To count cinder blocks, divide the wall area in square feet by 0.889 (the face of one 8 × 16 in block), which is the same as multiplying by 1.125. A 100 ft² wall needs 113 blocks, or 119 with 5% for waste and breakage, matching the CMHA (formerly NCMA) TEK 04-02A estimate of 119 per 100 ft². This CMU block calculator also subtracts openings and estimates mortar bags. For fully or partially grouted walls it adds the core-fill grout in cubic feet, cubic yards and bags.
It is for DIYers building a garden or retaining wall, homeowners pricing a block foundation or shop, and masons who want a quick takeoff. You need the wall's length and height, the total area of any doors and windows, and the block size. The figures are planning estimates. Corners, pilasters, bond beams and non-modular lengths add units, and you should count those separately as TEK 04-02A advises.
How to use the cinder block wall calculator
- Enter the wall length and height in feet. For several walls, add their lengths if they are the same height.
- Enter openings as the total square feet of doors and windows. A 3 × 7 ft door is 21 ft².
- Pick the block width (6, 8, 10 or 12 in nominal) and face size. Standard blocks have an 8 × 16 in face. Half-high (4 × 16) and half-length (8 × 8) units need twice as many per square foot.
- Set the waste percentage. TEK 04-02A uses about 5%.
- Choose the mortar you will use and the grout option: fully grouted, partially grouted at a set cell spacing, or hollow.
CMU block formula
Net wall area = length × height − openings. Blocks = net area × 144 ÷ (nominal face height × nominal face length) × (1 + waste). With an 8 × 16 in face that is net area × 1.125 × 1.05. Nominal dimensions include one 3/8 in mortar joint. According to CMHA's CMU-TEC-001, specified (actual) dimensions are typically 3/8 in less than nominal, so an 8 × 8 × 16 block measures 7-5/8 × 7-5/8 × 15-5/8 in. That is why you count with nominal sizes. Each block plus its joint fills exactly 8 × 16 in of wall.
| Nominal size (W × H × L) | Actual size | Blocks per ft² | Per 100 ft² (+5%) |
|---|---|---|---|
| 6 × 8 × 16 in | 5-5/8 × 7-5/8 × 15-5/8 in | 1.125 | 119 |
| 8 × 8 × 16 in | 7-5/8 × 7-5/8 × 15-5/8 in | 1.125 | 119 |
| 10 × 8 × 16 in | 9-5/8 × 7-5/8 × 15-5/8 in | 1.125 | 119 |
| 12 × 8 × 16 in | 11-5/8 × 7-5/8 × 15-5/8 in | 1.125 | 119 |
| 8 × 4 × 16 in (half-high) | 7-5/8 × 3-5/8 × 15-5/8 in | 2.25 | 237 |
| 8 × 8 × 8 in (half-length) | 7-5/8 × 7-5/8 × 7-5/8 in | 2.25 | 237 |
Wall width does not change the count per square foot. It changes the weight of each block and how much grout the cores hold.
Cinder blocks for common wall sizes
| Wall (L × H) | Area | Blocks (+5%) | Courses | 80 lb mortar bags |
|---|---|---|---|---|
| 10 × 3 ft | 30 ft² | 36 | 5 | 3 |
| 20 × 3 ft | 60 ft² | 71 | 5 | 5 |
| 20 × 4 ft | 80 ft² | 95 | 6 | 6 |
| 30 × 4 ft | 120 ft² | 142 | 6 | 9 |
| 40 × 4 ft | 160 ft² | 189 | 6 | 12 |
| 24 × 6 ft | 144 ft² | 171 | 9 | 11 |
| 50 × 6 ft | 300 ft² | 355 | 9 | 23 |
| 30 × 8 ft | 240 ft² | 284 | 12 | 18 |
| 40 × 8 ft | 320 ft² | 378 | 12 | 24 |
| 60 × 8 ft | 480 ft² | 567 | 12 | 36 |
| 100 × 8 ft | 800 ft² | 945 | 12 | 60 |
| 120 × 4 ft | 480 ft² | 567 | 6 | 36 |
Mortar: how many bags?
TEK 04-02A's Table 2 gives rule-of-thumb yields for face-shell bedding, where mortar goes only on the face shells, as in most hollow-block work. These include nominal waste:
- Preblended mortar, 80 lb bag: about 16 blocks. A 3,000 lb bulk bag lays about 420.
