Concrete Block Calculator
Calculate the number of concrete blocks (CMU), mortar bags, and courses needed for your wall. See also Concrete Calculator, Brick Calculator, and Mortar Calculator.
How to Use the Concrete Block Calculator
Enter the wall length and height in feet. The standard CMU (concrete masonry unit) is 8x8x16 inches nominal (actual 7-5/8 x 7-5/8 x 15-5/8 inches). The mortar joint thickness is typically 3/8 inch, which brings the modular size to exactly 8x8x16 inches. The calculator includes a 5% waste factor for breakage and cuts. Mortar bag count assumes one 80 lb bag covers approximately 12 standard blocks.
For walls with openings (doors, windows), calculate the full wall first, then subtract the blocks in the opening area. A standard door opening removes approximately 14 blocks, and a standard window removes about 8 blocks. Add 2-3 lintel blocks above each opening for structural support.
Block Wall Construction Tips
- Start with a level concrete footing at least twice the width of the block wall.
- Lay corner blocks first and use a mason's line to keep courses straight.
- Stagger vertical joints by half a block length (running bond pattern).
- Install horizontal joint reinforcement (ladder wire) every 2-3 courses.
- Fill cores with grout at corners, ends, and every 4 feet for reinforced walls.
- Cap the wall with solid cap blocks or a poured concrete bond beam.
Formula
Courses = Wall Height / ((Block Height + Joint) / 12)
Blocks per Course = Wall Length / ((Block Length + Joint) / 12)
Total Blocks = Courses x Blocks/Course x 1.05 (waste)
Mortar Bags = Total Blocks / 12
Example Calculation
Wall: 20 ft long x 8 ft tall, standard 8x8x16 blocks
Courses = 8 / ((8 + 0.375) / 12) = 11.5 → 12 courses
Blocks/course = 20 / ((16 + 0.375) / 12) = 14.6 → 15
Total = 12 x 15 x 1.05 = 189 blocks
Mortar = 189 / 12 = 16 bags (80 lb)
Block Size Reference Table
| Nominal Size | Actual Size | Weight | Blocks/sq ft | Use |
|---|---|---|---|---|
| 8x8x16 | 7-5/8 x 7-5/8 x 15-5/8 | 35-40 lbs | 1.125 | Standard walls |
| 8x4x16 | 7-5/8 x 3-5/8 x 15-5/8 | 24-26 lbs | 2.25 | Half-height courses |
| 12x8x16 | 11-5/8 x 7-5/8 x 15-5/8 | 50-55 lbs | 1.125 | Foundation walls |
| 6x8x16 | 5-5/8 x 7-5/8 x 15-5/8 | 28-30 lbs | 1.125 | Interior partitions |
| 4x8x16 | 3-5/8 x 7-5/8 x 15-5/8 | 22-24 lbs | 1.125 | Veneer, non-structural |
Frequently Asked Questions
How many concrete blocks do I need per square foot?
Standard 8x8x16 blocks require 1.125 blocks per square foot of wall area (including mortar joints). For quick estimates, multiply wall square footage by 1.125 and add 5-10% for waste and cuts.
How much mortar do I need per block?
One 80 lb bag of mortar mix lays approximately 12 standard blocks (face shell bedding) or 8 blocks (full mortar bedding). For a 100-block wall, plan on 8-12 bags of mortar depending on joint thickness and bedding type.
Do I need to fill concrete blocks with concrete?
Not all cells need filling. Fill cells containing rebar (typically every 4 feet and at corners). For retaining walls and structural applications, fill all cells with grout. Non-structural garden walls may not require any cell filling. Check local building codes.
What is the difference between a cinder block and concrete block?
Modern "cinder blocks" are actually concrete masonry units (CMU). True cinder blocks used coal cinders as aggregate and are no longer manufactured. Today's CMUs use Portland cement, sand, and gravel. The terms are used interchangeably in common speech.
How tall can a concrete block wall be without reinforcement?
Unreinforced block walls are limited to about 4 feet in height for freestanding walls. Walls over 4 feet require vertical rebar (typically #4 at 48" OC) grouted into the cells, plus horizontal joint reinforcement every 2-3 courses. Retaining walls always need reinforcement.
How long does it take to lay concrete blocks?
An experienced mason lays 60-80 standard blocks per hour. A DIYer should expect 20-30 blocks per hour. A 20-foot long, 8-foot tall wall (approximately 180 blocks) takes a professional about 2.5 hours and a homeowner a full day.