Concrete masonry units — CMU, often still called cinder blocks — are made to a modular size so walls lay out on a predictable grid. That makes the count easy: each block covers the same face area, so the number of blocks depends only on the wall area. This calculator adds the extras that matter on site: mortar bags, grout for the cells that hold reinforcing bars, and the number of vertical bars.
How to use the concrete block calculator
- Choose US or Metric units.
- Enter the wall length and height, or a known area, and subtract any openings.
- Choose the block size. Width affects grout volume, not the block count.
- Set the extra blocks allowance.
- Choose how often cells are grouted — the spacing of vertical rebar in your plans.
- Check the blocks per bag rating of your mortar and the extra allowance.
Block formulas
Each standard block has two cells 8 inches apart, so grouting one cell every 48 inches fills one cell in six. In metric the nominal block is 400 × 200 mm, which works out to 12.5 blocks per square meter.
Worked example: a 40 × 4 ft garden wall
Area: 40 × 4 = 160 ft². Blocks: 160 × 1.125 = 180; with 5% extra → 189 blocks.
Layout: 6 courses of 30 blocks.
Mortar: 180 ÷ 13 × 1.1 = 15.2 → 16 bags of 80-lb premix.
Grout in cells every 48 in: 180 × 0.25 ft³ × ⅙ × 1.1 = 8.25 ft³ (about 14 bags of 80-lb mix), with 11 vertical bars.
Common block sizes
| Nominal size | Actual size (in) | Typical use | Grout to fill both cells |
|---|---|---|---|
| 4 × 8 × 16 | 3⅝ × 7⅝ × 15⅝ | Veneer, partitions | about 0.08 ft³ |
| 6 × 8 × 16 | 5⅝ × 7⅝ × 15⅝ | Interior and garden walls | about 0.17 ft³ |
| 8 × 8 × 16 | 7⅝ × 7⅝ × 15⅝ | Foundations, most walls | about 0.25 ft³ |
| 10 × 8 × 16 | 9⅝ × 7⅝ × 15⅝ | Taller or retaining walls | about 0.32 ft³ |
| 12 × 8 × 16 | 11⅝ × 7⅝ × 15⅝ | Retaining and basement walls | about 0.40 ft³ |
Grout volumes are typical for two-cell hollow units; check the manufacturer’s data for your blocks. Corners, wall ends and openings use special units — corner, bullnose, half and lintel blocks — so count those separately when your supplier asks.
Mortar, grout and reinforcement
Mortar is what you trowel onto the face shells to bed each block. Grout is a more fluid mix, made with fine aggregate, poured into the cells around reinforcing steel. They are different products; do not fill cells with mortar. Horizontal joint reinforcement (ladder or truss wire) is often laid in the mortar every second course, every 16 inches.
The amount of steel and grout a wall needs is set by its height, the loads on it and the local building code. Retaining walls, basement walls and walls taller than about 4 feet usually need an engineered design and a permit.
Planning tips
- Lay out the wall in 8-inch increments to minimize cut blocks. A 40-foot wall is 30 full blocks long; a 41-foot wall needs cuts.
- Start on a level footing. The concrete calculator sizes the footing, and the rebar calculator estimates the steel.
- For brick or stone veneer over the block, use the brick calculator.
Frequently asked questions
How many concrete blocks are in a square foot?
A standard 8 × 8 × 16 block plus its ⅜-inch mortar joints covers 16 × 8 inches, or 0.889 ft², so it takes 1.125 blocks per square foot of wall. The block width (4, 6, 8, 10 or 12 inches) does not change the count. Half-high 8 × 4 × 16 units take 2.25 per ft².
How many blocks do I need for a 100 square foot wall?
100 × 1.125 = 112.5, so 113 standard blocks before waste. With 5% extra for breakage and cuts, buy about 119. Corners and openings use more cut or special units, so plan a little more for walls with several.
How many bags of mortar per 100 blocks?
An 80-lb bag of premixed mortar is typically rated for about 12–14 standard blocks, so 100 blocks take roughly 8 bags, or 9 with a 10% allowance for waste. Large jobs usually switch to masonry cement and bulk sand mixed on site.
What does 8 × 8 × 16 mean for a concrete block?
It is the nominal size in inches: width × height × length. The actual block is ⅜ inch smaller in each direction (7⅝ × 7⅝ × 15⅝) so that with mortar joints the wall lays out on an 8-inch grid.
Do I have to fill concrete blocks with concrete?
Only the cells your design calls for. Reinforced walls have vertical bars in some cells, and those cells are filled with grout. Retaining walls and some foundation walls are grouted solid. Hollow, ungrouted cells are normal elsewhere.