Home / Concrete & Masonry
Concrete & MasonryConcrete Block Calculator — How Many Blocks Do I Need?
Enter the wall’s length and height to estimate the number of standard 8×8×16-inch concrete blocks, mortar joints and a little waste included.
How this calculator works
We figure wall area (length × height) and multiply by about 1.125 standard 8×8×16 blocks per square foot (joints included), then add 5% for cuts and breakage.
What changes how much you need
- A standard 8×8×16 CMU covers about 0.89 sq ft of wall — roughly 1.125 blocks per sq ft.
- Half and corner blocks change counts at ends and openings.
- Plan about 3 bags of mortar per 100 blocks.
- Order 5–10% extra for cuts and breakage.
- Each course adds exactly 8 inches of height, joint included — so wall height in inches ÷ 8 gives your course count.
Blocks needed by wall size (standard 8×8×16 CMU)
Counts include 5% for cuts and breakage. Courses assume an 8-inch nominal block height; blocks per course assume a 16-inch nominal length, so both divide evenly into most wall dimensions.
| Wall | Wall area | Courses | Blocks per course | Blocks to buy |
|---|---|---|---|---|
| Garden wall · 8 × 4 ft | 32 sq ft | 6 | 6 | 38 |
| Retaining wall · 12 × 4 ft | 48 sq ft | 6 | 9 | 57 |
| Shed / shop wall · 20 × 8 ft | 160 sq ft | 12 | 15 | 189 |
| Garage wall · 24 × 8 ft | 192 sq ft | 12 | 18 | 227 |
| Long wall run · 32 × 8 ft | 256 sq ft | 12 | 24 | 303 |
| Basement wall · 40 × 8 ft | 320 sq ft | 12 | 30 | 378 |
Standard CMU sizes, weights, and coverage
Every full-size block presents the same 8 × 16-inch nominal face — about 0.89 sq ft — so the count per square foot of wall does not change with block thickness. Only the weight, the price, and the load it can carry change. Weights are for normal-weight hollow units; lightweight units run roughly 25% less.
| Nominal size | Actual size | Blocks per sq ft of wall | Approx. weight each | Typical use |
|---|---|---|---|---|
| 4 × 8 × 16 in | 3⅝ × 7⅝ × 15⅝ in | 1.125 | 22–26 lb | Veneer backup, low partitions |
| 6 × 8 × 16 in | 5⅝ × 7⅝ × 15⅝ in | 1.125 | 28–32 lb | Garden and screen walls |
| 8 × 8 × 16 in | 7⅝ × 7⅝ × 15⅝ in | 1.125 | 36–38 lb | The standard — foundations, structural walls |
| 10 × 8 × 16 in | 9⅝ × 7⅝ × 15⅝ in | 1.125 | 42–46 lb | Taller or more heavily loaded walls |
| 12 × 8 × 16 in | 11⅝ × 7⅝ × 15⅝ in | 1.125 | 50–55 lb | Basement and tall retaining walls |
| Half block · 8 × 8 × 8 in | 7⅝ × 7⅝ × 7⅝ in | 2.25 | 18–20 lb | Ends, corners, and openings |
Frequently asked questions
How many concrete blocks per square foot?
About 1.125 standard 8×8×16-inch blocks per square foot of wall, including mortar joints.
How much mortar for concrete block?
Roughly 3 bags of mortar per 100 blocks, depending on joint size and conditions.
What size is a "standard" concrete block?
Nominal 8×8×16 inches (actual 7⅝ each way, to allow for a 3⁄8-inch mortar joint).
How many blocks do I need for a 20 × 8 ft wall?
A 20 × 8 ft wall is 160 sq ft — 12 courses of 15 blocks, so about 180 blocks, or 189 once you add 5% for cuts and breakage. Enter your own length and height above for any wall size.
How many bags of mortar do I need per 100 blocks?
It depends which product you buy. With masonry cement plus your own sand, budget about 3 bags of 70-lb masonry cement and roughly 10 cubic feet (about half a ton) of masonry sand per 100 standard blocks. With pre-blended mortar mix, where the sand is already in the bag, one 80-lb bag lays only about 12–13 blocks — so the same 100 blocks takes closer to 8 bags. Joint thickness and how fully you butter the ends move both figures.
How many courses of block is an 8-foot wall?
Twelve. A standard block is 8 inches high including its 3⁄8-inch mortar joint, so every course adds exactly 8 inches: 96 ÷ 8 = 12 courses. A 4-foot wall is 6 courses, and a 6-foot wall is 9.
Does a thicker block change how many I need?
No. A 4-, 6-, 8-, 10-, or 12-inch block all present the same 8 × 16-inch nominal face, so the count stays at about 1.125 blocks per square foot of wall. Only the weight, the price, and the load the wall can carry change.
How much does a concrete block weigh?
A standard 8 × 8 × 16-inch normal-weight hollow block runs about 36–38 lb; lightweight units are roughly 26–30 lb. A 12-inch block is closer to 50–55 lb. Multiply by your block count before you plan the delivery — 200 standard blocks is around 3.5 tons.