Concrete Block Calculator

What Is a Concrete Block (CMU)?

The concrete block calculator above estimates the number of CMU blocks and mortar bags needed for any wall — a concrete masonry unit (CMU), commonly called a concrete block, cinder block, or simply block, is a precast masonry building unit made from Portland cement, aggregate, and water. Standard CMUs have nominal dimensions of 8×8×16 inches (actual dimensions 7.625×7.625×15.625 inches, with the remaining space taken up by a 3/8-inch mortar joint). They are hollow with two or three cores that can be left open, filled with grout for added strength, or filled with insulation for thermal performance.

CMUs are specified to a minimum compressive strength of 1,900 PSI per ASTM C90 — the standard that governs load-bearing masonry units. In practice, most blocks exceed 2,000 PSI. They are widely used for foundation walls, retaining walls, basement walls, garden walls, and commercial construction because they are strong, fire-resistant, dimensionally consistent, and relatively inexpensive compared to poured concrete.

How to Use This Concrete Block Calculator

This concrete block calculator estimates the number of standard 8×8×16 CMU (concrete masonry unit) blocks needed for a wall, along with the mortar bags and core fill concrete required. Enter the wall length and height to get your complete materials list instantly. For poured concrete walls, try the concrete calculator which handles slabs, columns, and footings.

How to Calculate Concrete Blocks for a Wall

To calculate how many concrete blocks you need, find the wall's total square footage, then divide by the coverage per block:

Blocks = (Wall Length × Wall Height) ÷ 0.89

One standard 8×8×16 CMU block covers approximately 0.89 square feet of wall face, accounting for the standard 3/8-inch mortar joint. Always round up to the nearest whole block and add 5–10% extra for breakage and cuts around corners, windows, and doors.

Example: 20-Foot Garden Wall, 4 Feet Tall

Wall area = 20 × 4 = 80 sq ft. Blocks needed = 80 ÷ 0.89 = 90 blocks. Add 10%: 99 blocks to order.

Mortar and Grout for CMU Block Walls

Mortar is used to bond blocks together at the horizontal (bed) joints and vertical (head) joints. As a general rule, plan for approximately 1 bag of 60 lb mortar mix per 33 blocks. Actual consumption depends on joint thickness and mason technique.

For structural walls, the hollow cores of the blocks are filled with grout (a fluid concrete mix) and vertical rebar. Core fill volume depends on the block size and how many cores are grouted — the calculator estimates this based on your wall dimensions.

Types of Concrete Blocks

• Standard CMU (8×8×16) — the most common block for walls, fences, and foundations; two hollow cores
• Half block (8×8×8) — used at corners and wall ends; also called half units
• Solid block — no cores; used for caps, windowsills, and applications requiring a solid face
• Lightweight CMU — 20–30% lighter than standard; easier to handle but slightly less strong
• Split-face block — textured exterior face for decorative applications like retaining walls and landscaping

Concrete Block Wall Construction Tips

• Start with a solid footer — pour a concrete footer below the frost line before laying the first course; use the concrete cost calculator to estimate material cost for the footer; the footer should be twice as wide as the block and at least 8 inches deep
• Use a chalk line for alignment — snap chalk lines on the footer to position the first course precisely; every course above will follow
• Running bond pattern — stagger vertical joints by half a block length for maximum wall strength; never align vertical joints from course to course
• Grout cores for structural walls — fill hollow cores with grout (and rebar) for retaining walls, basement walls, or anywhere required by code
• Control joints — install vertical control joints every 20–25 feet to manage thermal expansion and prevent uncontrolled cracking