Concrete block calculator (cinder block / CMU)
Enter the wall’s length and height and the block size to see how many blocks to buy, how many courses that makes and how much mortar to lay them.
Enter your measurements
More options: Openings · Extra
- Courses (rows)
- 6 courses
- Blocks per course (to the next half)
- 15
- Blocks per square foot
- 1.125 per sq ft
- Wall area (net of openings)
- 80 sq ft
- Mortar mix, 80 lb bags (incl. 15% extra)
- 8 bags
- Actual block size
- 7-5/8 × 7-5/8 × 15-5/8 in
6 courses of 8 in (block + ⅜ in joint) · 20 ft ÷ 16 in = 15 blocks per course
80 sq ft ÷ 0.889 sq ft per block face + 5% = 95 blocks
Mortar: 90 blocks ÷ 13 per 80 lb bag + 15% (BIA suggests 15–25%) = 8 bags
- Count corner, end and bond-beam units separately if your wall needs them; ask the supplier which shapes they stock.
Quick answers for common sizes
| Example | Answer |
|---|---|
| 8×8×16 block actual size | 7-5/8 × 7-5/8 × 15-5/8 in (194 × 194 × 397 mm) |
| How many blocks per square foot | 1.125 blocks per sq ft (119 per 100 sq ft with 5% extra) |
| How many cinder blocks for a 10×10 shed | 89 blocks for a 3-course wall (28 per course) |
Your wall at other heights
| Wall height | Blocks to buy | Courses (rows) | Mortar mix, 80 lb bags (incl. 15% extra) |
|---|---|---|---|
| 2 ft | 48 blocks | 3 courses | 4 bags |
| 3 ft | 79 blocks | 5 courses | 7 bags |
| 4 ft | 95 blocks | 6 courses | 8 bags |
| 6 ft | 142 blocks | 9 courses | 12 bags |
| 8 ft | 189 blocks | 12 courses | 16 bags |
A standard concrete block (CMU, often called a cinder block) is sold as 8 × 8 × 16 but actually measures 7⅝ × 7⅝ × 15⅝ in. The missing ⅜ in is the mortar joint, so each block plus its joints takes up exactly 8 × 16 in of wall — 8/9 of a square foot. That is why the count works out to 1.125 blocks per square foot.
How the block count works
courses = wall height ÷ 8 in (rounded up) blocks = wall area (sq ft) × 1.125 × (1 + waste) blocks per course = wall length ÷ 16 in
| Nominal size | Actual size (in) | Actual (mm) | Blocks per sq ft |
|---|---|---|---|
| 4×8×16 | 3⅝ × 7⅝ × 15⅝ | 92 × 194 × 397 | 1.125 |
| 6×8×16 | 5⅝ × 7⅝ × 15⅝ | 143 × 194 × 397 | 1.125 |
| 8×8×16 | 7⅝ × 7⅝ × 15⅝ | 194 × 194 × 397 | 1.125 |
| 10×8×16 | 9⅝ × 7⅝ × 15⅝ | 244 × 194 × 397 | 1.125 |
| 12×8×16 | 11⅝ × 7⅝ × 15⅝ | 295 × 194 × 397 | 1.125 |
| 8×4×16 half-high | 7⅝ × 3⅝ × 15⅝ | 194 × 92 × 397 | 2.25 |
CMHA’s estimating guide (TEK 04-02A) adds 5% for waste and breakage, which turns 112.5 blocks per 100 sq ft into 119. The full size chart is on 8×8×16 block actual size.
Worked examples
- 8×8×16 block actual size — the real dimensions, and an 8 × 10 ft wall.
- How many blocks per square foot — 1.125, and 119 per 100 sq ft with waste.
- How many cinder blocks for a 10×10 shed — a three-course foundation wall, measured on the centre line.
Mortar and grout
Quikrete’s Mortar Mix data sheet says an 80 lb bag lays up to 13 standard blocks; CMHA’s estimating guide gives about 16 per 80 lb bag of preblended mortar with face-shell bedding. Use the mortar calculator for the bag count. If the wall is grouted or reinforced, the block fill calculator works out the grout for the cores.
Tips
- Lay out the first course dry to see where cuts fall, and adjust the wall length to whole or half blocks where you can.
- Build the corners up a few courses first, checking level and plumb, then string a line and fill in between.
- Blocks are heavy and their weight varies with the mix and size — check the supplier’s figure, lift with your legs, and have the pallets dropped close to the wall.
- Cutting block with a saw makes silica dust. OSHA’s rule for saws is a blade fed continuously with water, or a saw with dust collection.
What you’ll need
- Mason’s line and line blocksBudget price
Bright braided line stretched between line blocks keeps each course of block or brick straight and level, and marks out slab edges.
See options on Amazon (opens in a new tab) - Brick trowelBudget price
Spreads and furrows the mortar bed and butters the ends of each block or brick. A 10–11 in trowel suits most hands.
See options on Amazon (opens in a new tab) - Mortar jointerBudget price
Tools mortar joints to a concave finish once they are thumbprint-hard, which compacts them and sheds water.
See options on Amazon (opens in a new tab) - Brick set chisel and hammerBudget price
Scores and splits brick and block for half-units and cuts. For many cuts, a masonry saw used wet is faster and keeps dust down.
See options on Amazon (opens in a new tab) - 4 ft levelMid-range price
Checks forms for level and slope, and plumbs block, brick and posts. A 4 ft level bridges small bumps that fool a short one.
See options on Amazon (opens in a new tab)
As an Amazon Associate we earn from qualifying purchases. Links to products are affiliate links — they cost you nothing extra and help keep these calculators free.
Frequently asked questions
How many cinder blocks do I need for a wall?
Multiply the wall’s area in square feet by 1.125 and add 5% for breakage. A wall 20 ft long and 4 ft high is 80 sq ft: 90 blocks, or 95 with the extra.
How many blocks are in a course?
Wall length in inches ÷ 16. A 20 ft wall is 240 in ÷ 16 = 15 blocks per course.
Is a cinder block the same as a concrete block?
In everyday use, yes — “cinder block” is the common name for a hollow concrete masonry unit (CMU). Modern blocks are made with concrete and aggregate rather than cinders.
How tall is a block wall with 6 courses?
Six courses of 8 in block (7⅝ in plus a ⅜ in joint) build 48 in, or 4 ft.
Related guides
Sources
- CMHA (formerly NCMA) CMU-TEC-001-23 — Concrete masonry unit shapes, sizes, properties and specifications (opens in a new tab) — Specified dimensions are 3/8 in less than nominal, so an 8×8×16 block is 7-5/8 × 7-5/8 × 15-5/8 in; nominal thicknesses 4–16 in; ±1/8 in tolerance.
- CMHA (formerly NCMA) TEK 04-02A — Estimating concrete masonry materials (opens in a new tab) — Each 8×16 unit covers 8/9 sq ft, so 112.5 units per 100 sq ft (119 with 5% waste); grout per 100 sq ft by unit width and core spacing (Table 3); cores filled per 80 lb grout bag (Table 4); one 80 lb bag of preblended mortar lays about 16 CMU with face-shell bedding (Table 2).
- Quikrete Mortar Mix (No. 1102) data sheet (opens in a new tab) — Each 80 lb bag lays up to 37 standard bricks or 13 standard 8×8×16 blocks.