Nominal module from CMHA CMU-TEC-001-23, mortar types from ASTM C270
A concrete block is not 8 by 16. It is made 3/8 in under nominal so the mortar joint puts the module back, which is exactly why 1.125 blocks per square foot is not a rule of thumb but the module itself. The count here is checked two ways. What the count cannot tell you is which of two identical-looking bags of mortar you need, and those two are 750 psi and 1,800 psi.
Block, course and mortar quantities
ft
ft
no.
ft
ft
%
Counts are for a plain rectangle of wall. Corners, returns and non-modular layouts raise both figures, and the source says so.
Specification summary
Generated from on .
Reopen that address to reproduce these figures exactly.
Entered
Result
The wall in running bond, at the nominal module
Worked example
A 20 ft by 8 ft wall in nominal 8 x 8 x 16 block, no openings, 5% for cuts and breakage, laid in Mason Mix.
Area. 20 × 8 = 160 sq ft.
By module. 20 ft is 240 in, so 240 ÷ 16 = 15 blocks a course. 8 ft is 96 in, so 96 ÷ 8 = 12 courses. 15 × 12 = 180.
By area. 160 × 1.125 = 180. The two methods agree exactly, and they have to: 1.125 is 144 ÷ (8 × 16).
With 5%.189 blocks.
Mortar. 189 ÷ 13 = 15 bags of 80 lb.
The calculator prints that cross-check on every wall without openings, because a block count that agrees with itself two ways is worth more than one that arrives from nowhere. Now change the length to 21 ft: the count barely moves, but 21 ft is 252 in, which is 15.75 blocks, so every single course now ends in a cut. Nudging a wall to land on the 16 in module is free at the drawing stage and expensive with a saw.
The formula
Two routes to the same number, which is the point:
n = A × 144/(8 × 16) = (12L/16) × (12H/8)
A
net wall area in sq ft, openings deducted
8 × 16
the NOMINAL face of the block, in, joint included. 144/128 = 1.125
12L/16
blocks per course. The 16 is the nominal length
12H/8
courses. The 8 is the nominal height
bags
blocks divided by 13, from the data sheet's own yield
The two routes are algebraically the same expression, so they agree for any wall with no openings, and the calculator prints both so you can see it. They part company the moment there is an opening in the wall, because area is reduced by the hole while the coursing runs past it.
Where 1.125 comes from, and what it silently assumes
Every block calculator on the internet uses 1.125 blocks per square foot and almost none of them say why. It is 144 divided by 128, the area of a nominal 8 by 16 face, and it is exact rather than approximate.
The reason it works is that the block is deliberately undersized. The Concrete Masonry & Hardscapes Association puts specified dimensions at 3/8 in less than nominal in each direction, so a nominal 8 x 8 x 16 unit is actually 7 5/8 x 7 5/8 x 15 5/8. Add a 3/8 in mortar joint on one side of each and the module closes back to 8 by 16. The unit and the joint were designed as one system.
So 1.125 is not a property of the block. It is a property of the block plus a 3/8 in joint. At a 1/2 in joint the module becomes 16 1/8 by 8 1/8 and the figure drops toward 1.10, which is a bag of mortar and a few blocks over a garage wall, and a coursing problem over a long one.
On top of that ASTM C90 permits the unit itself to vary by plus or minus 1/8 in on any overall dimension, which is why the count is a starting figure and the first course still gets dry-laid.
Nominal against specified, and what the module assumes
Nominal, in
Specified, in
Blocks per sq ft
Joint the module assumes
ASTM C90 tolerance
4 x 8 x 16
3 5/8 x 7 5/8 x 15 5/8
1.125
0.375 in (0 3/8 in)
plus or minus 0 1/8 in
6 x 8 x 16
5 5/8 x 7 5/8 x 15 5/8
1.125
0.375 in (0 3/8 in)
plus or minus 0 1/8 in
8 x 8 x 16
7 5/8 x 7 5/8 x 15 5/8
1.125
0.375 in (0 3/8 in)
plus or minus 0 1/8 in
10 x 8 x 16
9 5/8 x 7 5/8 x 15 5/8
1.125
0.375 in (0 3/8 in)
plus or minus 0 1/8 in
12 x 8 x 16
11 5/8 x 7 5/8 x 15 5/8
1.125
0.375 in (0 3/8 in)
plus or minus 0 1/8 in
Read 2026-09-08. Specified dimensions are nominal less 3/8 in, per CMU-TEC-001-23. Blocks per square foot is 144 divided by the nominal face, 8 x 16.
