How the block count works
Concrete masonry units (CMU) are sold by nominal size: the block plus one mortar joint. A standard 8 × 8 × 16 in block is actually 7-5/8 × 7-5/8 × 15-5/8 in, and the 3/8 in joint brings it to an even 8 in high and 16 in long in the wall. That makes counting simple: every course is 8 in tall, and every block covers 16 in of length.
- Courses = wall height in inches ÷ 8, rounded up.
- Blocks per course = wall length in inches ÷ 16, rounded up.
- Total = courses × blocks per course, plus a waste allowance.
blocks = ⌈height (in) ÷ 8⌉ × ⌈length (in) ÷ 16⌉ × (1 + waste %)
Metric blocks work the same way with 200 mm courses and 400 mm lengths. Each block face covers 0.889 sq ft, which is 1.125 blocks per square foot, or 12.5 blocks per m² in metric.
Worked example
A wall 40 ft long and 4 ft high: 48 in ÷ 8 = 6 courses, and 480 in ÷ 16 = 30 blocks per course. That is 180 blocks. With 5 percent extra you buy 189. Mortar at about 13 blocks per 80 lb bag comes to 14 bags.
In metric, a wall 10 m long and 1.2 m high takes 6 courses of 25 blocks: 150 blocks, or 158 with 5 percent extra, and about 21 bags of 20 kg premixed mortar.
Blocks per wall height
| Courses | Wall height (US block) | Wall height (metric block) |
|---|---|---|
| 3 | 2 ft | 600 mm |
| 6 | 4 ft | 1.2 m |
| 9 | 6 ft | 1.8 m |
| 12 | 8 ft | 2.4 m |
Plan wall heights and opening sizes in whole courses and half blocks where you can. It saves cutting, and cut blocks are where most waste comes from.
About the mortar estimate
Bagged premixed mortar (Type N or Type S, sand already included) is the simplest choice for small walls. Manufacturers commonly rate an 80 lb bag at about 13 standard blocks. Actual use depends on joint thickness, whether you mortar face shells only or the full bed, and how much drops. Treat the bag count as a starting point and buy a couple of extra bags. For larger walls, masons usually mix bagged masonry cement with sand on site, which needs a separate estimate.
Waste allowance
| Wall | Typical extra |
|---|---|
| Straight wall, few cuts | 3 to 5% |
| Corners, openings, pilasters | 5 to 10% |
The calculator counts a straight run. Corners use special or cut units, and door and window openings reduce the count; subtract the openings' width and height in courses, then add lintels or bond beam units separately.
Ordering tips
- Blocks usually come on pallets. Ask how many are on one, and whether you pay a deposit on the pallet.
- Order half blocks and corner blocks as their own line items instead of cutting full blocks.
- Have the pallets dropped as close to the wall as the truck can safely reach. Carrying blocks is the slowest part of the job.
- Keep blocks covered and dry before use; wet blocks shrink as they dry and can crack the wall.
Safety and code notes
- A block wall needs a proper concrete footing. Footing size, reinforcement and grout depend on wall height, load and soil, so check local code.
- Walls that retain soil, carry a roof, or stand over about 4 ft high usually need engineered design and a permit.
- Blocks are heavy. Lift with your legs, wear gloves and eye protection when cutting, and use a saw with dust control or water.
- Fresh mortar is caustic. Avoid skin contact and wash it off promptly.
Comparing block sizes before you buy? See the guide to concrete block sizes. For the footing itself, use the concrete calculator.
Frequently asked questions
How many concrete blocks are in a square foot?
A standard 8 × 16 in block face covers 0.889 sq ft, so a wall takes 1.125 blocks per square foot, or 112.5 blocks per 100 sq ft, before waste.
How many blocks do I need for a 24 ft wall that is 6 ft high?
9 courses of 18 blocks: 162 blocks before waste, or 171 with 5 percent extra.
How many blocks does a bag of mortar lay?
An 80 lb bag of premixed mortar is commonly rated at about 13 standard 8 × 8 × 16 in blocks. Check the bag, and buy a few extra.
How many metric blocks per square meter?
A 400 × 200 mm nominal block gives 12.5 blocks per square meter.