Concrete Block Calculator
Enter your wall length and height to get the number of concrete blocks you need — plus courses, bags of mortar, and optional grout to fill the cores. Works for CMU and cinder block.
Free · no sign-up ·
You need · incl. +5% waste
- Wall area
- 160 ft²
- Blocks / course
- 15
- Blocks / ft²
- 1.18
- Exact blocks
- 180
Mortar defaults to ~3 bags per 100 blocks — adjust the blocks-per-bag field for your joint size. Grout fill estimates filling every core; partial fills (only rebar cells) use less.
Structural walls, retaining walls, and walls over a few feet usually need engineering, rebar in grouted cores, and a permit. Check your local building code before you build.
The formula
Blocks = Wall area ÷ block face coverage (a nominal 16×8 in. block covers about 0.89 ft², or ~1.125 blocks per ft²). Courses = wall height ÷ block height; mortar bags = blocks ÷ blocks-per-bag.
How to estimate a concrete block wall
Concrete block, cinder block, and CMU (concrete masonry unit) are different names for the same thing, and the count works the same for all of them. The trick is that block sizes are nominal: a "16×8" block is actually 15⅝ × 7⅝ inches, and the missing ⅜ inch is the mortar joint. So a standard block — joint included — covers a 16 × 8 inch face, which is about 0.89 square feet, or roughly 1.125 blocks per square foot of wall. Multiply your wall area by 1.125 (and add waste) and you have your block count.
You also need mortar to lay them and, for many walls, grout to fill the cores. A rule of thumb is about three 80-lb bags of mortar per 100 standard blocks — the calculator defaults to that, and you can adjust the blocks-per-bag figure for your mortar and joint size. If the wall is structural or a retaining wall, you will often grout-fill some or all of the cores (and add rebar): turn on core fill and the calculator estimates the grout in cubic yards from the block count.
Order about 5% extra block for cuts at openings, corners, and the odd broken unit, and lay up from a level footing with the first course bedded in mortar. Walls over a few feet, anything retaining soil, and anything carrying a structural load generally need engineering, rebar, and a permit — check locally before you build. For the gravel base and footing under the wall, use the Gravel or Crushed Stone calculators.
Frequently asked questions
How many concrete blocks do I need?+
Multiply your wall area (length × height in feet) by about 1.125 — a standard 16×8-inch block covers roughly 0.89 square feet including the mortar joint. A 20 ft × 8 ft wall (160 sq ft) needs about 180 blocks, or 189 with a 5% waste allowance. Enter your wall size above for an exact count.
How many blocks are in a square foot?+
About 1.125 standard 16×8-inch blocks per square foot of wall. Half-height (16×4) blocks run about 2.25 per square foot. The calculator computes it exactly from your block size.
Is a concrete block the same as a cinder block or CMU?+
In everyday use, yes — they all refer to the rectangular masonry units used to build walls, and the count is identical. "Cinder block" is an older term for lighter units made with cinders; "CMU" (concrete masonry unit) is the technical name. This calculator works for all of them.
How much mortar do I need for concrete block?+
Plan on roughly three 80-lb bags of mortar mix per 100 standard blocks. For a 189-block wall that is about 6 bags. The calculator defaults to that ratio and lets you change the blocks-per-bag figure for your mortar and joint thickness.
How many blocks does a bag of mortar lay?+
About 30 to 40 standard blocks per 80-lb bag of mortar mix, depending on joint size and waste. The calculator uses 33 by default — adjust it up for thin joints or down for thick ones.
How much grout to fill block cores?+
A standard 8-inch CMU holds roughly 0.26 cubic feet of fillable core. Filling every core in a 189-block wall takes about 1.8 cubic yards of grout. Turn on core fill in the calculator to estimate it — partial fills (only cells with rebar) use less.
How many courses will my block wall be?+
Divide the wall height by the block height: a standard 8-inch block gives 1.5 courses per foot, so an 8-foot wall is 12 courses. The calculator shows the courses and the blocks per course for your dimensions.
How much extra block should I order?+
Add about 5% for cuts at openings and corners, broken units, and breakage on site. The calculator includes an adjustable waste allowance.
Do I need a permit or rebar for a block wall?+
Often, yes — structural walls, retaining walls, and walls over a few feet typically need engineering, vertical rebar in grouted cores, and a permit. Requirements vary by location and load, so check your local building code before you build.
Is this concrete block calculator free?+
Yes — it is completely free, needs no sign-up, and runs entirely in your browser, so nothing you enter is stored or sent anywhere.