Table 4-3. - Average Concrete Masonry Units and Mortar Per 100 Sq Ft of wall

required for every 100 square feet of a concrete in this masonry wall.

NOTE: Actual wall length is measured from the outside edge to outside edge of units and equals the nominal length minus 3/8" (one mortar joint).

NOTE: For concrete masonry units 7 5/8" and 3 5/8" in height laid with 3/8" mortar joints. Height is measured from center to center of mortar joints.

NOTE: Mortar is based on 3/8" joint with a face-shell mortar bed and 10% allowance for waste.

As a Builder, you might find yourself in the field without the tables handy. To solve that problem, we will cover two methods for estimating concrete masonry units (CMUs) without the tables. The first method is CHASING THE BOND, which is using the 3/4 rule and the 3/2 rule. Remember when estimating, always use OUTSIDE measurements to calculate the number of blocks required per course. In most Seabee construction, 8-inch by 8-inch by 16-inch block is used. Using the 3/4 rule (three full block per 4 feet in length) or .75, multiply the length of the wall by .75. For example, a retaining wall that is 10 feet high by 100 feet in length (1,000 sf) will require 75 block for the first course.

Length of course in feet x rule 3/4 = number of CMU per course

Using the 3/2 rule (three full block per 2 feet in height), multiply the height of the wall by 1.5. For example, the height of the retaining wall is 10 feet. Multiply (10) by the rule 3/2 (1.5) which will equal 15 block high (courses high). See the following formula:

Height of wall in feet x rule 3/2 = courses high

Then, to find the total number of full block (FB) in the retaining wall, multiply the number of block in length by the number of block in height, which, example, is 75 CMUs in length times 15 courses high, which equals 1,125 (FB). Let's take another example:

Given: A building 20-feet long by 8-feet wide by 8-feet high

.75 x 20 x 2 (sides) = 30 (8" x 8" x 16" block)

.75 x 8 x 2 (sides) = 12 (8" x 8" x 16" block)

Or you can find the total linear feet (LF) of the building and multiply by .75.

20 x 2 (sides) + 8 x 2 (sides) = 56 LF

56 x .75 =42 FB

1.5 x 8 = 12 courses high

42 FB x 12 courses = 504 total FB

The second method of estimating CMU is theSQUARE FOOT METHOD. It is usually the quickestand simplest method but NOT the most accurate. However, you, the estimator, will use this method quite frequently. Remember in the first example, the retaining wall was 10 feet high and 100 feet in length. All you do is multiply L x H = SF in this example; the answer is 1,000 square feet (SF). To find the number of 8" x 8" x 16" block required, you must determine the square footage of one CMU which is .89 SF per block. Next, you divide 1,000 SF by .89 SF/CMU which equals 1,124 FB. You calculated the block for 1,000 SF and the difference was (1) less block figuring by the SF method. See the following formula:

Total SF divided by SF/CMU = total number of CMU

Now calculate the 20 ft x 20 ft x 8 ft building:

20 x 8 = 160 SF x 2 (sides) = 320 SF