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Concrete block calculator diagram showing block wall dimensions and running bond pattern 20 feet 8 feet
LIVE CALCULATION

Results

Blocks Needed
198
blocks
20 ft × 8 ft wall · Standard 8×8×16 blocks

Project Summary

Wall Area (Gross)160 sq ft
Block Face Area0.89 sq ft
Blocks (before waste)180 blocks
Blocks (with waste)198 blocks

Mortar Estimate

Mortar Volume0.31 cu yd
Bags of Mortar (80 lb)17 bags

Cost Estimate

Blocks$297.00
Mortar$110.50
Total Material Cost$407.50

Calculation Assumptions

Block Size8 × 8 × 16 in
Wall Dimensions20 ft × 8 ft
Mortar Joint⅜ in
Mortar MixPre-mixed bags
Mortar Bag80 lb bags
Waste Allowance10%
PatternRunning Bond

What You Get

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Written & Reviewed by Editorial Team.

MyBuilderBase is an independent construction calculator and information project. Content is researched and prepared with the assistance of AI tools, followed by editorial review for clarity, consistency, calculation logic, and usability.

Editorial review is not professional engineering, architectural, or building-code approval.

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Last updated: September 12, 2026

What Is a Concrete Block Calculator

A concrete block calculator estimates the number of concrete masonry units (CMUs) needed to build a wall, along with the mortar required to hold them together. You enter the wall dimensions and block size, and the calculator determines the total block count, mortar volume, and material costs.

Whether you are building a retaining wall, privacy fence, foundation, or structural partition, this tool eliminates guesswork and helps you order the right amount of materials the first time.

Concrete block wall under construction showing running bond pattern with mortar joints

How to Calculate Concrete Blocks

Calculating concrete blocks for a wall follows a straightforward process. Here is the step-by-step method:

  • Step 1: Measure the wall — Measure the total length and height of the wall in feet. If you have doors, windows, or other openings, note their dimensions separately.
  • Step 2: Calculate wall area — Multiply length × height to get total square footage. Subtract the area of any openings. For example, a 20 ft × 8 ft wall = 160 sq ft.
  • Step 3: Find block face area — For a standard 8×16 block, the face area is 16 in × 8 in = 128 sq in, which equals 0.89 sq ft. Divide the wall area by this number.
  • Step 4: Add waste allowance — Add 5-10% for cuts, breakage, and waste. Round up to the nearest whole block. It is always better to have a few extra than to run short.

Formula: Blocks = (Wall Area ÷ Block Face Area) × (1 + Waste %)

Standard Block Sizes

Concrete blocks come in several standard sizes. The most common is the 8×8×16 nominal size, which measures 7⅝ × 7⅝ × 15⅝ inches actual. The difference accounts for the mortar joint.

Size (Nominal) Actual Dimensions Face Area Weight
6 × 8 × 165⅝ × 7⅝ × 15⅝ in0.89 sq ft~22 lbs
8 × 8 × 167⅝ × 7⅝ × 15⅝ in0.89 sq ft~35 lbs
10 × 8 × 169⅝ × 7⅝ × 15⅝ in0.89 sq ft~45 lbs
12 × 8 × 1611⅝ × 7⅝ × 15⅝ in0.89 sq ft~55 lbs

Mortar Calculation

Mortar fills the joints between blocks and bonds them together. The amount needed depends on the joint thickness and the total number of blocks.

Rule of thumb: For ⅜-inch joints, a standard 8×8×16 block needs about 0.042 cubic feet of mortar — its bed and head joints total roughly 72 cubic inches. That works out to about 9 bags of 80-lb pre-mixed mortar per 100 blocks; manufacturer yield charts (11–13 blocks per bag) agree.

The calculator computes the exact joint volume from your block dimensions and joint thickness, so thicker joints are handled automatically — no rule-of-thumb bump needed. Type S or Type N mortar mix is commonly used for block walls, with Type S preferred for structural applications. For bag counts and site-mix cement-and-sand splits on any wall size, the Mortar Calculator works from the same joint-volume math.

Why our cement and sand figures look leaner than old rule-of-thumb charts: traditional guides (3 cement bags per 100 blocks) pad heavily for field waste and sloppy proportioning. We compute the true joint volume instead and let you control the cushion with the Waste Allowance field — and the result matches manufacturer yield charts, which list 11–13 eight-inch blocks per 80-lb bag.

