The Block Wall Calculator calculates how many concrete blocks (cinder blocks or CMUs) you need for a wall by dividing the wall’s net face area by the face area of one block, then adding waste.
A standard 8 × 16 in block covers 128 in² (about 0.889 ft²) including the mortar joint, so a wall needs 1.125 blocks per square foot before waste. The calculator also estimates mortar bags and has a metric mode for 440 × 215 mm blocks.
Block Wall Calculator
How many concrete, cinder or CMU blocks — and bags of mortar — you need for a block wall, with openings and waste.
Formula
Blocks = (L × H − openings) ÷ (block length × block height) × (1 + waste %)
Mortar bags = blocks × bags per 100 blocks ÷ 100 (rounded up)
Metric: Blocks = net area ÷ ((block length + joint) × (block height + joint)) × (1 + waste %)
Where L = wall length, H = wall height. US block sizes are nominal (they already include a ⅜ in mortar joint); metric sizes are actual, so the joint is added.
How the Calculation Works
Step 1: Find the wall face area and subtract doors, windows or other openings.
Step 2: Divide by the face area of one block including its joint.
Step 3: Add the waste allowance and round up to whole blocks. Mortar is estimated from the block count.
Variables Explained
| Input | Meaning | Unit |
|---|---|---|
| Wall length, height | Face size of the wall | ft, m |
| Block length, height | Nominal size (US) or actual size (metric) | in, mm |
| Openings | Area of doors and windows | ft², m² |
| Waste | Extra for breakage and cuts | % |
| Mortar bags per 100 blocks | From your mortar bag label | bags |
Example Calculation
US 8 × 16 blocks
Given: wall 20 ft × 6 ft, no openings, 5% waste, 3 mortar bags per 100 blocks
Net area: 120 ft²; block face: 16 × 8 in = 0.889 ft²
Blocks: 120 ÷ 0.889 = 135 × 1.05 = 141.75
Result: 142 blocks and 5 bags of mortar
Metric 440 × 215 mm blocks
Given: wall 6 m × 1.8 m, 10 mm joints, 5% waste
Block + joint: 0.45 × 0.225 = 0.10125 m² (≈ 9.88 blocks per m²)
Result: 10.8 ÷ 0.10125 × 1.05 ≈ 112 blocks
Common Block Sizes
| Block (nominal) | Face area incl. joint | Blocks per 100 ft² |
|---|---|---|
| 8 × 16 in | 128 in² (0.889 ft²) | 112.5 |
| 8 × 8 in (half block) | 64 in² (0.444 ft²) | 225 |
| 4 × 16 in | 64 in² (0.444 ft²) | 225 |
Counts are before waste. The block’s width (4, 6, 8, 10 or 12 in) changes the weight and fill volume, not the number of blocks.
Material Estimation Notes
Mortar
Mortar use depends on joint thickness, block type and workmanship. The default of 3 bags per 100 blocks is a starting point — check the coverage printed on your mortar bags and adjust the input.
Cores, caps and reinforcement
Grout or concrete fill, cap blocks and rebar are separate items. Use the Block Wall Core Fill Calculator for fill volume.
Waste Factor Considerations
The right waste allowance depends on the number of cuts, corners and openings and how blocks are delivered. Straight, long walls need less; walls with many openings need more.
Frequently Asked Questions
How many blocks do I need per square foot?
For standard 8 × 16 blocks, 1.125 blocks per ft² before waste.
How do you calculate concrete blocks for a wall?
Divide the net wall area by one block’s face area (including the joint) and add waste.
How many blocks are in a 20 by 6 foot wall?
About 135 blocks, or 142 with 5% waste.
How many bags of mortar do I need for 100 blocks?
It depends on the mix and joint size; the bag label gives a coverage figure. Enter it in the calculator.
Is a cinder block the same as a concrete block?
People use the terms interchangeably. Modern blocks are usually concrete masonry units (CMUs).