Cinder Block Calculator
Enter your wall length and height to get how many standard concrete (cinder) blocks and bags of mortar you'll need, including an allowance for waste.
Standard concrete blocks (often called cinder blocks or CMUs) measure a nominal 16 × 8 inches on the face, which includes the mortar joint. That works out to about 1.125 blocks per square foot of wall.
How to calculate how many cinder blocks you need
Find the wall area, then divide by the face area of one block. Add about 10% for cuts at corners and openings and the odd broken block.
Wall area = Length × Height
Blocks = Wall area ÷ 0.889 sq ft (≈ 1.125 per sq ft)
With waste = Blocks × 1.10
Blocks = Wall area ÷ 0.889 sq ft (≈ 1.125 per sq ft)
With waste = Blocks × 1.10
| Item | Rule of thumb |
|---|---|
| Blocks per sq ft | ≈ 1.125 (standard 16 × 8 in block) |
| Waste allowance | 10% for cuts, corners, breakage |
| Mortar | ≈ 1 bag (80 lb) per 30 blocks |
Worked example
A wall 30 ft long and 8 ft high:
| Wall area = 30 × 8 | 240 sq ft |
| Blocks = 240 ÷ 0.889 | ≈ 270 |
| With 10% waste | ≈ 297 blocks |
| Mortar = 270 ÷ 30 | ≈ 9 bags |
Feet or metres?
Use the Feet / Metres toggle to enter your wall in metres; the block count stays based on the standard 16 × 8 inch block.
Frequently asked questions
How many cinder blocks are in a square foot?
About 1.125. A standard 16 × 8 inch block covers 0.889 sq ft of wall including the mortar joint, so one square foot needs roughly 1.125 blocks.
How much mortar do I need for cinder blocks?
Roughly one 80 lb bag of mortar per 30 standard blocks, using a 3/8-inch joint.
Should I add extra for waste?
Yes — add about 10% for cuts at corners and openings and for the occasional broken block. This calculator already includes it.
Related calculators
Estimates for standard 16 × 8 in blocks and general walls. For structural, load-bearing, or reinforced walls, follow an engineer's specification.