- Masonry cement, 70 lb bag, plus sand: about 30 blocks. Use 1 ton of masonry sand per 8 bags, which works out to roughly 240 blocks per ton of sand.
- Portland cement-lime, 94 lb bag: about 62 blocks, using about half a 50 lb bag of hydrated lime and 4¼ ft³ of sand per bag of cement.
Skill, weather and layout move these numbers a lot, which is why the TEK calls mortar one of the most often misestimated materials. Round up and keep receipts for returns.
Block fill calculator: grout volume for core fill
Table 3 of TEK 04-02A lists the grout needed to fill the vertical cells of hollow single-wythe walls. Cells sit 8 in apart in a standard block, so "every 8 in" means fully grouted. Larger spacings are partial grouting, where only the cells with rebar are filled. Values are cubic feet per 100 ft² of wall and assume two-core units and 3% waste:
| Grouted cell spacing | 6 in wall | 8 in wall | 10 in wall | 12 in wall | 8 in wall, yd³ |
|---|---|---|---|---|---|
| 8 in (solid) | 25.6 | 36.1 | 47 | 58.9 | 1.337 |
| 16 in | 12.8 | 18.1 | 23.5 | 29.5 | 0.67 |
| 24 in | 8.6 | 12.1 | 15.7 | 19.7 | 0.448 |
| 32 in | 6.4 | 9.1 | 11.8 | 14.8 | 0.337 |
| 40 in | 5.2 | 7.3 | 9.4 | 11.8 | 0.27 |
| 48 in | 4.3 | 6.1 | 7.9 | 9.9 | 0.226 |
| 56 in | 3.7 | 5.2 | 6.8 | 8.5 | 0.193 |
| 64 in | 3.2 | 4.6 | 5.9 | 7.4 | 0.17 |
| 72 in | 2.9 | 4.1 | 5.3 | 6.6 | 0.152 |
| 80 in | 2.6 | 3.7 | 4.7 | 5.9 | 0.137 |
| 88 in | 2.4 | 3.3 | 4.3 | 5.4 | 0.122 |
| 96 in | 2.2 | 3.1 | 4 | 5 | 0.115 |
| 104 in | 2 | 2.8 | 3.7 | 4.6 | 0.104 |
| 112 in | 1.9 | 2.6 | 3.4 | 4.3 | 0.096 |
| 120 in | 1.8 | 2.5 | 3.2 | 4 | 0.093 |
Grout per block when fully grouted is about 0.23 ft³ for 6 in, 0.32 ft³ for 8 in, 0.42 ft³ for 10 in and 0.52 ft³ for 12 in units. For bagged grout, TEK 04-02A Table 4 gives a second check: an 80 lb bag (about 0.66 ft³) fills about 3.6 cores of a 6 in unit, 2.7 of an 8 in, 2.2 of a 10 in and 1.8 of a 12 in. The calculator shows both bag counts. The core-yield method usually gives more bags, so treat it as the cautious figure for small bagged jobs.
Worked examples
Garden wall: 20 ft long, 3 ft high, hollow 8 in block
Area 60 ft² × 1.125 × 1.05 = 71 blocks, laid in 5 courses. Mortar: 71 ÷ 16 = 5 bags of 80 lb preblended mix.
Foundation: 120 ft around, 4 ft high, 8 in block grouted at 48 in
480 ft² × 1.125 × 1.05 = 567 blocks. Grout: 480 ÷ 100 × 6.1 = 29.3 ft³ = 1.08 yd³, or 45 bags of 80 lb grout by volume (67 by TEK core yield). Mortar with masonry cement: 19 bags of 70 lb and 2.38 tons of sand.
Shop wall: 30 × 10 ft with a 3 × 7 ft door, fully grouted
Net area 300 − 21 = 279 ft², so 330 blocks. Full grout at 36.1 ft³ per 100 ft² = 100.7 ft³ = 3.73 yd³. At that volume, a ready-mix grout delivery usually beats 153 bags.
Common mistakes
- Counting with actual instead of nominal size. Dividing by 7-5/8 × 15-5/8 in leaves out the mortar joint and overcounts blocks by about 5%.
- Forgetting openings, or subtracting them twice. Subtract each door and window once. Lintels and sills are separate pieces.
