Block sizes
A standard block is called 8 × 8 × 16 in, and it is a little smaller than that: 7 5/8 × 7 5/8 × 15 5/8 in. The 3/8 in that is missing each way is the mortar joint. A block maker’s own table says it in so many words: the name of a block is its size plus the joint.
That is why a block wall is so easy to count. Laid with its joints, a block takes up exactly 16 in of length and 8 in of height.
Blocks also come 4, 6, 10 and 12 in wide. The width is the thickness of the wall; the face is the same, and so is the count.
Counting by courses or by area
A course is one row of blocks. Divide the height of the wall by 8 in for the courses, and its length by 16 in for the blocks in each, and round both up.
Worked example. A wall 40 ft long and 4 ft high.
- Courses: 48 in ÷ 8 in = 6
- Blocks in a course: 480 in ÷ 16 in = 30
- Blocks: 6 × 30 = 180
- With 5% extra: 189
- Mortar: 180 ÷ 13 = 14 bags of 80 lb
By area it is the same number: one square foot of wall holds 1.125 blocks, and the 160 sq ft of that wall hold 180. Counting by courses is safer for a wall that is not a whole number of blocks: a wall 21 ft 4 in by 3 ft 4 in is 5 courses of 16, 80 blocks.
In a running bond each course is half a block off the one below, so every other course begins and ends on a half: 6 halves on the example. They are in the count, cut from whole blocks or bought as halves.
For a door or a window, enter the area of the opening and its blocks come off: a door of 21 sq ft leaves 157 blocks in the same wall.
Mortar
Quikrete’s data sheet says an 80 lb bag of its mortar mix lays up to 13 standard blocks. “Up to” is the maker’s word: thick joints, mortar dropped and mortar left on the board all come out of it, so the calculator rounds up and you may round up again.
Cinder block or concrete block?
In a store they are the same thing. A cinder block was made with coal cinders as its aggregate, which made it lighter; blocks are made of concrete today, and the old name stayed in everyday speech. The trade’s name for both is CMU, concrete masonry unit. Whatever the label says, the face is 8 × 16 in with its joints, and this calculator counts it.
What this calculator doesn’t do
It counts blocks and mortar. It does not say whether a wall needs steel in it, grout in its cores, a footing of what size, or what it takes to hold back earth. A wall that carries a load or retains soil is designed to your local code, and often by an engineer.
Questions about block walls
How many blocks in a square foot?
1.125, for the standard 8 × 16 in face: 112.5 blocks in 100 sq ft.
How many blocks per bag of mortar?
Up to 13 standard blocks for an 80 lb bag of mortar mix, by Quikrete’s data sheet. The 180 blocks of the example take 14 bags.
What’s the difference between cinder and concrete blocks?
The aggregate they were made with, and mostly the name. For counting there is none: same face, same joints.
How many blocks for a wall 40 ft long?
30 in each course. At 4 ft high that is 6 courses and 180 blocks, or 189 with 5% extra.
Sources
- Angelus Block, Dimensions and Sizes for CMU. A nominal 8 x 8 x 16 in block is specified at 7.625 x 7.625 x 15.625 in: nominal dimensions are the specified ones plus the thickness of the mortar joint. Widths of 4, 6, 10 and 12 in are made to the same height and length. Read on .
- Quikrete, Mortar Mix (No. 1102), data sheet. Each 80 lb bag lays up to 37 standard bricks or 13 standard blocks of 8 x 8 x 16 in. Read on .
Updated . How we calculate. Found a mistake? Tell us.