Concrete Block Calculator
Estimate blocks needed, mortar bags, and total cost for any CMU (concrete masonry unit) wall project. Standard 8x8x16 blocks cover 0.889 sq ft each.
Reviewed & updated for 2026 by Rakesh Choudhary, PhD · How we calculate
Concrete block sizes reference
| Block size | Face area | Blocks/100 sq ft | Typical use |
|---|---|---|---|
| 8 × 8 × 16 (most common) | 0.889 sq ft | ~113 blocks | Foundation, garage, retaining walls |
| 4 × 8 × 16 | 0.889 sq ft | ~113 blocks | Partition walls, half-thickness |
| 6 × 8 × 16 | 0.889 sq ft | ~113 blocks | Medium walls |
| 12 × 8 × 16 | 0.889 sq ft | ~113 blocks | Large foundation walls |
| Half-block (8 × 4 × 16) | 0.444 sq ft | ~225 blocks | Corner & end finishes |
Block counts for common projects
Most homeowners are not building skyscrapers. The five projects below cover roughly 90% of residential CMU work. Use these as a sanity check after running the calculator.
| Project | Typical dimensions | Blocks (8x8x16) | Mortar bags |
|---|---|---|---|
| Garden retaining wall | 20 ft × 3 ft | ~60 blocks | 3 bags |
| Detached garage walls | 24 × 24 × 8 ft | ~770 blocks | 29 bags |
| Basement foundation (single side) | 40 ft × 8 ft | ~320 blocks | 12 bags |
| Privacy wall around yard | 100 ft × 6 ft | ~600 blocks | 23 bags |
| Storm shelter walls | 8 × 8 × 8 ft | ~256 blocks | 10 bags |
All numbers include a 10% waste factor and use the rule of thumb that one 80-pound bag of pre-mixed mortar lays roughly 27 standard CMUs. Real projects with lots of corners, openings, or curves can use 15-20% more.
Mortar, rebar, and grout: what you actually need
A finished CMU wall is not just blocks. For any wall taller than three or four courses, you will also need mortar to glue them together, rebar for vertical reinforcement, and (often) grout to fill the cores around the rebar. Forgetting any one of these turns a 12-hour weekend job into a return trip to the lumberyard.
- Mortar uses Type N for above-grade walls (chimneys, garden walls) and Type S for below-grade or load-bearing walls (foundations, retaining walls). One 80-lb bag of pre-mix covers 25 to 30 standard blocks with 3/8-inch joints.
- Rebar #4 (½-inch) vertical bars go in the cores every 32 to 48 inches for non-reinforced walls and every 16 to 24 inches for taller or retaining walls. Local code overrules these defaults — always check.
- Grout fills the rebar-containing cores. One cubic foot of grout fills roughly 20 cores in standard 8x8x16 block, so a 200-block wall with rebar every other core needs about 5 cubic feet of grout.
- Bond beam blocks (U-shaped CMU) run horizontally every 8 to 10 feet vertically, filled with two horizontal rebars and grout. They tie the wall together and resist cracking.
- Sand and cement if you mix mortar yourself instead of buying pre-mix: roughly 1 part Portland cement, 1/4 part lime, 3 parts mason sand. Pre-mix is more expensive per pound but eliminates ratio mistakes.
A typical 30 ft × 8 ft garage wall using 270 blocks needs about 8 bags of mortar, 80 linear feet of #4 rebar, and 6 cubic feet of grout for partial core fill. Add roughly $250-$400 to the bare block cost for all this material.
CMU vs poured concrete: which is right for your project
CMU and poured concrete walls cost roughly the same for a finished structure, but they fail in different ways and suit different projects. The decision usually comes down to project size, DIY skill level, and how the wall will be loaded.
| Factor | Concrete block (CMU) | Poured concrete |
|---|---|---|
| DIY friendly | Yes, with practice | No, needs forms and pumping |
| Material cost | $3-$5 per sq ft | $5-$8 per sq ft |
| Labor cost | $4-$8 per block installed | $5-$12 per sq ft (forms + pour) |
| Speed for small projects | Faster (no forms needed) | Slower setup, faster pour |
| Speed for large projects | Slower (every block by hand) | Faster (single pour) |
| Waterproofing | Mortar joints can leak | More watertight |
| Insulation | Hollow cores can be filled or foam-injected | Solid; needs exterior insulation |
Choose CMU for retaining walls under 6 feet, garages, garden walls, storm shelters, and any project you want to do yourself. Choose poured concrete for full house foundations, walls over 10 feet, basement walls in high water-table areas, and any project where speed of pour matters more than DIY accessibility.
FAQs
How do I calculate concrete blocks needed?
Formula: Number of blocks = (Wall length × Wall height) / (Block face area). Standard 8x8x16 CMU has face area of 8" × 16" = 0.889 sq ft. For a 30 ft × 8 ft wall: (30 × 8) ÷ 0.889 = 270 blocks. Add 10% for waste/breakage.
How many concrete blocks per square foot?
Standard 8x8x16 CMU: 1.125 blocks per square foot of wall. 4x8x16 CMU: 1.125 blocks per square foot. 6x8x16 CMU: 1.125 blocks per square foot. (All standard blocks have same 8" × 16" face dimensions; thickness differs.) For 100 sq ft of wall, that is about 113 blocks, or roughly 124 once you add 10% for waste.
How much mortar do I need for concrete blocks?
Approximately 1 bag of premix mortar (80 lb) per 25-30 blocks. For a 200-block wall: 7-8 bags of mortar. Premium estimate: 1 bag per 20 blocks if you're learning or working in difficult conditions. Always order 10-15% extra.
What's the cost of concrete blocks?
Standard 8x8x16 CMU costs $1.50-$3.50 per block in 2026, depending on region and quantity. Mortar adds about $4-$8 per bag. For a 200-block wall: $300-$700 in materials + labor of $4-$8 per block installed = total $500-$2,000+ depending on complexity.
What sizes of concrete blocks are available?
Common CMU sizes (nominal dimensions, actual is 3/8" smaller for mortar joint): 8x8x16 (most common, full-size load-bearing wall), 4x8x16 (partition walls), 6x8x16 (medium walls), 12x8x16 (large foundation walls), 8x4x16 (half-height). All share the 16-inch length and most share 8-inch height, the depth varies for load-bearing requirements.
How tall can a concrete block wall be?
Without reinforcement (rebar + mortar fill): max 4 feet for retaining walls, 8 feet for non-retaining. With proper engineering (rebar every 32-48" vertical, bond beams every 8-10 ft), CMU walls can reach 20+ ft. Always consult an engineer for retaining walls over 4 feet or load-bearing walls over 10 feet.