How many concrete blocks do I need per square foot?

A standard 8×8×16 inch CMU block with a 3/8-inch mortar joint covers about 0.89 square feet of wall face, so plan for roughly 1.125 blocks per square foot. To size a job, take the wall square footage and divide by 0.89, then add 5–10% for breakage and cuts around corners and openings. For half-blocks (8×8×8), double the count for any course where you need them.

How many bags of mortar do I need per 100 concrete blocks?

Plan for about 3 bags of 60 lb mortar mix (Type N or Type S) per 100 standard 8×8×16 CMU blocks laid with 3/8-inch bed and head joints — roughly 1 bag per 33 blocks. Tooled joints, larger blocks, or a less efficient mason can push that to 1 bag per 28 blocks, so add 10–15% to avoid a mid-day supply run. Use Type S mortar for below-grade and structural work and Type N for above-grade non-loadbearing walls.

What is the difference between running bond and stack bond?

Running bond staggers each course by half a block so the vertical head joints never line up — this interlocks the units and is the strongest, most common pattern for structural CMU walls. Stack bond aligns every block directly above the one below, creating continuous vertical joints that are decorative but structurally weak; it requires extra horizontal joint reinforcement (ladder or truss wire every course) to resist cracking. For any loadbearing or retaining application, use running bond.

Do I need to fill concrete block cells with grout?

Only structural walls need grouted cells. Freestanding garden or partition walls under 4 feet can be left hollow, but any retaining wall, basement wall, or loadbearing wall must have vertical rebar set in the cores, which are then filled with fluid grout. Code typically requires grouting cells at corners, wall ends, around openings, and at regular spacing (often every 32–48 inches).

What is the difference between loadbearing and non-loadbearing block?

Loadbearing CMU carries roof, floor, or wall loads above it and must meet the minimum 1,900 PSI compressive strength and reinforcement requirements of ASTM C90. Non-loadbearing block (partition or veneer) only supports its own weight and can use lighter, lower-strength units. The two look similar, but loadbearing walls are grouted and reinforced and usually require an engineered design and permit, especially over 4 feet tall.

How many courses of block are in a foot of wall height?

Each standard 8-inch-high CMU course (block plus 3/8-inch bed joint) adds 8 inches of height, so 1.5 courses equal one foot of wall — three courses make 24 inches and six courses make 48 inches. To find total courses, take the wall height in inches and divide by 8. Plan a bond beam or lintel block course over door and window openings to carry the load above.

Concrete Block Calculator — CMU Block & Mortar Estimator

What Is a Concrete Block (CMU)?

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. The workhorse unit is the standard 8×8×16-inch (nominal) two-core hollow block, which actually measures 7.625×7.625×15.625 inches; the missing 3/8 inch in each direction is filled by the mortar joint. Because the units are modular, every dimension on a CMU wall is a multiple of 8 inches, which is why masons plan layouts around 16-inch lengths and 8-inch courses.

Standard loadbearing CMU is specified to a minimum compressive strength of 1,900 PSI per ASTM C90, the governing standard for concrete masonry units. The hollow cores can be left open in non-structural walls, filled with grout and vertical rebar in structural walls, or filled with loose insulation for thermal performance. CMU is the default for foundation walls, basement walls, garden walls, and commercial partitions because it is strong, fire-rated, dimensionally consistent, and faster to lay than forming and pouring concrete.

How to Use This Concrete Block Calculator

Enter the wall length and height and this calculator returns the number of standard 8×8×16 CMU blocks, the bags of mortar for the bed and head joints, and the core-fill grout volume if you are grouting cells. Subtract openings (doors and windows) and add 5–10% for breakage. For a poured concrete wall instead of block, use the concrete wall calculator; for a stepped or battered landscape wall, use the retaining wall calculator.

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How to Calculate Concrete Blocks for a Wall

Find the wall's net face area (length × height, minus any openings), then divide by the coverage of one block:

Blocks = (Wall Length × Wall Height) ÷ 0.89

One standard 8×8×16 CMU covers about 0.89 square feet of wall face with a 3/8-inch joint. You can also work by course: each 8-inch course needs 0.75 blocks per linear foot of wall, and there are 1.5 courses per foot of height. Always round up and add 5–10% for breakage and cuts.

Example: 20-Foot Garden Wall, 4 Feet Tall

Wall area = 20 × 4 = 80 sq ft. Blocks = 80 ÷ 0.89 = 90 blocks. That is six courses high (48 in ÷ 8 in) at 15 blocks per course. Add 10%: 99 blocks to order, plus a half-block at one end of every other course to maintain the running bond.

Block Types: Standard, Half, Bond Beam, Lintel and Corner

A real CMU wall uses several specialized units beyond the standard stretcher block. Knowing which unit goes where prevents cuts and keeps the bond clean:

Standard stretcher (8×8×16) — the field block with two hollow cores; the bulk of every wall
Half block (8×8×8) — starts or ends alternating courses to keep a running bond without cutting
Corner / return block — has one finished (closed) end so corners present a clean face in both directions
Bond beam block — a channel-shaped unit with knockout webs that holds horizontal rebar and grout to form a continuous reinforced beam, typically at the top course and over openings
Lintel block — a U-shaped unit set over door and window openings to carry the load above the span

Mortar, Joints, Bond Pattern and Reinforcement

Blocks are bedded in 3/8-inch bed joints (horizontal) and head joints (vertical) of Type N or Type S mortar — plan about 1 bag of 60 lb mix per 33 blocks. Lay them in a running bond, offsetting each course by half a block so head joints never stack; this interlock is what gives the wall its strength. Stack bond (aligned joints) is decorative only and needs continuous joint reinforcement.

Structural walls add reinforcement two ways: horizontal ladder or truss wire ties laid in the bed joints every other course, and vertical rebar set in grouted cells at corners, ends, openings, and regular spacing. Filling those cells with grout turns a hollow wall into a reinforced one — see the concrete wall calculator for solid-pour comparisons and the concrete cost calculator to price the grout. Install vertical control joints every 20–25 feet to manage shrinkage and thermal movement.

Loadbearing vs Non-Loadbearing Walls

A loadbearing wall carries structure above it and must use C90-rated units, grouted reinforced cells, and usually an engineered design and permit (especially over 4 feet). A non-loadbearing partition or veneer only holds itself up and can use lighter units with minimal reinforcement.

Sources & References

ASTM C90: Standard Specification for Loadbearing Concrete Masonry Units — ASTM International
ACI 318-19: Building Code Requirements for Structural Concrete — American Concrete Institute
NCMA TEK 15-5B: Segmental Retaining Wall Design — National Concrete Masonry Association

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Concrete Block Calculator