TONS & YARDS | OFFICIAL CMU MASONRY TAKEOFF
Project: Commercial / Residential Masonry Schedule | Standard 3/8-in Mortar Joint Spec | Generated:
Cinder Block (CMU) & Mortar Calculator
Calculate precise concrete masonry unit (CMU) counts, course layers, 80-lb pre-mixed mortar bags, and core fill grout cubic yards for foundation stems, structural retaining walls, and partition barriers.
1. Masonry Wall Parameters
2. Order Bill of Materials (BOM)
Engineering Mathematics: CMU Block & Mortar Calculations
Unlike poured concrete, concrete masonry unit (CMU) takeoffs operate on discrete modular geometry governed by ASTM C90 (Standard Specification for Loadbearing Concrete Masonry Units). Standard modular blocks incorporate a uniform 3/8-inch (0.375 in) bed and head mortar joint.
Primary CMU Surface Density Formula
Standard stretcher units have actual physical dimensions of 7-5/8" H × 7-5/8" W × 15-5/8" L. With 3/8″ mortar joints, their nominal face dimensions become exactly 8" H × 16" L:
Mortar Bag Estimating Formula (80-lb Pre-Mix)
Field masonry benchmarks confirm that one standard 80-lb pre-mixed bag of ASTM C270 Type S or Type N masonry cement/sand lays between 30 to 33 standard 8×8×16 CMU blocks when applying standard 3/8″ face-shell bed joints and full head joints:
Worked Field Engineering Scenario: 40′ × 8′ Foundation Stem Wall
A masonry contractor is bidding an 8-inch thick foundation stem wall measuring 40 feet in length and 8 feet in height with structural vertical rebar cores grouted at 32 inches on-center:
- Surface Area Calculation: (40text{ ft} times 8text{ ft} = 320text{ sq ft}).
- Modular Course Geometry: Height requires ((8 times 12) / 8 = 12text{ courses}). Length requires ((40 times 12) / 16 = 30text{ blocks per course}).
- Raw CMU Count: (320text{ sq ft} times 1.125 = 360text{ blocks}). Adding 5% cut waste yields (lceil 360 times 1.05 rceil = mathbf{378text{ CMU units}}).
- Mortar Bag Demand: (lceil 378 / 33 rceil = mathbf{12text{ bags (80-lb) of ASTM C270 Type S mortar}}).
- Core Grout Volume: At 32″ O.C., one core every two blocks is filled. With core volume of 0.08 cu ft per core, 378 blocks (times 0.04 = 15.12text{ cu ft}). Converted to cubic yards: (15.12 / 27 = mathbf{0.56text{ cu yds of fine grout}}).
ASTM C90 CMU Specifications & Core Volume Matrix
Concrete masonry units vary significantly in weight and core capacity depending on aggregate classification (Normal Weight, Medium Weight, Light Weight) and nominal wall width.
| Nominal CMU Size (W × H × L) | Actual Dimensions (in) | Blocks / Sq Ft | Avg Weight (lbs) | Solid Grout Volume / 100 Blocks | Primary Application |
|---|---|---|---|---|---|
| 4″ × 8″ × 16″ | 3-5/8 × 7-5/8 × 15-5/8 | 1.125 | 22 – 26 lbs | 0.28 cu yds (7.5 cu ft) | Veneer walls, fire separation screens |
| 6″ × 8″ × 16″ | 5-5/8 × 7-5/8 × 15-5/8 | 1.125 | 28 – 34 lbs | 0.43 cu yds (11.6 cu ft) | Light residential basement, interior partition |
| 8″ × 8″ × 16″ (Standard) | 7-5/8 × 7-5/8 × 15-5/8 | 1.125 | 33 – 42 lbs | 0.59 cu yds (16.0 cu ft) | Standard foundation stems, load-bearing walls |
| 10″ × 8″ × 16″ | 9-5/8 × 7-5/8 × 15-5/8 | 1.125 | 40 – 48 lbs | 0.74 cu yds (20.0 cu ft) | Commercial earth-retaining walls, deep basements |
| 12″ × 8″ × 16″ | 11-5/8 × 7-5/8 × 15-5/8 | 1.125 | 48 – 58 lbs | 0.93 cu yds (25.0 cu ft) | Heavy industrial, cantilevered retaining > 8 ft |
Professional Masonry Field Protocol: Footings, Bonds & Rebar
Poured Concrete Footing Tolerances
Concrete footings must be poured wider than the masonry wall (typically twice the wall width, e.g., 16″ wide footing for an 8″ CMU wall) and below local frost depth. Footing surface must be level within 1/4″ across 10 feet. Keyed joints or vertical starter dowels (#4 or #5 rebar) must extend at least 18 inches above the footing to lap into the CMU vertical cores.
3/8-Inch Bed & Head Mortar Rules
Lay the first course on a full bed of mortar (1/2″ to 3/4″ thick) to compensate for minor footing irregularities. Subsequent courses use face-shell bedding: apply mortar strips along the outer edges of the block top shells only (do not mortar across cross webs unless grouting solid). Tool joints with a concave jointer when mortar is “thumbprint hard” to seal against water penetration.
High-Lift vs. Low-Lift Grout Pouring
Low-lift grouting is placed in lifts up to 5 feet as the wall is erected. High-lift grouting allows walls up to 24 feet high to be grouted in a single operation, provided cleanout openings are maintained at the base of every vertical core to remove fallen mortar droppings before pumping. Consolidate grout thoroughly with a mechanical pencil vibrator to eliminate honeycombing.
OSHA 29 CFR 1926.706 Masonry Wall Bracing Mandate
Under OSHA federal safety regulations, all masonry walls greater than 8 feet in height must be adequately braced to prevent collapse until permanent structural supports (such as floor joists, bond beams, or roof diaphragms) are in place. Maintain a limited access zone (LAZ) equal to the height of the wall plus 4 feet along the entire length of unbraced masonry.
Frequently Asked Questions: CMU Takeoffs & Mortar Specification
How many standard 8x8x16 cinder blocks do I need per square foot of wall?
You need exactly 1.125 standard CMU blocks per square foot of wall surface area. Standard 8x8x16 blocks have nominal face dimensions of 8 inches high by 16 inches long when factoring in the standard 3/8-inch mortar joint (actual block size is 7-5/8 × 15-5/8 inches). Each block covers 0.8889 square feet (8/12 × 16/12), yielding 1 / 0.8889 = 1.125 blocks per sq ft.
How many bags of mortar do I need for 100 cinder blocks?
For 100 standard 8x8x16 CMU blocks, you will need approximately 3 to 3.5 bags (80-lb each) of Type S or Type N pre-mixed masonry mortar. One 80-lb bag lays approximately 30 to 33 standard blocks with 3/8-inch bed and head joints, assuming professional tooling and standard 10% trowel drop/waste.
What is the difference between Type S and Type N mortar for CMU blocks?
Type S mortar achieves a minimum 28-day compressive strength of 1,800 psi and is formulated for below-grade foundations, structural retaining walls, load-bearing walls, and high-wind/seismic zones. Type N mortar achieves 750 psi and is formulated for above-grade, non-load-bearing exterior walls, interior partition walls, and decorative chimneys.
How much concrete grout is required to fill reinforced CMU block cores?
A standard hollow two-core 8x8x16 CMU has approximately 0.16 cubic feet of internal core volume. If grouting solid, 100 blocks require 16 cubic feet (0.59 cubic yards) of grout. If grouting cores reinforced with rebar at 32 inches on-center (every second block), 100 blocks require approximately 8 cubic feet (0.30 cubic yards) of fine or coarse grout.