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Concrete Block & Mortar Calculator

Estimate CMU block count, 80 lb mortar bags, and optional core fill volume for a straight hollow-block wall. Enter wall dimensions and a waste factor for a jobsite-ready masonry takeoff on standard 8×8×16 units.

Wall Dimensions

ft
ft
%

Live Results

Total Blocks Needed

396

360 gross + 10% waste · 8×8×16 standard CMU

Wall Area

320 sq ft

Mortar Bags (80lb)

36

~11 blocks per bag

Core Fill Volume

—

Not included

How to Use This Calculator

  1. Enter wall dimensions. Measure wall length and height in feet for the block wall area.
  2. Select block size. Use standard 8×8×16 in CMU dimensions (nominal) — the calculator accounts for mortar joints.
  3. Set waste factor. Use 5–10% for straight walls. Increase for corners, lintels, and partial blocks at openings.
  4. Review block, mortar, and fill totals. Order blocks by count, mortar by 80 lb bags, and plan core fill grout for reinforced cells separately.

Formula & Example

An 8×8×16 concrete masonry unit (CMU) is a nominal size — the name contractors and suppliers use on invoices and material lists. Actual block dimensions are ⅜ inch undersized on each face to account for a standard ⅜-inch mortar joint. A nominal 8×8×16 block measures approximately 7⅝″ × 7⅝″ × 15⅝″ in the field.

When laid in running bond with ⅜″ joints, each unit covers 0.8889 sq ft of wall face — not the 1.11 sq ft you would get from raw nominal dimensions (8″ ÷ 12 × 16″ ÷ 12). This calculator uses that industry-standard face coverage constant for all block count math.

This version is fixed to standard 8×8×16 hollow CMU. Other nominal sizes — 4-inch, 6-inch, 10-inch, and 12-inch widths — exist for partition walls, veneers, and specialty applications, but each requires different face coverage and mortar yield factors. A block size selector is intentionally omitted in v1 to keep takeoffs consistent and avoid mixing nominal dimensions with actual installed coverage.

Wall Area (sq ft) = Wall Length × Wall Height

Gross Blocks = CEILING(Wall Area ÷ 0.8889 sq ft)

Total Blocks = CEILING(Gross Blocks × (1 + Waste % ÷ 100))

Mortar Bags (80 lb) = CEILING(Total Blocks ÷ 11)

Core Fill (cu yd) = Wall Area × 0.0025 (when enabled)

Joint Thickness Distribution

Mortar joint width directly affects how many blocks fit per course and how much wall area a given block count covers. The industry default for CMU construction is a ⅜-inch (10 mm) joint on bed joints (horizontal) and head joints (vertical). This thickness balances structural bond strength, weather resistance, and efficient laying speed.

Thicker joints — common in restoration work or when using irregular units — reduce blocks per course and increase mortar consumption. Thinner joints save mortar but require tighter tolerances and more skilled laying. At the standard ⅜″ assumption baked into the 0.8889 sq ft footprint, an 8-foot-tall wall requires approximately 12 courses of block plus mortar lift.

Head joint alignment in running bond staggers vertical joints by half a block length each course, which distributes load paths and improves lateral stability. Your mortar bag estimate assumes consistent joint thickness across the full wall height and length — add extra bags if your spec calls for wider joints or if you expect significant cut-block waste at openings.

Worked Example

A 40 ft × 8 ft wall = 320 sq ft. At 1.125 blocks/sq ft = 360 blocks base. With 10% waste: 396 blocks. Mortar: 396 ÷ 11 ≈ 36 bags of 80 lb mortar mix.

Frequently Asked Questions

What mortar mix type should I use — Type N, Type S, or Type M?▾
Mortar is classified by compressive strength and intended application. Type N (750 psi) is the general-purpose mix for above-grade exterior walls, veneers, and non-load-bearing partitions — the most common choice for residential CMU. Type S (1,800 psi) is specified for below-grade work, retaining walls, and structural load-bearing walls where higher flexural and bond strength is required. Type M (2,500 psi) is reserved for heavy commercial foundations, pilasters, and high-lateral-load applications. Match your mortar type to the wall's structural role and local building code — this calculator estimates bag quantity only, not mix design.
How do corner blocks and pilasters affect my takeoff?▾
This calculator assumes a single straight running-bond wall with standard stretcher units — it does not add dedicated corner blocks, pilaster units, or lintel blocks to the count. Corner and pilaster blocks are specialty units with different face dimensions and mortar requirements. At each 90° turn you typically need two corner blocks per course instead of one stretcher, and pilasters add solid or partially filled units at concentrated load points. Calculate each straight wall segment separately, then add corner, pilaster, and lintel units manually based on your elevation drawings — similar to how you would handle fence corners and gate posts on a multi-segment fence run.
What does "filling cores" mean and when is it required?▾
Hollow CMU blocks have two vertical cores (cells) that can remain empty or be filled with grout (a flowable concrete mix). Core filling creates a continuous grout column that, combined with vertical rebar, provides structural reinforcement against lateral loads and uplift. It is typically required for rebar-reinforced load-bearing walls, retaining walls, shear walls, and any wall subject to seismic or high-wind design loads per IBC and local code. In running bond, cores are often filled on an alternating pattern — every other core in each course — which this calculator's 0.0025 cu yd/sq ft constant assumes. Enable the Fill Cores toggle when your plan set calls for grouted cells; leave it off for simple garden walls or non-structural partitions.
How many blocks per square foot of wall?▾
Standard 8×8×16 in CMU yields approximately 1.125 blocks per sq ft of wall surface (8 sq ft per block). Always round up and add waste for cuts at openings and control joints.
How many blocks does one bag of mortar lay?▾
An 80 lb bag of Type S or N mortar typically lays 10–12 standard CMU blocks at 3/8 in joints. Order extra bags for lintels, piers, and partial blocks that use more mortar.

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