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.
A worked example: a 24-foot garden wall
A block wall 24 feet long and 4 feet high has 96 ft² of face. At 1.125 blocks per square foot that is 108 blocks, and with 5% waste the order is 114 blocks. The wall is 6 courses high, because 48 inches divided by 8 is 6, and mortar comes to about 4 bags.
The wall does not stop at the blocks. A block wall needs a concrete footing below it, usually about twice as wide as the block and at least as deep as the block is wide, set below the frost line where you live. For an 8-inch block, that is a footing about 16 inches wide and 8 inches deep, which for 24 feet of wall is 24 × 1.33 × 0.67 ≈ 21 ft³, or about 0.8 cubic yards of concrete. Our concrete calculator sizes it exactly, and the rebar calculator counts the steel.
Courses for common wall heights
| Wall height | Courses (8 in) | Blocks per 10 ft of wall |
|---|---|---|
| 16 in (1.33333 ft) | 2 | 15 |
| 24 in (2 ft) | 3 | 23 |
| 32 in (2.66667 ft) | 4 | 30 |
| 40 in (3.33333 ft) | 5 | 38 |
| 48 in (4 ft) | 6 | 45 |
| 64 in (5.33333 ft) | 8 | 60 |
| 72 in (6 ft) | 9 | 68 |
| 96 in (8 ft) | 12 | 90 |
Heights that are multiples of 8 inches avoid cutting the top course. Blocks per 10 feet are before waste.
Mistakes that cost money
- Rushing the first course. Every course takes its level from the one below. Lay the first course in a full mortar bed, check it for level and line, and the rest of the wall goes up fast.
- Leaving out the steel. Retaining walls and tall freestanding walls need vertical rebar grouted into the cores and often a bond beam. Local building codes set the spacing; check before you build.
- Mixing more mortar than you can use. Mortar starts to stiffen within about 90 minutes. Mix small batches you can lay in that time.
- Ignoring drainage behind a retaining wall. Water pressure is what pushes retaining walls over. Gravel backfill and a drain pipe at the base are part of the job, not extras.
What it costs
A standard 8 × 8 × 16 gray concrete block usually sells for about $2 to $4 in the US, and an 80 lb bag of mortar mix for $10 to $15. For the example wall, the blocks and mortar come to roughly $228 to $516, before the footing, rebar, grout and any rental tools. Prices vary by region and supplier.
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.