How many concrete blocks do I need for a 20 × 8 ft wall?
For the worked example (20 × 8 ft wall, standard 8 × 8 × 16 in block, ⅜ in joints) the material list is Concrete blocks: 189 blocks (calculated 180 blocks, based on 15.63 in × 7.63 in face, 0.38 in joints, 5% extra). Quantities to buy are whole units rounded up from the calculated amounts. Enter your own measurements in the calculator above for your exact list.
How to use this calculator
- Enter the wall length and height, and the total area of any openings.
- Pick the standard 8 × 8 × 16 in block, or enter your block’s actual face length and height.
- Check the mortar joint and the allowance. Optionally enter how many blocks one bag of your mortar lays.
- Copy, print or share the material list.
How this is calculated
- Module = (block face length + joint) × (block face height + joint)
- Blocks = net wall area ÷ module
- Buy = round up (blocks × (1 + allowance ÷ 100))
- Net wall area = length × height − openings
- Mortar bags = round up (blocks to buy ÷ blocks per bag), only if you enter a yield
Geometry is deterministic. Material coverage, density, installation waste and product yield can vary. We expose those assumptions so you can replace them with the values printed on the products you are actually buying. Treat every result as an estimate. Items on your list are labelled Calculated (straight from your measurements) or Estimated (depends on an assumption), and “Buy” quantities are whole units rounded up.
Sources
- CMHA (formerly NCMA) – TEK 05-12, Modular Layout of Concrete Masonry. Actual concrete masonry unit dimensions are typically 3/8 in less than nominal so the 4 or 8 in module is kept with 3/8 in mortar joints. Used for the standard block face and the 3/8 in joint. Mortar yield is left to you.
Last reviewed: . See the methodology.
Assumptions
These starting values are used until you change them. They are typical, not universal. Check the label on the product you are buying.
| Assumption | Starting value | Where it comes from |
|---|---|---|
| Standard 8 × 8 × 16 block face (specified size) | 15⅝ × 7⅝ in | Manufacturer / industry sourced – 3/8 in under the nominal 16 × 8 in face, so the unit plus a 3/8 in joint fits a 16 × 8 in module (CMHA). |
| Inches in a foot | 12 | Mathematical – Depth in inches ÷ 12 gives feet |
| Block face length (actual) | You enter it | You enter it (product / supplier) |
| Block face height (actual) | You enter it | You enter it (product / supplier) |
| Mortar joint | 0.375 in | Manufacturer / industry sourced |
| Extra allowance | 5% | Common planning default |
| Blocks laid per bag of mortar (optional) | You enter it | You enter it (product / supplier) |
Worked example
A 20 × 8 ft wall in standard 8 × 8 × 16 in block with ⅜ in joints and 5% extra:
- Concrete blocks: 189 blocks (180 before the 5% allowance) · 15.63 in × 7.63 in face, 0.38 in joints, 5% extra
Common questions
Nominal or actual block size?
The calculator uses the actual face plus the joint. A nominal 8 × 16 in block is smaller than that, by the joint, so the two together make a 16 × 8 in module. Custom lets you enter any actual size.
Why is there no default mortar yield?
Bags per wall depend on the mortar, the block and how it is laid. Enter blocks per bag from your product; otherwise mortar is listed as a reminder.
Is this a wall design?
No. It counts blocks from the wall face. Reinforcement, footings and wall limits are decided by your plans and local requirements.