- Treating corners, ends and bond beams as stretchers. Count special units separately, as TEK 04-02A recommends.
- Filling the wrong cells. A partially grouted wall fills only the cells with rebar. Check the drawing before ordering grout for every cell.
- Using concrete bag yields for grout. Grout bags yield differently. Use the yield printed on the grout bag.
For the footing under the wall, use the concrete calculator (footing mode). To price a slab beside it, try the concrete slab cost calculator, and for gravel backfill behind a retaining wall, the cubic yard calculator.
Frequently asked questions
How many cinder blocks do I need per square foot?
A standard block has an 8 × 16 in nominal face, which is 128 in² or 8/9 ft². That works out to 1.125 blocks per square foot, or 119 per 100 ft² with the 5% waste allowance in CMHA TEK 04-02A.
What are the actual dimensions of an 8x8x16 block?
7-5/8 × 7-5/8 × 15-5/8 in. CMHA explains that specified (actual) dimensions are typically 3/8 in less than nominal, so a 3/8 in mortar joint brings each block back to the 8 in module. ASTM C90 allows ±1/8 in on width, height and length.
How many blocks for a 20 ft wall 3 ft high?
The wall is 60 ft². At 1.125 blocks per ft² plus 5% waste, buy 71 blocks. That is 5 courses of 15 blocks. With preblended mortar at about 16 blocks per 80 lb bag (TEK 04-02A), you need 5 bags.
How much mortar per block?
CMHA TEK 04-02A (Table 2, face-shell bedding) gives approximate yields: one 80 lb bag of preblended mortar lays about 16 conventional blocks. One 70 lb bag of masonry cement, mixed with sand, lays about 30, with 1 ton of sand for every 8 bags. One 94 lb bag of portland cement with lime and sand lays about 62.
How much grout to fill a cinder block wall?
For a fully grouted 8 in wall, TEK 04-02A Table 3 gives 36.1 ft³ of grout per 100 ft² of wall (two-core units, 3% waste), which is 1.34 yd³. A 6 in wall takes 25.6 ft³, a 10 in wall 47 ft³ and a 12 in wall 58.9 ft³ per 100 ft².
How much grout per block for core fill?
Divide the fully grouted figure by the 112.5 blocks in 100 ft². For an 8 in block that is about 0.32 ft³ per block. For a 12 in block it is 0.52 ft³. Both include TEK 04-02A's 3% waste.
What does partially grouted mean?
Only the cells with vertical rebar are filled, for example every 32 or 48 in. That cuts grout sharply: an 8 in wall grouted at 48 in on center takes 6.1 ft³ per 100 ft², against 36.1 ft³ when every cell is filled. The rebar spacing comes from your engineer or local code.
How many bags of grout per yard?
TEK 04-02A says an 80 lb bag of preblended grout yields about 0.66 ft³, so a cubic yard takes 41 bags. A 3,000 lb bulk bag yields about 25 ft³. On bigger walls, ready-mix grout by the yard is usually more practical.
Can I fill block cores with regular concrete?
Masonry grout is a high-slump mix made to flow into narrow cells around rebar. CMHA TEK 09-04A says grout for concrete masonry should comply with ASTM C476, although the governing building code may allow other options. For a structural wall, use a product labeled as masonry grout or have a ready-mix plant batch grout, and check with your building department.
Are cinder blocks and concrete blocks the same?
In everyday speech, yes. Most blocks sold today are concrete masonry units (CMU) made with cement and aggregate. Historic cinder blocks used coal cinders as aggregate. The counting method is the same for both: wall area × units per square foot.
Sources & method
- CMHA (NCMA) TEK 04-02A, Estimating Concrete Masonry Materials: units per 100 ft², mortar (Table 2), grout volumes (Tables 3–5)
- CMHA CMU-TEC-001-23, Concrete Masonry Unit Shapes, Sizes, Properties, and Specifications: specified = nominal − 3/8 in; ASTM C90 tolerances
- CMHA TEK 09-04A, Grout for Concrete Masonry: grout should comply with ASTM C476
- NIST Handbook 44 (2026), Appendix C: 1 ft² = 144 in²; 1 yd³ = 27 ft³
Results are estimates for general information. Found an error? It helps everyone — see our methodology.