Why 1.125 blocks per square foot works, and when it stops
A concrete block is not 8 in by 16 in. The Concrete Masonry & Hardscapes Association states the rule directly:
Specified dimensions of concrete masonry units are typically 3/8 in. (9.5 mm) less than nominal dimensions, so that a 4 or 8 in. (102 or 203 mm) module is maintained with 3/8 in. (9.5 mm) mortar joints.
So a nominal 8 x 8 x 16 unit is manufactured at 7 5/8 x 7 5/8 x 15 5/8, and the 3/8 in mortar joint puts the module back to 8 by 16. The unit and the joint are designed as one thing.
Which is where the familiar figure comes from. A nominal 8 by 16 face is 128 square inches, and 144 ÷ 128 = 1.125 blocks per square foot. It is not a rule of thumb, it is the module, and it is exact.
It is exact only at a 3/8 in joint. Lay the same units to a 1/2 in joint and every course grows: the module becomes 16 1/8 by 8 1/8, the count per square foot drops to about 1.10, and over a long wall the coursing no longer lands where the drawing says it does. That is why bond patterns, openings and lintel bearings are all set out in nominal dimensions.
ASTM C90 then allows the unit itself to vary by plus or minus 1/8 in on any overall dimension. Fifteen units in a 20 ft run, each at the low end of that tolerance, is nearly 2 in of wall you did not plan for. It is why masons dry-lay the first course before mixing anything.
Two bags, the same yield, and 2.4 times the strength
This is the one worth stopping on. Quikrete sells two 80 lb masonry bags that are, on every quantity measure, identical:
Product
Mortar type
ASTM C270 minimum
Yield per 80 lb bag
Quikrete Mason Mix (Type S)
Type S
1,800 psi
13 blocks or 37 bricks
Quikrete Mortar Mix (Type N)
Type N
750 psi
13 blocks or 37 bricks
Same bag weight. Same 13 blocks. Same 37 bricks. Both cite ASTM C270 and ASTM C1714. And one of them has to reach 750 psi while the other has to reach 1,800, because one is Type N and the other is Type S.
The full ASTM C270 minimums, as the data sheets themselves reproduce them:
Mortar type
Minimum compressive strength
Type M
2,500 psi
Type S
1,800 psi
Type N
750 psi
A quantity calculator cannot tell these two apart, because on quantity there is nothing to tell apart. The bag count this page gives you is the same either way. Which bag is a specification question, not an estimating one, and it is decided by where the wall is: below grade, retaining soil, or carrying load is not Type N territory. Ask your building department or your engineer, and read the bag, because both products say other types are available by special request and the shelf label is not the specification.
Landing the wall on the module
The cheapest decision on a block wall is made before anything is ordered: make the length a multiple of 16 in and the height a multiple of 8 in.
A 20 ft wall is 240 in, which is exactly 15 blocks. A 21 ft wall is 252 in, which is 15.75, so every course ends in a quarter block and somebody spends the day at the saw. The material difference is trivial. The labour difference is not, and neither is the look of a wall with a cut in every course at the same end.
Height works the same way in 8 in steps. Eight feet is 12 courses exactly. Eight foot six is 12.75, so the top course is ripped down its length, which is slower and weaker than a whole unit.
The calculator flags both cases and tells you the nearest lengths that land clean.
What the block count does not include
Corners. In running bond every corner needs the bond to turn, which means a half block or a purpose-made corner unit in alternate courses. A rectangle of wall with four corners is not four straight walls added up.
Lintels and bond beams. Openings need something over them, and a bond beam course is a different unit with a channel in it. Neither is counted here.
Grout and reinforcement. A reinforced wall has vertical bar in filled cells and the grout to fill them. That volume is deliberately not calculated on this page: the association's manual publishes a grout estimation table and the copy obtainable from here does not extract cleanly enough to transcribe with confidence. Under-filling a reinforced wall is not a rounding error.
The footing. Block walls sit on concrete. The concrete calculator handles that, and it will also tell you the strength and air entrainment the code wants for it.
The association's own warning about counting blocks
The CMHA manual that sets out how to estimate masonry quantities opens that section with this:
Estimating materials quantities in this section should be used with caution and checked against rational judgment. Design issues such as non-modular layouts or numerous returns and corners can significantly increase the number of units and the volume of mortar or grout required to construct a project.
Worth reading twice, because it is the body that publishes the method telling you where the method breaks. A rectangle of wall is easy. Corners, returns and anything non-modular are not, and they push both the unit count and the mortar up. Every corner in running bond needs a half block somewhere, openings want the coursing to land on them, and a wall whose length is not a multiple of 16 in means cutting.
