Cinder block calculator: the math
Every standard block — 4, 6, 8, 10 or 12 in wide — has the same face: 8 in high and 16 in long once you include the 3/8 in mortar joint. That face is 0.889 sq ft, so a wall needs 1.125 blocks per square foot, whatever the block's width.
| Wall | Area | Blocks (+5%) | Courses | 80 lb mortar bags |
|---|---|---|---|---|
| 10 × 4 ft | 40 sq ft | 48 | 6 | 4 |
| 20 × 4 ft | 80 sq ft | 95 | 6 | 8 |
| 40 × 4 ft | 160 sq ft | 189 | 6 | 16 |
| 10 × 10 ft | 100 sq ft | 119 | 15 | 10 |
| 20 × 8 ft | 160 sq ft | 189 | 12 | 16 |
| 40 × 8 ft | 320 sq ft | 378 | 12 | 32 |
Stores and contractors use "cinder block" and "concrete block" interchangeably. Real cinder blocks are rare today; the gray 8 × 8 × 16 in block at the home center is a concrete masonry unit (CMU).
Cinder block wall calculator: courses, caps and openings
Courses are 8 in each, so a 4 ft wall is six courses and an 8 ft wall twelve. If your height isn't a multiple of 8 in, round up — cutting a whole course lengthwise is far more work than one more course.
Cap blocks finish the top of a garden or screen wall. Solid caps come 16 in long, so you need one per block along the top course; the calculator adds them when Add cap blocks is on. Cap blocks are set in mortar or masonry adhesive.
Openings come out of the area at about 21 sq ft per door and 15 sq ft per window, but you'll need lintels over them and half blocks at the jambs — order a few extra halves.
Corners use the same blocks turned 90 degrees every other course, so they don't change the count.
Mortar, footings and rebar
QUIKRETE rates an 80 lb bag of premixed Mortar Mix at up to 12 standard blocks — call it 9 bags per 100 blocks once you allow for droppings. Use Type N for above-grade garden walls and Type S below grade or anywhere the wall holds back soil; the mortar calculator explains the difference.
Our firm advice: a block wall that retains soil or is part of a building is a structural wall. It needs a footing below the frost line, vertical rebar in grouted cores and often horizontal joint reinforcement, all sized by your local code or an engineer. The block count above doesn't include grout to fill cores — ask your supplier for their grout yield if your plans call for it. For a short landscape wall, segmental retaining-wall blocks that stack without mortar are often easier.
