Concrete Block Calculator
Your estimate
- Blocks per 100 ft² / per m²
- 112.5
- Net wall area
- 240 ft²26.67 yd² · 22.3 m²
Also called a cinder block or CMU calculator — enter the wall size, subtract doors and windows, get blocks and mortar.
Updated 2026-10-20 · Based on: standard block, brick and paver sizes with their mortar joints.
How to use the Concrete Block Calculator
- Enter the wall length and height.
- Pick the block size (8 × 8 × 16 in is the standard CMU).
- Type the area of any doors and windows to subtract.
- Read the blocks (with 5 % extra) and the mortar bags.
Formula
A standard block with its ⅜ in joint covers 16 × 8 in = 0.889 ft², so you need 1.125 blocks per square foot (112.5 per 100 ft²).
Blocks = (wall area − openings) ÷ face area of one block × (1 + waste). One 80 lb bag of mortar mix lays about 30 blocks.
Worked example
With the values preset in the calculator above:
- Length
- 40 ft
- Height
- 6 ft
- Block size
- 8 × 8 × 16 in (standard)
- Doors & windows (area)
- 0 ft²
- Extra for waste
- 5 %
- Mortar bag
- 80 lb bags
- Blocks (with waste)
- 284
- Blocks per 100 ft² / per m²
- 112.5
- Net wall area
- 240 ft² (26.67 yd²)
- Mortar bags
- 10
How many cinder blocks do I need?
A wall 40 ft long and 6 ft high is 240 ft², which takes 284 standard blocks with 5 % extra and about 10 bags of 80 lb mortar.
Frequently asked questions
How many cinder blocks in a square foot?
1.125 standard 8×8×16 blocks per square foot, or 112.5 per 100 ft², because each block plus joint covers 0.889 ft².
How many blocks per bag of mortar?
Roughly 30 standard blocks per 80 lb bag of mortar mix, a little less for beginners or wide joints.
What size is a standard concrete block?
Nominal 8 × 8 × 16 in; actual 7⅝ × 7⅝ × 15⅝ in, so a ⅜ in mortar joint brings it to the nominal size.
More calculators
Estimates only — check quantities with your supplier and local building code before you order.