Mortar calculation setup showing measuring tape, mortar mix, and calculation notepad

Tips for Building Block Walls

  • Start with a level foundation — Ensure the first course is perfectly level and straight. Use a string line and level. Uneven first courses compound errors as the wall rises.
  • Use a running bond pattern — Offset each course by half a block (8 inches) for maximum strength. This is the most common and structurally sound pattern for block walls.
  • Mix mortar to the right consistency — Mortar should be thick enough to hold its shape but workable enough to spread easily. Aim for a peanut butter consistency. Mix only what you can use in 30-45 minutes.
  • Check plumb frequently — Use a level to check plumb every 2-3 courses. Correct small deviations immediately before they become large problems at the top of the wall.

Material Costs

Concrete block costs vary by region, size, and supplier. Here are typical 2026 price ranges:

Material Typical Cost
Standard 8×8×16 block$1.25 – $2.00 each
Split-face (decorative) block$2.50 – $4.00 each
Mortar mix (80 lb bag)$5.00 – $8.00 each
Rebar (½ inch × 10 ft)$4.00 – $6.00 each
Grout fill (per cubic yard)$100 – $150

Worked Example

Let us calculate blocks for a 20-foot long, 8-foot tall privacy wall using standard 8×8×16 blocks with 10% waste allowance.

Step 1: Calculate wall area

20 ft × 8 ft = 160 sq ft

Step 2: Find block face area

16 in × 8 in = 128 sq in = 0.89 sq ft

Step 3: Calculate blocks before waste

160 ÷ 0.89 = 180 blocks

Step 4: Add 10% waste allowance

180 × 1.10 = 198, round up to 198 blocks

Step 5: Calculate mortar needed

198 ÷ 100 × 2.5 bags = 5 bags (80 lb)

Frequently Asked Questions

How many concrete blocks do I need for a wall? ▼
Divide the total wall area by the face area of one block. For standard 8×16 blocks (0.89 sq ft face), a 100 sq ft wall needs about 113 blocks. Always add 5-10% for waste and cuts.
How much mortar do I need per block? ▼
Each standard 8×8×16 block requires about 0.042 cubic feet of mortar at ⅜-inch joints — the bed and head joints total roughly 72 cubic inches per block, or about 1.5 cubic yards per 1,000 blocks. Thinner or thicker joints scale this proportionally.
What is the standard size of a concrete block? ▼
The standard concrete masonry unit (CMU) measures 8×8×16 inches nominal, with actual dimensions of 7⅝ × 7⅝ × 15⅝ inches. This allows for a ⅜-inch mortar joint to reach the 8×16 face size.
How many bags of mortar mix do I need for 100 blocks? ▼
For 100 standard 8×8×16 blocks with ⅜-inch joints, plan on about 9 bags of 80-lb mortar mix (about 4.2 cubic feet of mortar). Manufacturer yield charts list 11–13 eight-inch blocks per bag, putting the range at 8–10 bags; the waste allowance covers the spread.
Can I build a load-bearing wall with concrete blocks? ▼
Yes, concrete block walls can be load-bearing when properly reinforced with rebar and filled with grout. The wall thickness, block strength, and reinforcement must meet local building codes. Consult a structural engineer for load-bearing applications.
What is the difference between CMU and cinder block? ▼
CMU (Concrete Masonry Unit) is the general term for concrete blocks. Cinder blocks specifically use coal cinders or fly ash as aggregate, while modern CMUs use standard concrete mix. CMUs are stronger and more common in construction today.

References

  1. Concrete Masonry & Hardscapes Association, "Estimating Concrete Masonry Materials," TEK 04-02A, cmha.org
  2. Concrete Masonry & Hardscapes Association, "Concrete Masonry Basement Wall Construction," TEK 03-11, cmha.org
  3. Concrete Masonry & Hardscapes Association, "Mortars for Concrete Masonry," TEK 09-01A, cmha.org
  4. ASTM International, "Standard Specification for Loadbearing Concrete Masonry Units," ASTM C 90, astm.org
  5. ASTM International, "Standard Specification for Mortar for Unit Masonry," ASTM C 270, astm.org
  6. International Code Council, "International Building Code (IBC)," Chapter 21 — Masonry, iccsafe.org

Disclaimer: The concrete block calculator results on this page are planning estimates only. Actual block counts vary with block dimensions, bond pattern, and cutting waste, and mortar quantities vary with joint thickness and site practice. Confirm block size, mortar yield, and pricing with your supplier, and have load-bearing or reinforced walls designed or reviewed by a qualified engineer.

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