How to use the cinder block calculator
- Enter the wall length and height in feet. For several walls, add the lengths together if they share the same height.
- Subtract openings such as doors and windows, in square feet.
- Choose the block face. Standard concrete blocks have a nominal face of 16 × 8 inches regardless of thickness, so 4", 6", 8", 10" and 12" block all use the same count. Half-high block is 16 × 4.
- Set waste to cover broken blocks and cuts: 5% for simple walls, 10% with many corners or openings.
- Set blocks per bag of mortar from your product's label to estimate mortar bags.
The formula
A standard block is actually 15⅝ × 7⅝ inches on its face. With a 3/8 inch mortar joint it fills a nominal 16 × 8 inch space, which is 128 square inches or 0.889 sq ft.
Blocks = (wall area − openings) ÷ 0.889 × (1 + waste %)
That is the same as 1.125 blocks per square foot of wall. You can also count by courses:
Courses = wall height (in) ÷ 8 · Blocks per course = wall length (in) ÷ 16
For mortar, QUIKRETE's data sheets for its Mortar Mix and Mason Mix state that an 80 lb bag lays up to about 13 standard 8 × 8 × 16 inch blocks. That is the default here; change it for other products or bag sizes.
Worked example
A garden wall is 40 ft long and 4 ft high, with no openings and 5% waste.
- Wall area: 40 × 4 = 160 sq ft
- Blocks without waste: 160 ÷ 0.889 = 180 blocks (check: 6 courses × 30 blocks)
- With 5% waste: 180 × 1.05 = 189 blocks
- Mortar: 189 ÷ 13 = 14.5 → 15 bags of 80 lb mortar mix
At $2.25 per block, the blocks cost about $425 before mortar, rebar, grout fill and footing concrete.
Things the block count does not include
- Footing: block walls need a concrete footing, typically twice the wall thickness in width, below the local frost line. Building codes set the details for structural walls.
- Core fill and rebar: retaining and load-bearing walls usually have reinforced, grout-filled cores. Grout volume depends on how many cores are filled.
- Special blocks: corners use bullnose or corner blocks, and the top course may use solid cap blocks. Count these separately; the total stays about the same.
- Gravel behind retaining walls: use the gravel calculator for drainage stone and the French drain calculator for a perforated pipe at the base.
Tips
- Design to block dimensions. Making the wall length a multiple of 16 inches and the height a multiple of 8 inches avoids cutting blocks.
- Plan your lifting. A hollow 8-inch normal-weight block commonly weighs 35–40 lb; lightweight units are lighter. A pallet can exceed 3,000 lb, so check delivery access.
- "Cinder block" vs concrete block: blocks made with coal cinders are rare now. What stores sell as cinder block is a concrete masonry unit (CMU); the size and count are the same.
- Get a permit check. Many jurisdictions require engineering for retaining walls over about 4 ft or walls that carry a load.
Frequently asked questions
How many cinder blocks are in a square foot?
About 1.125 standard 8 × 16 inch blocks per square foot of wall, because each block covers 0.889 sq ft including its mortar joint.
How many blocks do I need for a 10 ft by 4 ft wall?
The wall is 40 sq ft, so 40 ÷ 0.889 = 45 blocks, or about 48 with 5% waste.
How many blocks does an 80 lb bag of mortar lay?
QUIKRETE lists up to about 13 standard 8 × 8 × 16 blocks per 80 lb bag of its Mortar Mix or Mason Mix. Thick joints, hot weather and waste reduce that number.
Do 6-inch and 12-inch blocks change the count?
No. The face of standard block is 16 × 8 inches nominal for all thicknesses, so the number of blocks per square foot is the same. Thicker block needs more mortar and costs more per unit.
How much does a cinder block weigh?
A standard hollow 8 × 8 × 16 inch concrete block usually weighs about 35–40 lb; lightweight aggregate block is roughly 25–30 lb. Check the supplier specification.