About this calculator
Concrete masonry units (CMU), often called cinder blocks, are sized so that one block plus a ⅜ in mortar joint takes up exactly 16 × 8 in of wall. That makes counting them straightforward: 1.125 blocks per square foot of wall. This calculator does the count, adds a breakage allowance and estimates the mortar you’ll need.
The drawing shows the wall to scale with every block in running bond, so you can see the courses and the half blocks at the ends. Below it you get the number of courses, the half blocks, whether your length fits the 8 in block module, and the cost at your local prices.
How to use it
- Enter the wall length and height.
- Subtract the area of openings (doors, windows) in square feet.
- Choose the block face size. The block’s width (4, 6, 8 or 12 in thick) doesn’t change how many you need, only their weight and price.
- Keep 5% extra for broken blocks and cuts, and add your prices to see the cost. Check the length check and the drawing: a length on the 8 in module needs no cut blocks.
The formula
Blocks per sq ft = 144 ÷ (16 × 8) = 1.125
Blocks = (wall area − openings) × blocks per sq ft × (1 + extra%)
Mortar bags ≈ blocks ÷ 13 (80 lb bags, 8×8×16 block)
Courses = wall height ÷ 8 in, rounded up
Block dimensions are nominal: an “8×8×16” block is actually 7⅝ × 7⅝ × 15⅝ in, and the ⅜ in joint brings it to 8 × 16 in on the wall. The mortar estimate uses the manufacturer’s figure of about 13 standard blocks per 80 lb bag of mortar mix.
Concrete masonry is laid out on an 8 in module, both across and up. In running bond each course is offset half a block from the one below, so every other course starts and ends with a half block. A wall whose length is a multiple of 8 in needs only whole and half blocks; any other length leaves a piece to cut at one or both ends of every course. Unlike brick, block courses can’t be squeezed to hit a height, so a height that isn’t a multiple of 8 in builds up to the next whole course or needs the top course cut.
Worked example
These are the calculator’s starting values and the results they produce:
| Wall length | 20 ft |
|---|---|
| Wall height | 4 ft |
| Area of openings to subtract | 0 sq ft |
| Block face size | Standard 16 in long × 8 in high |
| Extra for breakage and cuts | 5 % |
| Concrete block, each | $2.50 |
| Mortar mix, per 80 lb bag | $10.00 |
Tips before you buy
- Lay out walls in multiples of 8 in (length and height) to minimize cutting. The length check shows the nearest lengths that need no cuts.
- Half blocks are sold next to the full ones. Buying them saves cutting, and you can order about half as many fewer full blocks.
- Corners use the same number of blocks as straight runs, but you may want corner or bond-beam blocks for specific courses.
- Retaining walls and foundations usually need rebar and grout-filled cores — ask your supplier or engineer for quantities.
- An 8 in hollow block weighs roughly 35–40 lb; plan deliveries and staging accordingly.
Frequently asked questions
How many concrete blocks are in a square foot?
1.125 standard 8×8×16 blocks per square foot of wall (one block per 128 sq in including the mortar joint). For 100 sq ft of wall you need 113 blocks before waste.
How many bags of mortar per concrete block?
About one 80 lb bag of mortar mix for every 13 standard blocks, according to Quikrete. That’s roughly 8 bags per 100 blocks.
How many blocks do I need for a 20 ft × 4 ft wall?
The wall is 80 sq ft, so 90 blocks, or 95 with 5% extra, plus about 8 bags of mortar mix. At $2.50 a block and $10 a bag that’s about $317.50.
How many courses of block are in a 4 ft wall?
6 courses of standard 8 in high block: 48 ÷ 8 = 6. On a 20 ft wall each course is 15 blocks (240 ÷ 16), and the 3 courses that start half a block in need a half block at each end, 6 half blocks in all.
What if my wall isn’t a multiple of 8 in long?
Then every course ends in a piece you have to cut. A 20 ft 3 in wall with 6 courses has 6 cut pieces; at 20 ft or 20 ft 8 in it needs only whole and half blocks. If you can, move the wall’s end to the 8 in module.
Sources
Areas and volumes are standard geometry. The published figures this calculator relies on come from these sources; other defaults (waste allowances, typical densities and prices) are common trade figures worth checking with your supplier.
- Concrete Masonry & Hardscapes Association TEK 05-12: Modular Layout of Concrete Masonry: nominal 8 × 16 in block faces, actual sizes ⅜ in smaller for ⅜ in joints, the 8 in module and half blocks in running bond
- Quikrete Mortar Mix data sheet 1102: an 80 lb bag of mortar mix lays about 13 standard 8 × 8 × 16 in blocks