How to use the concrete block calculator
Enter the wall's length and height and the calculator counts blocks, rows and the mortar to lay them. Subtract any doors or windows, and use Number of walls for repeated walls, like the four sides of a shed foundation (but see the corner note below). The block width doesn't matter for the count: 4, 6, 8, 10 and 12 inch blocks all share the same 16 × 8 inch face.
The formula
blocks per ft² = 144 ÷ (16 × 8) = 1.125
blocks = (length × height − openings) × 1.125 × (1 + extra %)
courses = height (in) ÷ 8, rounded up
mortar bags (80 lb) = blocks ÷ 13, rounded up
The "8 × 8 × 16" name is the nominal size. The block itself is 7⅝ × 7⅝ × 15⅝ inches, and the ⅜ inch mortar joint brings each one up to a 16 × 8 inch footprint in the wall. That's why the maths works on 16 × 8.
Worked example: a garden wall 40 ft long and 4 ft high
- Wall area: 40 × 4 = 160 ft².
- Blocks: 160 × 1.125 = 180.
- Add 5% extra: 180 × 1.05 = 189 blocks.
- Layout: 4 ft = 48 in ÷ 8 = 6 courses of 40 ft ÷ 16 in = 30 blocks.
- Mortar: 189 ÷ 13 = 14.5, so 15 bags of 80 lb mortar mix.
Corners and multiple walls
When walls meet at a corner, the corner block is shared. Measuring each wall's full outside length counts the corners twice, which slightly overestimates (fine, it covers waste). For a tighter count, measure one wall's outside length and the other wall's inside length.
Other things you'll need
- Footing. Block walls sit on a poured concrete footing, typically twice the block width and at least as deep as the block is wide, and below the frost line. Use the footing mode in the concrete calculator, and check your local code.
- Reinforcement and grout. Taller walls, retaining walls and anything structural need rebar in the cores filled with grout, sized by an engineer or the code. That's outside this count.
- Cap blocks for the top course, if the wall is decorative.
- Half blocks or a saw for ends. Running bond means every other course starts with a half block.
Common mistakes
- Using the actual block size in the maths. 15⅝ × 7⅝ gives 1.21 blocks per ft² and overbuys by about 7%. Use the nominal 16 × 8.
- Forgetting the courses below grade. If the first course sits below the soil, include it in the height.
- Mixing too much mortar at once. Mix what you can lay in about an hour and a half; retempering old mortar weakens it.
Questions people ask
How many concrete blocks do I need per square foot?
1.125 standard blocks. An 8 × 8 × 16 block is really 7⅝ × 7⅝ × 15⅝ inches; with a ⅜ inch mortar joint it covers 16 × 8 inches, which is 0.889 square feet.
How many blocks are in a 10 ft wall?
A wall 10 ft long and 4 ft high is 40 square feet: 45 blocks, or 48 with 5% extra. It's 6 courses of 7½ blocks, so you'll cut some or use half blocks at the ends.
How much mortar do I need for concrete blocks?
QUIKRETE says one 80 lb bag of its Mortar Mix lays up to 13 standard 8 × 8 × 16 blocks. For 189 blocks that's 15 bags. Real-world coverage depends on joint size and how much drops, so buy a bag or two extra.
Do 6 inch and 12 inch blocks change the count?
No. 4, 6, 8, 10 and 12 inch wide blocks all have the same 16 × 8 inch face, so the count is the same. Width changes the weight, the mortar used and the wall's strength.
How much does a concrete block weigh?
A standard hollow 8 × 8 × 16 block typically weighs around 35 to 40 lb, varying with the aggregate and the maker. Lightweight blocks are less.
Sources and assumptions
- QUIKRETE Mortar Mix (No. 1102) data sheet: an 80 lb bag lays up to 13 standard 8 × 8 × 16 blocks
- NCMA: nominal CMU face 16 × 8 in including a ⅜ in mortar joint (actual 15⅝ × 7⅝ in)
- Structural block walls (foundations, retaining walls) need engineering and reinforcement per local code. This counts materials only.
Last reviewed 27 October 2026. Every formula on this page is checked against the worked example above in our automated tests. Found a problem? Tell us.