So treat the figure above as the quantity for a plain rectangle of wall, add for cuts and corners, and dry-lay the first course.
Grout for filled cells is not calculated here. The same manual carries a grout volume table and this page does not reproduce it, because the copy obtainable from here does not extract cleanly enough to transcribe a cell with confidence. An under-filled reinforced wall is not a rounding error, so the figure is absent rather than approximate.
Where these figures came from
Read 2026-09-08. Unit dimensions and tolerances are from the trade association's own tech note; mortar yields and types are from the manufacturer's data sheets.
CMHA tech note CMU-TEC-001-23, Concrete Masonry & Hardscapes Association
Figure 2 gives nominal 16 x 8 x 8 in against specified 15 5/8 x 7 5/8 x 7 5/8 in. Overall dimensions may vary by no more than plus or minus 1/8 in from the specified dimensions. ASTM C90 minimum net area compressive strength is 2,000 psi.
Carried for that sentence, which is the association warning about its own estimating method. Its grout volume table is NOT transcribed here, see the note on the page.
Frequently asked questions
How many blocks do I need for a 20x8 wall?
180 nominal 8 x 8 x 16 blocks, before an allowance for cuts and breakage. That is 15 blocks a course and 12 courses, and it is also 160 sq ft times 1.125, which is the same arithmetic written two ways. With a 5% allowance you would buy 189.
How many concrete blocks per square foot?
1.125, and it is exact rather than a rule of thumb. A nominal block face is 8 in by 16 in, which is 128 square inches, and 144 divided by 128 is 1.125. It holds only for a 3/8 in mortar joint, which is the joint the block's undersize is designed around.
What are the actual dimensions of an 8 inch concrete block?
7 5/8 by 7 5/8 by 15 5/8 in. CMHA puts specified dimensions at typically 3/8 in less than nominal so that the 8 in module is maintained with a 3/8 in mortar joint. ASTM C90 then allows plus or minus 1/8 in variation on any overall dimension.
How many blocks will one bag of mortar lay?
Thirteen standard 8 x 8 x 16 blocks, or 37 standard bricks, per 80 lb bag. That figure is printed identically on the data sheets for both Quikrete Mortar Mix and Quikrete Mason Mix, so the bag count does not depend on which of the two you buy.
What is the difference between Mortar Mix and Mason Mix?
The mortar type, and it is a large difference. Quikrete Mortar Mix is Type N and Quikrete Mason Mix is Type S. Under ASTM C270 the minimum compressive strengths are 750 psi and 1,800 psi respectively. The bags are both 80 lb and both lay 13 blocks, so nothing about quantity distinguishes them.
Does the mortar joint thickness change how many blocks I need?
Yes. The 1.125 per square foot figure assumes a 3/8 in joint, which is what the block's 3/8 in undersize is designed for. Lay to a 1/2 in joint and the module grows to roughly 16 1/8 by 8 1/8, dropping the count nearer 1.10 per square foot and moving where the coursing lands.
Why does my wall need cut blocks?
Because its length is not a multiple of 16 in or its height is not a multiple of 8 in. A 21 ft wall is 15.75 blocks a course, so every course ends in a cut. Adjusting to 20 ft or 21 ft 4 in lands on the module and removes the cutting entirely.
Check these numbers yourself
Concrete Masonry Unit Shapes, Sizes, Properties and SpecificationsCMHA tech note CMU-TEC-001-23, Concrete Masonry & Hardscapes Association. Figure 2 gives nominal 16 x 8 x 8 in against specified 15 5/8 x 7 5/8 x 7 5/8 in. Overall dimensions may vary by no more than plus or minus 1/8 in from the specified dimensions. ASTM C90 minimum net area compressive strength is 2,000 psi.
Annotated Design and Construction Details for Concrete MasonryCMHA CMU-MAN-001-03. Carried for that sentence, which is the association warning about its own estimating method. Its grout volume table is NOT transcribed here, see the note on the page.
How every figure here is verified: Sources & Method. Who builds this: About. Found something wrong? Tell us and it gets fixed or removed.
Figures on this page last checked against the source documents on 2026-09-08. Codes are amended locally; confirm against the edition your jurisdiction enforces.
Counts are for a plain rectangle of wall. Read 8 September 2026. Corners, returns, lintels, bond beams and any non-modular layout raise both the block and the mortar figures, and the association that publishes this estimating method says so itself. Grout for filled cells is not calculated here. Mortar type is a specification decision, not an estimating one.