Estimating a CMU Block Wall
Concrete masonry units are sized so the arithmetic stays simple. A standard 8×8×16 in block covers about 1.125 sq ft of wall face once the 3/8 in mortar joint is counted, which means one square foot of wall takes a little over one block. Give the calculator a length and a height and it returns block count, mortar, courses, rebar, and grout — the full material list for a straight run of wall.
A Worked Example
Say you are building a 20 ft long, 8 ft tall wall. That is 20 × 8 = 160 sq ft of face. At 1.125 sq ft per block you need 160 ÷ 1.125 ≈ 142 blocks, and adding 5% for cuts and breakage rounds you to about 150 blocks to order. The wall stands 96 in ÷ 8 = 12 courses tall. For mortar, 150 blocks at roughly one bag per 30–35 blocks means 4–5 bags of Type S. If the wall is reinforced, you would add vertical #4 bars at the corners and every 48 in with grout in those cells.
Where Reinforcement Comes In
The block count is the easy part; the reinforcement is what varies. A low free-standing garden wall may need little more than blocks and mortar, while a retaining wall or foundation carries vertical rebar doweled up from the footing, horizontal bond-beam steel, and grouted cells to lock it all together. Grout runs about half a cubic foot per filled core, and the spacing of steel is dictated by wall height, soil load, and your local code rather than a fixed rule.
CMU Wall Calculator
Estimate blocks, mortar, rebar, and grout fill for concrete block walls.
CMU Wall Material Estimates by Wall Size
8" standard blocks, all cores grouted, includes 5% waste
| Wall Size | Blocks | Mortar (bags) | Grout (cu ft) | Rebar (ft) |
|---|---|---|---|---|
| 20' × 4' | 95 | 12 | 47 | 52 |
| 30' × 6' | 209 | 25 | 104 | 98 |
| 40' × 8' | 378 | 45 | 189 | 168 |
| 50' × 8' | 473 | 56 | 236 | 200 |
| 60' × 10' | 709 | 84 | 354 | 310 |
| 80' × 8' | 756 | 89 | 378 | 320 |
How to Use This Calculator
- Enter Wall Length and Height: Type in the length and height of the wall in feet. Length × height gives the wall face area the block count is built from.
- Pick Block Size and Fill: Choose the block width (8 in is standard) and whether cores are fully grouted, filled only at rebar cells, or left hollow. This drives the grout and rebar figures.
- Add a Price per Block: Enter your local block price to get a materials cost alongside the quantities. Leave the default if you just want counts.
- Calculate and Order: Run the numbers to see blocks, mortar bags, courses, rebar, and grout. Round up and add a little cushion before placing the order.
How It Works
The math behind a block wall comes down to face area. A standard 8×8×16 in CMU covers about 1.125 sq ft of wall once you include the 3/8 in mortar joint, so the count is simply wall area divided by 1.125. From there the calculator layers on mortar, courses, rebar, and grout fill so you can price the whole job from two numbers: length and height.
The basic rule:
- Blocks per square foot: 1.125 blocks/sq ft (8 in × 16 in nominal face including a 3/8 in mortar joint; the block itself is 7⅝ in × 15⅝ in)
- Courses: wall height in inches ÷ 8 in per course. An 8 ft wall is 96 in ÷ 8 = 12 courses
- Mortar: roughly 1 bag of Type S per 30–35 blocks for 3/8 in joints (about 8.5 blocks per 80 lb bag on full-bed structural work)
- Grout fill: about 0.5 cubic feet per 8 in block core, two cores per standard block
- Rebar: vertical #4 bars near corners and every 48 in on center, with horizontal bond-beam steel roughly every 48 in of height
- Waste: add about 5% to the block count for breakage, cuts, and corners
Treat every figure as a planning estimate. Doors, windows, corners, control joints, and pilasters all shift the real count, and reinforcement spacing is set by your local code and wall height rather than by a rule of thumb. Confirm rebar and grout requirements with the engineer or building official on any load-bearing or retaining wall before you order.
Tips & Considerations
- Design wall heights to a multiple of 8 in so the top course lands full-height and you avoid cutting every block on the last row.
- Order about 5% extra block — corners, cuts, and the occasional cracked unit add up fast on a long wall.
- Buy one or two more bags of mortar than the estimate; a full-bed structural mix uses more per block than face-shell bedding.
- Grout only what the code requires — filling every core on a wall that only needs rebar cells wastes material and money.
- Dowel vertical rebar up out of the footing before the concrete sets so the steel ties cleanly into the grouted cells above.
Frequently Asked Questions
How many CMU blocks do I need per square foot?
About 1.125 standard blocks (8×8×16 in) per square foot of wall face. That single figure already accounts for the 3/8 in mortar joint around each block. Multiply your wall area in square feet by 1.125, then add roughly 5% for cuts and breakage. A 160 sq ft wall works out to 160 × 1.125 ≈ 180 blocks, or about 189 with waste.
How much mortar do I need for a CMU wall?
Plan on about one bag of Type S mortar for every 30–35 standard blocks laid with 3/8 in joints. So a 150-block shed wall needs roughly 4–5 bags. If you are laying a full mortar bed rather than face-shell bedding, mortar goes further per bag (closer to 8–9 blocks per 80 lb bag). Buy an extra bag or two — running short mid-course is far more disruptive than a leftover bag.
Do I need rebar in a CMU wall?
For anything load-bearing, below grade, or holding back soil, yes. Typical practice runs vertical #4 bars in grouted cells near each corner and every 48 in on center, tied to horizontal bond-beam steel every few courses. Short, free-standing garden walls under about 3 ft are sometimes allowed without vertical steel, but retaining walls and foundation walls always need engineered reinforcement. Your local code and wall height set the actual spacing.
How many courses of block will my wall be?
Divide the wall height in inches by 8, since each standard course (block plus its bed joint) is 8 in tall. An 8 ft wall is 96 ÷ 8 = 12 courses; a 4 ft wall is 6 courses. Because block heights come in 8 in increments, designing walls to a multiple of 8 in avoids cutting blocks down the final course.
Do all CMU walls need to be filled with grout?
No. Non-structural partition walls are often left hollow. Structural walls, retaining walls, and walls in seismic or high-wind zones usually need cores grouted — at minimum every cell that contains rebar, and sometimes all cells (fully grouted). Figure roughly 0.5 cubic feet of grout per filled 8 in core when you estimate. Check your local code for which cells must be solid.
What size CMU block should I use?
8 in wide block is the default for most residential and light commercial walls. Step up to 12 in for tall retaining walls and heavily loaded foundations, drop to 6 in for lighter non-load-bearing partitions, and reserve 4 in units for veneer and decorative work. All of these share the same 8 in × 16 in face, so the per-square-foot block count stays about 1.125 regardless of width.
Do I need a footing under a block wall?
Yes — CMU walls bear on a poured concrete footing. A common rule sizes the footing at twice the wall width and at least 8 in deep, so an 8 in wall sits on a 16 in wide × 8 in deep footing, with the bottom below the local frost line. Vertical rebar is usually doweled up out of the footing so it ties into the grouted cells above.