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Concrete Block Calculator: CMU Blocks, Mortar & Sand

Calculate standard 8×8×16 inch CMU concrete blocks, mortar bags, and masonry sand needed for wall construction.

Quick Answer:Standard 8×8×16 CMU blocks require 1.125 blocks per square foot of wall surface area (accounting for standard 3/8-inch mortar joints).

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How to Use the Concrete Block Calculator (CMU Wall & Mortar)

1Enter the length of your wall in feet.
2Enter the total height of your wall in feet.
3Enter the square footage of any doors, windows, or gates to subtract from the gross wall area.
4Select your cutting waste allowance (10% is recommended for cutting around corners and bond beams).
5Review the total CMU block count, 70-lb mortar bags, and masonry sand requirements.

Formula & How It Works

CMU Wall Surface Area Method

Blocks = (Net Wall Area in Sq Ft) × 1.125 × (1 + Waste %)

A standard 8×8×16 block with a 3/8-inch joint covers 16 inches by 8 inches = 128 square inches (0.8889 sq ft). Thus, each square foot requires 1 / 0.8889 = 1.125 blocks.

Worked Examples

30ft × 8ft Basement Wall with Small Window

Input: L = 30 ft, H = 8 ft, Openings = 20 sq ft, Waste = 10%

Explanation: Gross area = 240 sq ft. Net area = 220 sq ft. Exact blocks = 220 × 1.125 = 247.5. With 10% waste = 273 blocks.

Result: 273 Blocks (9 bags mortar, 2,730 lbs sand)

Garden Retaining Wall (40ft long, 3ft high)

Input: L = 40 ft, H = 3 ft, Openings = 0 sq ft, Waste = 10%

Explanation: Net area = 120 sq ft × 1.125 = 135 blocks. With 10% waste = 149 blocks.

Result: 149 Blocks (5 bags mortar, 1,490 lbs sand)

Small Storage Shed Foundation (10ft × 4ft)

Input: L = 10 ft, H = 4 ft, Waste = 10%

Explanation: 40 sq ft × 1.125 = 45 blocks. With 10% waste = 50 blocks.

Result: 50 Blocks (2 bags mortar)

Frequently Asked Questions

How many concrete blocks do I need per square foot?▼

For standard 8×8×16 blocks with 3/8-inch mortar joints, you need 1.125 blocks per square foot.

How much mortar do I need for 100 concrete blocks?▼

Approximately three 70-lb bags of Type N or Type S masonry cement and 1,000 lbs of masonry sand are needed per 100 standard blocks.

What is the difference between nominal and actual block size?▼

Nominal dimensions include the 3/8-inch mortar joint (8" × 8" × 16"). The actual manufactured block measures 7-5/8" × 7-5/8" × 15-5/8".

How many blocks are on a pallet?▼

A standard wooden pallet typically holds 72 to 90 standard 8x8x16 blocks depending on the manufacturer.

Is this calculation private?▼

Yes, all estimations run 100% locally in your web browser.

Quick Reference & Conversion Table

Block Nominal Size	Actual Dimensions (less 3/8" joint)	Blocks per 100 Sq Ft	Mortar Bags per 100 Blocks
8×8×16 Standard	7-5/8" × 7-5/8" × 15-5/8"	112.5 blocks	3.0 bags (70 lb)
4×8×16 Partition	3-5/8" × 7-5/8" × 15-5/8"	112.5 blocks	2.5 bags (70 lb)
12×8×16 Foundation	11-5/8" × 7-5/8" × 15-5/8"	112.5 blocks	4.0 bags (70 lb)

⚠️ Common Pitfalls to Avoid

•Forgetting to deduct large garage door and window openings from the total wall square footage.
•Omitting waste for corner cuts, half blocks, and lintels.
•Forgetting rebar and core-fill grout required by local seismic or retaining wall building codes.

💡 Practical Real-World Uses

•Building structural basement foundation walls and crawlspace piers.
•Constructing landscape retaining walls and perimeter privacy walls.
•Estimating masonry mortar and sand delivery quantities for masonry contractors.

Hollow CMU Blocks vs Solid Concrete Blocks

✓Hollow CMU blocks feature two large cavities (cores) that accept vertical steel rebar and concrete grout for structural strength.
✓Solid blocks are denser and used primarily for capping walls, heavy gravity retaining bases, or firebreak construction.

Last reviewed & updated: 2026-11-01

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