Concrete Block Calculator
A concrete block calculator determines the number of CMUs (Concrete Masonry Units), mortar, and rebar required for a retaining wall or foundation. It simplifies the math for standard 8x8x16 inch blocks.
How to use this calculator
Estimate your monthly mortgage payment (PITI: Principal, Interest, Taxes, and Insurance). For the most accurate result, include optional expenses like property taxes, home insurance, and private mortgage insurance (PMI).
Introduction to Concrete Block Calculator
How to Use the Concrete Block Calculator
- Measure Wall Dimensions: Input the total length and height of the wall in feet.
- Select Block Size: The default is the standard 8x8x16 unit. If using 4-inch or 12-inch wide blocks, adjust the dimensions accordingly.
- Choose Reinforcement Level: Specify if you need rebar in every core, every second core, or just at the top (bond beam).
- Account for Waste: Add 5-10% extra for blocks that may be damaged during delivery or require custom cuts.
- Review Material List: The tool provides the total block count, number of mortar bags, and total linear feet of rebar.
Understanding Concrete Block
The "Standard" concrete block is known as an 8x8x16 CMU. However, its actual dimensions are 7 ⅝" x 7 ⅝" x 15 ⅝". When you add a standard ⅜" mortar joint, the block occupies exactly 8 inches of height and 16 inches of length in the wall. This makes the math simple: every square foot of wall surface area requires 1.125 blocks. For a structural wall, the "cores" (the holes in the blocks) are often filled with grout and reinforced with steel rebar to provide tensile strength against soil pressure or wind loads.
For retaining walls, the base is the most critical component. The first course of blocks should be buried (embedded) in the ground to prevent the wall from sliding. A general rule is to bury one full course for every 3 feet of wall height. Additionally, you must provide a way for water to escape from behind the wall using "weep holes" or a perforated drainage pipe. Our calculator factors in the total height, including the buried course, to ensure you have enough material for a stable, long-lasting structure. Furthermore, for walls higher than 4 feet, most local building codes require a design stamped by a professional structural engineer to ensure the safety of the public and the surrounding property.
When calculating mortar and grout, remember that they serve different purposes. Mortar is the "glue" that holds the blocks together and absorbs seasonal movement. Grout is a much more fluid concrete mix that is poured into the hollow cores to provide structural mass and lock the rebar in place. A common mistake is using mortar to fill the cores; mortar is too thick and will create "voids" or air pockets that weaken the wall. Our calculator provides independent estimates for both materials to ensure your wall meets professional standards.
Formula & Calculation Method
The number of blocks is determined by the total surface area divided by the area of a single block face (including mortar).
Example Calculation:
A wall 20 feet long and 4 feet high is 80 sq ft. 80 × 1.125 = 90 blocks.
Tips for Accurate Results
- 1Always start your first course on a perfectly level concrete footing or a 6-inch layer of compacted gravel.
- 2Use a "Bond Beam" block (a block with a horizontal channel) for the top course to hold horizontal rebar and grout for extra strength.
- 3Check your local building codes; most walls over 3 or 4 feet high require a permit and a design from a structural engineer.
- 4Fill the cores with "Grout" (a fluid concrete mix), not just mortar, for structural reinforcement.
- 5Apply a waterproof membrane to the back of any retaining wall before backfilling to prevent water from seeping through the blocks.
- 6Stagger your vertical joints (Running Bond pattern) so that each block sits on the center of the two blocks below it.
- 7Use a masonry saw for clean, professional cuts around corners or steps.
- 8When building a foundation, ensure you install "Anchor Bolts" in the top course of grout to secure the wood sill plate.
- 9Install "Control Joints" every 20-30 feet in long walls to allow for thermal expansion and contraction without random cracking.
- 10Always wear heavy-duty gloves and steel-toed boots when handling blocks, as they are heavy and have rough edges.
Key Facts About Concrete Block
- •Standard CMUs are designed to fit perfectly into 8-inch and 16-inch increments.
- •A standard 8x8x16 block weighs approximately 38 pounds.
- •Blocks provide excellent fire resistance and sound insulation.
- •Type S mortar is the standard for structural block walls due to its high compressive strength.
- •Proper drainage behind a block retaining wall is essential to prevent structural failure.
- •Block walls can be finished with stucco, stone veneer, or paint for improved aesthetics.
- •ASTM C90 is the industry standard for load-bearing concrete masonry units.
- •Concrete blocks continue to gain strength for years after they are manufactured.
- •A 100-foot long block wall will expand approximately 3/8 of an inch during a typical summer day.
- •Modular block design allows for the easy integration of electrical and plumbing lines within the wall cores.
Frequently Asked Questions
How many blocks are in a square foot of wall?
There are 1.125 standard 8x8x16 blocks per square foot of wall surface.
How much mortar do I need for 100 blocks?
On average, you will need about 3 to 4 bags (80lb) of Type S masonry cement and 1,000 lbs of sand for every 100 blocks.
What is the difference between a "Cinder" block and a "Concrete" block?
Cinder blocks use coal cinders as aggregate, making them lighter and less strong. Modern "Concrete" blocks use crushed stone and are much more durable for structural work.
How high can I build a block wall without rebar?
Most engineers recommend horizontal and vertical reinforcement for any wall over 2 feet high, especially if it is holding back soil.
What is "Grouting" a block wall?
Grouting is the process of filling the hollow cores of the blocks with a fluid concrete mix to create a solid, reinforced wall.
How many blocks are in a pallet?
A standard pallet of 8x8x16 blocks typically contains 72 to 90 blocks, depending on the manufacturer.
What is a "Half-Block"?
Half-blocks are 8x8x8 units used at the ends of alternating courses to maintain a running bond pattern without having to cut full blocks.
Can I use blocks for a fire pit?
Yes, but you must line the inside with fire bricks and high-temperature refractory mortar. Standard concrete blocks can crack or even explode when exposed to direct, intense heat.
What is "Type S" mortar?
Type S is a high-strength mortar specifically designed for structural masonry and applications at or below grade, such as foundations and retaining walls.
How deep should my block wall footing be?
Footings should always be placed below the local frost line (typically 12" to 42" depending on the region) to prevent shifting due to freeze-thaw cycles.
Related Calculators
Concrete Calculator
Calculate concrete volume in cubic yards and bags. Our concrete slab calculator makes it easy to estimate materials for your patio or footing.
Gravel Calculator
Calculate the amount of gravel or crushed stone you need. Get project estimates in cubic yards and tons for any landscape or driveway project.
Roofing Materials Calculator
Estimate your roofing materials needs. Calculate the number of shingle bundles and roofing squares required based on your roof's area and pitch.
Drywall Calculator
Estimate your drywall project materials. Calculate the number of drywall sheets, screws, and joint compound you need for your walls or ceiling.
Related Articles
What is AFUE? Understanding Furnace Efficiency Ratings
A complete guide to AFUE (Annual Fuel Utilization Efficiency). Learn what the percentage means, the difference between 80% and 95% furnaces, and how to choose the right one.
Ductwork Design (Manual D): The Unseen Key to HVAC Performance
Your HVAC system is only as good as its ductwork. Learn what ACCA Manual D is, why it's critical for airflow, and how improper ducts waste thousands.
Heat Pump vs. Gas Furnace: Which is Right for Your Climate Zone?
An analysis of heat pumps and gas furnaces. Learn the pros, cons, and operating costs of each to determine the best heating system for your home's climate.