How it works
Two measurements give you blocks, courses and the mortar to lay them.
Measure the wall
Length and height in feet. Measure each wall separately and add the lengths.
Standard block assumed
8 × 8 × 16 inch CMU, which works out to 1.125 blocks per square foot with the joint.
Get blocks and mortar
Block count, how many courses high, and the bags of mortar to lay them.
Why 1.125 and not 1.125 exactly
A standard concrete block is called 8 × 8 × 16, but like lumber those are nominal figures. The block itself measures 15⅝ by 7⅝ inches, and the missing ⅜ inch on each dimension is the mortar joint. Laid up, each block plus its joint occupies exactly 8 by 16 inches of wall face — which is 0.888 ft², or 1.125 blocks per square foot.
That is why the nominal size is the useful one. You do not need to model the joints separately; they are already baked into the figure. A 40 by 8 foot wall is 320 ft², which is 360 blocks before any allowance.
The course count is the wall height divided by 8 inches. It matters because block walls work best when the height lands on a whole course. A wall specified at 7 feet 10 inches means cutting every block in the top row, which is slow and looks poor. Adjust the height to a multiple of 8 inches where you can.
Mortar runs at roughly three bags per hundred blocks for a standard 3/8 inch joint, which is what the figure here assumes. Cold weather, wide joints or a sloppy hand all push it up. Corners, half blocks and bond beams are counted here as whole blocks — that is deliberate, since you buy them whole either way.
Frequently asked questions
How many blocks per square foot?
1.125 for standard 8 × 8 × 16 inch block, including the mortar joint. Multiply your wall area in square feet by 1.125.
How many blocks in a 40x8 wall?
About 378 including 5% waste, plus roughly 12 bags of mortar.
How much mortar per 100 blocks?
About three bags for a standard 3/8 inch joint. Wider joints and cold weather use more.
What size is a concrete block really?
15⅝ by 7⅝ inches. It is called 8 × 16 because that is the space it takes up once the mortar joint is added.
More material calculators
Estimates are for planning only. Confirm quantities with your supplier before ordering.