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Concrete Block Calculator

Calculate how many concrete blocks (CMU) you need for your wall project. Includes door & window opening deductions — the only calculator that accounts for openings automatically.

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Quick Rule: 1.125 blocks per sq ft for standard 8"×8"×16" blocks

A 10ft × 8ft wall (80 sq ft) needs ~90 blocks + 3 bags of mortar ≈ $150-$180 in materials

✓Door/Window Deduction
✓6 Block Sizes
✓Mortar & Cost Estimate

📐 Wall Dimensions

ft
ft

Recommended: 10% for standard walls, 15% for complex layouts

$

🚪 Door & Window Openings

Unique Feature!

Click buttons above to add doors or windows. We'll deduct them from the total.

📊 Materials Needed

Total Blocks Needed

185

8" x 8" x 16" (Standard)

Mortar Bags

6

80 lb bags

Est. Total Cost

$326

blocks + mortar

Calculation Breakdown

Gross Wall Area160.0 sq ft
Net Wall Area160.0 sq ft
Block Face Area0.952 sq ft
Base Blocks Needed169
+ Waste Factor (10%)+17
Total Blocks185

💰 Cost Breakdown

185 blocks × $1.5$277.50
6 mortar bags × $8$48.00
Estimated Total$325.50

🧱 Complete Guide to Concrete Block Calculation

Concrete blocks, also known as concrete masonry units (CMU) or cinder blocks, are one of the most versatile and cost-effective building materials for walls, foundations, and structures. This calculator helps you determine exactly how many blocks you need, accounting for openings like doors and windows — a feature most other calculators lack.

How to Calculate Concrete Blocks

The basic formula is simple: divide your net wall area (total wall minus openings) by the face area of a single block. For standard 8"×8"×16" blocks, each block covers approximately 0.89 square feetof wall surface when including the mortar joint.

📐 Example Calculation

For a 20ft × 8ft wall (160 sq ft) with a 3ft × 7ft door opening (21 sq ft):
Net area = 160 − 21 = 139 sq ft
Blocks needed = 139 ÷ 0.89 = 156 blocks
With 10% waste = 172 blocks

Why Account for Door & Window Openings?

Most online calculators simply multiply wall area by blocks per square foot, resulting in over-ordering by 10-30% on walls with openings. Our calculator lets you add specific openings (doors, windows, vents) and automatically deducts them from the total. This can save hundreds of dollars on large projects.

Choosing the Right Block Size

The most common CMU size is 8"×8"×16", suitable for most residential and commercial walls. Use thicker blocks (10" or 12") for load-bearing walls, basements, or retaining walls.Thinner blocks (4" or 6") work well for partition walls and non-structural applications.

🧱 Quick Facts

Blocks/sq ft: 1.125 (standard)

Mortar/100 blocks: 3 bags

Standard size: 8"×8"×16"

Weight: 35-40 lbs each

💡 Our Advantage

This is the only concrete block calculator that automatically deducts door and window openings. Save money by ordering exactly what you need!

Frequently Asked Questions

To calculate blocks needed: 1) Measure your wall's length and height in feet, 2) Calculate wall area (length × height), 3) Subtract any door/window openings, 4) Divide by the block's face area (typically 0.89 sq ft for standard 16"×8" blocks), 5) Add 5-10% for waste. For example, a 20ft × 8ft wall (160 sq ft) with a 3ft × 6.67ft door (20 sq ft) needs: (160-20) ÷ 0.89 × 1.10 = ~173 blocks.

The concrete block calculation formula is: Number of Blocks = (Wall Area - Opening Area) ÷ Block Face Area × (1 + Waste Factor). For standard 8"×16" blocks, the face area is 128 sq inches or 0.89 sq ft. So for a 100 sq ft wall: 100 ÷ 0.89 = 112.4 blocks, plus 10% waste = 124 blocks.

For standard 8"×8"×16" concrete blocks: you need approximately 1.125 blocks per square foot, or about 12.1 blocks per square meter. This accounts for the 3/8" mortar joint between blocks. Smaller 8"×8" blocks require 2.25 blocks per square foot.

On average, you need 3 bags (80 lb each) of mortar mix per 100 standard concrete blocks. This equals about 1 cubic foot of mortar per 30 blocks. For precise calculations: divide your total block count by 33.3 to get the number of mortar bags needed. Always buy an extra bag or two for waste and repairs.

First calculate the total wall area, then subtract the area of all openings (doors and windows). For a door opening, use the rough opening size (typically 2-3 inches larger than the door). Our calculator automatically handles this - just add your openings and it will deduct them from the total block count.

The most common concrete block size in the US is 8" × 8" × 16" (nominal). The actual dimensions are 7-5/8" × 7-5/8" × 15-5/8" to allow for 3/8" mortar joints. Other common sizes include 4", 6", 10", and 12" thick blocks, all typically 8" high × 16" long.

Concrete blocks cost $1-$5 each depending on size and type. A typical wall costs $10-$15 per square foot installed, including blocks, mortar, and labor. DIY material costs are roughly $5-$8 per square foot. Factors affecting cost include block type, wall height, reinforcement needs, and local labor rates.

🧱 Disclaimer: This calculator provides estimates for planning purposes only. Actual material requirements may vary based on wall complexity, block type, mortar joint thickness, and construction methods. Always consult with a professional contractor for precise estimates on structural projects. Prices shown are approximate US averages and vary by location.