Concrete Block Calculator
A standard 16 × 8 in concrete block with a ⅜ in mortar joint covers 8/9 ft² of wall, so you need 1.125 blocks per square foot of net wall area. A 40 × 8 ft wall with one door takes about 336 blocks, or 354 with 5% waste. For mortar and grout too, use the Block Wall Calculator.
How it's calculated: Blocks = (wall length × height − openings) × 1.125, then your waste allowance is added.
Result appears here
Fill in the fields and press Calculate.
Formula, calculation, assumptions and sources
Net wall area = (Length × Height) − openings
Blocks = Net wall area (ft²) × 1.125 (standard 16×8 in CMU + 3/8-in joint)
Run the calculation to see each step with your own numbers.
A 40 ft × 8 ft wall with one 3 ft × 7 ft door, default 5% waste.
Gross area = 320 ft²; net = 320 − 21 = 299 ft²
Blocks = 299 × 1.125 = 336.4 exact
With 5%: order 354 blocks
- Standard 16 in × 8 in nominal CMU face with a 3/8-in mortar joint (ASTM C90).
- Running bond. Stack bond and specialty units give slightly different counts.
- Corner, bond-beam and half blocks are counted as ordinary blocks — order the specials separately.
- Does not include mortar, rebar, grout for filled cells, or a bond beam. Use the Block Wall and Mortar calculators for those.
- Is 1.125 blocks per square foot exact?
- It's exact for a 16×8 in nominal face (a 15-5/8 in unit + 3/8 in joint). A standard 8×8×16 CMU covers 8/9 ft² of wall face, so you need 9/8 = 1.125 per ft².
- What about mortar and rebar?
- Not included here — this is the block count. The Mortar Calculator covers mortar; the Rebar Calculator covers vertical/horizontal steel.
- How do I count blocks per course?
- Each standard block is 8 in tall and covers 16 in of wall length. A 40 × 8 ft wall has 12 courses of 30 blocks, or 360 blocks in total. That agrees with the area method: 320 ft² × 1.125 = 360 blocks. Deduct openings and add waste the same way.
- How many blocks for a 40 ft × 8 ft wall?
- About 336 blocks for the 299 ft² left after deducting one 3 × 7 ft door, or 354 with the default 5% waste. Corner, half and bond-beam blocks are counted as ordinary blocks, so order those specials separately.
Blocks per area
Geometry: a standard 16×8 in nominal CMU face (ASTM C90; a 15-5/8 × 7-5/8 in unit plus a 3/8-in mortar joint) covers 128 in², so 144 ÷ 128 = 1.125 blocks per ft² of wall.
Block dimensions
ASTM C90 Standard Specification for Loadbearing Concrete Masonry Units.
About the Concrete Block Calculator
The Concrete Block Calculator estimates how many concrete masonry units (CMUs) a wall needs, from the wall area and the block face size, with an editable allowance for cuts and breakage. It counts blocks only — mortar and any core grout are separate. For a full block wall including mortar and fill, see the Block Wall Calculator; for grout to fill the cores, see the Concrete Block Fill Calculator.
What you enter
- Wall length
- Wall height
- Openings to deduct — Total area of doors and windows. Leave 0 if none.
- Waste allowance — Default 5%. Adjust to your own allowance — the exact figure is always shown too.
What you get back
- Net wall area — the exact result of the calculation.
- Blocks needed — shown as the exact figure and, separately, a recommended figure with your waste allowance added.
The formula, a worked example, the assumptions behind it and the sources are shown in full below the calculator.
Related calculators
- Block Wall Calculator — Blocks, mortar and fill for a concrete block wall.
- Mortar Calculator — Mortar quantity for a brick or block wall.
- Concrete Block Fill Calculator — Grout volume needed to fill the cores of a concrete block wall.
- Brick Calculator — Number of bricks for a wall of a given area and brick size.
Part of the Concrete calculators.