blocks per square foot?1.125 blocks per square foot for standard 8×16 face blocks, accounting for mortar joints. This applies to all block widths (4", 6", 8", 12") since they all share the same face size.
- How much mortar do I need for concrete block?Approximately one 60lb bag of mortar per 28 blocks — about 25–30 blocks per bag depending on joint thickness and your mixing ratio. Buy a few extra bags; mortar is cheap and running short mid-course is frustrating.
- Do I need rebar in a concrete block wall?For structural walls, retaining walls, and walls over 4 feet, rebar is typically required by building code. Vertical rebar runs through hollow cores (then grouted). Always pull a permit for structural masonry — an inspector will catch missing rebar.
- What's the difference between Type S and Type N mortar for block?Type S mortar (1800 PSI) is for below-grade and exterior applications — foundations, retaining walls, masonry in contact with ground. Type N mortar (750 PSI) is for above-grade, interior, and non-structural applications. When in doubt, use Type S.