Concrete blocks (CMU - Concrete Masonry Units) are a versatile building material for walls, foundations, and structures. Accurately estimating the number of blocks needed prevents project delays and ensures you have adequate materials on site. Our free block calculator simplifies this process by accounting for standard block dimensions and mortar joints.

Simply enter your wall length and height, and our calculator instantly provides the number of standard 8x8x16 blocks needed, along with the wall area and estimated cost.

Standard Block Dimensions

The standard concrete block (CMU):

• Nominal size: 8 × 8 × 16 inches
• Actual size: 7-5/8 × 7-5/8 × 15-5/8 inches
• The difference allows for mortar joints
• Height includes one 3/8-inch mortar joint
• Length includes one 3/8-inch mortar joint

Other common block sizes:

• 6 × 8 × 16 inches (6-inch wide)
• 10 × 8 × 16 inches (10-inch wide)
• 12 × 8 × 16 inches (12-inch wide)
• 4 × 8 × 16 inches (4-inch wide)

Block Types

Standard (Stretcher):

• Most common type
• Two cores
• Used for general wall construction

Half Block (Half-High):

• 8 × 4 × 16 inches nominal
• Used for pattern bands
• Ends bond beams

Corner Block:

• Solid ends for corner reinforcement
• Facilitates vertical bar placement

Bond Beam:

• Knock-out web
• Horizontal reinforced beam within wall

Lintel Block:

• U-shaped for reinforced concrete beams
• Used over openings

The Wall Area Method

Standard calculation:

1. Calculate wall area: Length × Height

2. Subtract opening areas (doors, windows)

3. Divide by block face area (8 × 16 = 128 sq in = 0.89 sq ft)

4. Add waste factor (10-15%)

Face area of standard block:

• Nominal: 8 × 16 = 128 square inches
• In square feet: 128 ÷ 144 = 0.89 sq ft

Blocks per square foot:

• 1 ÷ 0.89 = 1.125 blocks per sq ft
• Approximate: 1.13 blocks per sq ft

Example Calculations

20-foot long, 8-foot high wall:

• Wall area: 20 × 8 = 160 sq ft
• Blocks needed: 160 × 1.125 = 180 blocks
• With 10% waste: 198 blocks

50-foot long, 10-foot high wall with 4×6 window:

• Wall area: 50 × 10 = 500 sq ft
• Window area: 4 × 6 = 24 sq ft
• Net area: 500 - 24 = 476 sq ft
• Blocks needed: 476 × 1.125 = 535 blocks
• With 10% waste: 589 blocks

Deducting Openings

Standard deduction method:

• Calculate total opening area
• Subtract from total wall area
• Calculate blocks for net area

Openings to deduct:

• Windows and doors
• Vents and utility openings
• Large openings for equipment
• Pass-throughs

Not Deducting Small Openings

Sometimes don't deduct:

• Small openings under 2 sq ft
• Openings that will be filled with block
• Where waste factor covers the difference

Recommended Waste Percentages

| Project Type | Waste Factor |

|--------------|--------------|

| Simple rectangular wall | 10% |

| Walls with openings | 10-15% |

• Complex layouts | 15% |
• DIY projects | 15% |

Wage sources:

• Broken blocks during handling
• Cutting blocks for openings and corners
• Miscuts and mistakes
• Defective blocks in delivery
• Future repairs (keep extra)

Estimating Mortar

General guidelines:

• Standard blocks: 3 bags per 100 blocks
• Includes 80lb bags of masonry cement
• Plus masonry sand (approximately 1/3 cu yd per 100 blocks)

Mortar joint size:

• Standard: 3/8 inch
• Range: 1/4 to 1/2 inch
• Affects total mortar needed

Mixing Mortar

Typical mix:

• 1 part masonry cement
• 3 parts masonry sand
• Water to workable consistency
• Clean, potable water

Foundation Requirements

Footer for block walls:

• Minimum: 8 × 16 inches (twice wall width)
• Reinforced: 2 #4 bars typical
• Below frost line
• Level and square

Starting the First Course

Critical first course:

• Establish level and straight reference
• Use string line for alignment
• Check corner build: 3-4-5 triangle
• Full mortar bed on footing
• Establish bond pattern

