Why block arithmetic is unusually clean
Concrete block is the one masonry unit where the numbers come out tidy,
and it is deliberate: nominal sizes already contain the mortar joint, so
the coursing works in whole inches.
nominal face = 8 × 16 in = 0.888 ft²
blocks per ft² = 1 ÷ 0.888 = 1.125
blocks per course = ceil(wall length in inches ÷ 16)
courses = ceil(wall height in inches ÷ 8)
The block is smaller than its name
An 8 by 8 by 16 block actually measures 15 5/8 by 7 5/8 inches. The
missing three eighths in each direction is the joint, and that is why a
course of ten reaches exactly 160 inches. Build with the real dimension
and every tenth block puts you nearly four inches out.
Common mistakes
- Setting out from the actual block size. The nominal size is what courses.
- A wall height that does not divide by 8. It means a cut course, planned or not.
- Ordering corner units in the main count. They are a separate line at the supplier.
- Forgetting grout. Filled cores are a separate volume, not part of the block count.
Footings and facing
A block wall stands on a footing, and the
concrete calculator sizes that
pour with bag counts. If the block is a backing for brick veneer, the
brick calculator counts the
face — remember it gives one wythe, so the two are separate orders.
Blocks, courses and mortar by wall size
Standard 8 × 8 × 16 nominal block. Corner units are counted here as whole blocks; order them as a separate line.
Common questions
How many concrete blocks do I need per square foot?
Exactly 1.125 for standard 8 by 8 by 16 inch block. That number is unusually tidy because nominal block sizes already include the mortar joint: a nominal 8 by 16 inch face is 0.888 square feet, and one divided by 0.888 is 1.125. Nine blocks cover eight square feet.
Is a concrete block really 8x8x16?
No — the block itself is 15 5/8 by 7 5/8 inches, three eighths short in each direction. That missing three eighths is the mortar joint, and the nominal size is what the coursing is designed around. It is why ten blocks in a course reach exactly 160 inches rather than 156.
How many courses will my wall be?
The height in inches divided by 8, rounded up. An 8 foot wall is 96 inches, so twelve courses exactly — which is why 8 foot is such a common wall height in block. Heights that do not divide by 8 mean cutting a course or adjusting the joint, and the second is much harder than it sounds.
How much mortar does block need?
One 80 lb bag of Mason Mix lays up to 13 standard 8 by 8 by 16 inch blocks, which is the yield the manufacturer publishes and what this uses. Block still takes less mortar per square foot than brick, because there are far fewer joints in the same area — one block covers the ground of about six bricks — but the figure is substantially higher than the three bags of 70 lb per hundred blocks this page used to quote.
Do I need to fill the cores?
That depends on what the wall is doing, and it is a structural question rather than an arithmetic one. Many walls have only the cells containing rebar grouted; some are fully filled. This calculator counts blocks and mortar — if you are grouting cores, work the fill volume out separately and add it.
Does this include corner blocks?
They are counted as whole blocks, which is right for quantity but not for ordering. Corner units have one finished end and are a separate line at the supplier, so count your corners and split them out of this total when you place the order.
Sources
- Default waste allowance
- 5%
Nominal sizes already include the mortar joint, which is why a nominal 8 x 16 in face is 0.888 ft2 and one divided by that is exactly 1.125 blocks per square foot. Mortar is the published Mason Mix yield of 13 blocks per 80 lb bag, replacing an earlier 3 bags of 70 lb per hundred blocks that was roughly two and a half times short.
Unit conversions are exact by definition and need no source.
How we choose and cite these figures