Table 4-3.Average Concrete Masonry Units and Mortar Per 100 Sq Ft of wall
required for every 100 square feet of a concrete
length by the number of block in height, which, in this
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,
example, is 75 CMUs in length times 15 courses high,
which equals 1,125 (FB). Lets take another example:
Given: A building 20-feet long by 8-feet wide by
.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
always use OUTSIDE measurements to calculate the
The second method of estimating CMU is the
number of blocks required per course. In most Seabee
SQUARE FOOT METHOD. It is usually the quickest
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
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
and 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
Now calculate the 20 ft x 20 ft x 8 ft building:
20 x 8 = 160 SF x 2 (sides) = 320 SF