Block Laying Pattern

Running bond (standard):

• Stagger joints by half block
• Most common pattern
• Strongest wall
• Use half blocks at corners

Building Corners

Corner construction:

• Build corners first (lead)
• Stretch string between corners
• Fill in between
• Check level frequently
• Corner poles helpful for tall walls

Horizontal Reinforcement

Bond beams:

• Every 4 feet vertically (typical)
• Use bond beam blocks
• 1-2 #4 bars continuous
• Grout cores solid

Vertical Reinforcement

When required:

• Tall walls (engineering based)
• Seismic areas
• High wind areas
• Parapet walls

Placement:

• In grouted cores
• #4 or #5 bars typical
• Dowelled into foundation
• Spacing varies by design

Grouting

Solid grouting when:

• Reinforced cells
• High strength required
• Weather exposure (solid walls)
• Engineering requirements

Essential Tools

For laying block:

• Mason's trowel
• Level (2-foot and 4-foot)
• Mason's line and line blocks
• Tape measure
• Square
• Hammer and chisel
• Brush for cleaning joints

Additional Equipment

Helpful items:

• Mortar boards
• Wheelbarrows or power buggy
• Scaffolding or sawhorses
• Power mixer (for large projects)
• Safety equipment

Standard Running Bond

• Most common
• Staggered joints
• Half blocks at corners
• Uses full and half blocks

Stack Bond

• Blocks aligned vertically
• Weaker than running bond
• Architectural applications
• Requires reinforcement

Decorative Patterns

• Scored blocks
• Painted finishes
• Different bond patterns
• Architectural features

Consider professional masons for:

• Structural walls
• Tall walls requiring scaffolding
• Complex designs
• Large projects
• Engineered walls
• Projects requiring permits

Sand for Mortar

Approximate requirements:

• 1/3 cubic yard per 100 blocks
• Or 0.0033 cubic yards per block
• Clean masonry sand
• No organic material

Wall Ties (for veneer)

• 1 tie per 2.25 sq ft (4×9 block spacing)
• Corrosion resistant
• Embedded in mortar joints

Reinforcement

• Horizontal: As per design
• Vertical: As per engineering
• Lap splices minimum 24 inches
• Corner bars required

Uneven Wall

Causes:

• Poor foundation preparation
• Inconsistent mortar bed thickness
• Not checking level frequently
• Twisted blocks

Prevention:

• Level foundation
• String line guides
• Check level constantly
• Use quality blocks

Weak Mortar

Causes:

• Wrong mix proportions
• Too much water
• Old cement
• Poor sand quality

Prevention:

• Follow mix design
• Use clean materials
• Don't add too much water
• Use fresh cement

Weather Damage

Protection needed:

• Cover walls during construction in rain
• Hot weather: dampen blocks, protect from sun
• Freezing: heat materials, protect walls
• Wind: protect fresh mortar

How many blocks for a 20×8 foot wall?

• Area: 160 sq ft
• Blocks needed: 180 blocks
• With 10% waste: 198 blocks
• Mortar: Approximately 6 bags

What's the standard block size?

• Nominal: 8 × 8 × 16 inches
• Actual: 7-5/8 × 7-5/8 × 15-5/8 inches
• Difference accounts for mortar joints

Should I fill block cores?

Fill cores when:

• Reinforcement required
• Extra strength needed
• Weather exposure (solid walls)
• Engineering specifies
• Not necessary for non-structural walls

How high can I build a block wall?

• Non-load-bearing: Typically limited by code
• Load-bearing: Engineering based
• Height-to-thickness ratios apply
• Reinforcement requirements increase with height
• Consult codes for specific project

Can I lay block in cold weather?

Yes, with precautions:

• Keep materials above 40°F
• Heat mixing water if needed
• Protect fresh work from freezing
• Use accelerators if approved
• Follow cold weather masonry practices

Accurate block estimation ensures you have adequate materials on site. Use our block calculator to determine your needs, and always add 10-15% waste for breakage, cutting, and miscuts.

Remember: Quality masonry work depends on proper preparation, good materials, and attention to detail. Take time to establish a level foundation and straight reference lines—these are the keys to a successful project.