Concrete Block Calculator
Blocks = wall area × 1.125, + 5 % waste. A wall 25 ft long and 4 ft high is 100 ft²: 119 blocks of 8 × 8 × 16 in and 10 bags of 80 lb mortar. Take doors and windows off the wall area first; corner and half blocks then come out of the total, not on top of it.
Concrete blocks, 8 × 8 × 16 in
119 blocks
1.125 blocks per ft² of wall + 5 % waste
Estimate only. Check dimensions, loads and local code requirements with a qualified professional before you build.
- Net wall area
- 100 ft²
- Blocks before waste
- 112.5
- Mortar (QUIKRETE Mason Mix)
- 10 bags of 80 lb
Corner, half and bond beam blocks are part of this count: ask the yard to split it by shape.
Worked examples
A garden wall 25 ft long, 6 courses high (4 ft)
- 25 × 4 = 100 ft²
- 100 × 1.125 = 112.5 blocks; + 5 % = 118.1, so 119 blocks
- 119 ÷ 13 = 9.2, so 10 bags of 80 lb mortar
Result: 119 blocks and 10 bags of mortar, the CMHA figure for 100 ft².
Checked by the automated test “concrete block — defaults: 25 ft wall, 4 ft high, 5 % waste → 100 ft², 119 blocks, 10 bags of mortar”.
The same wall with grouted cores every 32 in
- CMHA Table 3, 8 in wall, cores every 32 in: 9.1 ft³ per 100 ft²
- 100 ÷ 100 × 9.1 = 9.1 ft³; 9.1 ÷ 27 = 0.34 yd³
Result: 9.1 ft³ (0.34 yd³) of grout.
Checked by the automated test “concrete block — grout in the cells every 32 in, 8 in wall, 100 ft² → 9.1 ft³ (0.34 yd³)”.
How it works
Net wall area = length × height − doors and windows
Blocks = net area × 1.125 × (1 + waste), rounded up (an 8 × 16 in face covers 8/9 ft²)
Mortar bags (80 lb) = blocks ÷ 13, rounded up
Grout = net area ÷ 100 × the CMHA figure for the wall width and core spacing
Assumptions
- Standard blocks of 8 × 8 × 16 in nominal: the real block is 3/8 in smaller each way, and the 3/8 in mortar joint makes up the difference, so the nominal face is what each block covers.
- 5 % waste and breakage, the allowance CMHA builds into its 119 blocks per 100 ft².
- Mortar: QUIKRETE Mason Mix lays up to 13 standard blocks per 80 lb bag; a slow or uneven job uses more.
- Grout figures (CMHA TEK 04-02A, Table 3) are for two-core hollow blocks in a single wythe and include 3 % waste.
Reference table
| Wall area | Blocks | Mortar bags |
|---|---|---|
| 10 ft² | 12 | 1 |
| 25 ft² | 30 | 3 |
| 50 ft² | 60 | 5 |
| 75 ft² | 89 | 7 |
| 100 ft² | 119 | 10 |
Before you order
- Measure in courses. Each course of 8 in blocks adds 8 in of height: 6 courses make 4 ft, 9 courses make 6 ft.
- Ask the yard to split the count. Corner, half and bond beam blocks come out of the same total; tell the supplier how many corners and wall ends you have.
- Footing first. The wall sits on a concrete footing: work out its volume with the concrete calculator.
Common mistakes
- Dividing by the real block size A block is 7 5/8 × 15 5/8 in, 3/8 in under nominal, and the mortar joint makes up the rest. Divided by the real face, 100 ft² gives 121 blocks before waste instead of 113.
- Leaving doors and windows in CMHA subtracts every opening from the wall area. A 3 × 4 ft window is 12 ft²: 13.5 blocks you do not need.
- Counting mortar like bricks An 80 lb bag of QUIKRETE Mason Mix lays up to 37 bricks but only 13 blocks. Counted as bricks, 119 blocks would get 4 bags instead of 10.
- Adding corner and half blocks on top CMHA counts corner, pilaster and bond beam units separately and subtracts them from the total: they replace standard blocks, they are not extra.
- Treating the count as a design CMHA warns that non-modular layouts and many corners raise the count, and that estimating is often best left to a professional. A retaining wall or foundation needs its footing and steel set by one.
Frequently asked questions
How many concrete blocks do I need?
Multiply the wall area in ft² by 1.125, then add 5 % for breakage. A wall 40 ft long and 6 ft high is 240 ft²: 270 blocks, 284 with the extra.
How many cinder blocks do I need?
Count them like concrete blocks: an 8 × 16 in face covers 8/9 ft², so 113 blocks per 100 ft² of wall, 119 with 5 % extra.
How much mortar do I need for block?
One 80 lb bag of QUIKRETE Mason Mix lays up to 13 standard blocks, so 119 blocks take 10 bags. Mixing your own, CMHA counts about 30 hollow blocks per 70 lb bag of masonry cement, plus sand.
How much concrete does it take to fill cinder blocks?
Cores are filled with grout. CMHA counts 36.1 ft³ (1.34 yd³) per 100 ft² of 8 in wall with every core filled, and 9.1 ft³ with cores every 32 in, for two-core blocks with 3 % waste.
Sources
Formula, examples and sources checked on by the Yardstick Math team. How we check our formulas.
- CMU-TEC-001-23, Concrete Masonry Unit Shapes, Sizes, Properties, and Specifications (specified dimensions 3/8 in less than nominal) — Concrete Masonry & Hardscapes Association (CMHA), accessed
- TEK 04-02A, Estimating Concrete Masonry Materials (119 units per 100 ft²; grout per 100 ft², Table 3) — Concrete Masonry & Hardscapes Association (CMHA, formerly NCMA), accessed
- Mason Mix #1136, specification data (up to 13 standard blocks per 80 lb bag) — QUIKRETE, accessed