This concrete block calculator shows how many concrete or cinder blocks (CMU) to buy for your wall, how many courses you’ll lay and how many 80 lb bags of mortar to pick up.
Quick answer: a standard 8 × 16 in block covers 8/9 ft² of wall, so you need 1.125 blocks per square foot. That is 113 blocks per 100 ft², or 119 with the 5% waste allowance the Concrete Masonry & Hardscapes Association uses, plus one 80 lb bag of mortar mix for every 13 blocks.
How to use the concrete block calculator
- Enter the wall length and height. For a closed foundation, measure the perimeter along the center of the wall so corners aren’t counted twice.
- Add up the area of doors, windows and other openings if you want them left out.
- Pick the block face. Most walls use standard 8 × 16 in blocks; half blocks and half-high blocks cover half as much face each.
- Keep a few percent extra for broken and cut blocks, and add prices if you want a materials total.
Block and mortar formulas
Wall area (ft²) = length × height − openings Blocks per ft² = 144 ÷ (nominal face height × nominal face length, in inches) Blocks = wall area × blocks per ft² × (1 + extra %), rounded up Mortar bags (80 lb) = blocks ÷ 13, rounded up
A standard block’s nominal face is 8 × 16 in, or 128 in², so one square foot (144 in²) takes 144 ÷ 128 = 1.125 blocks.
Nominal sizes include the mortar joint: the Concrete Masonry & Hardscapes Association notes that specified block dimensions are
typically 3/8 in smaller than nominal, so that a 4 or 8 in module is kept with 3/8 in mortar joints. An “8 × 16” block therefore
measures about 7⅝ × 15⅝ in on its face.
The mortar figure comes from QUIKRETE’s Mortar Mix data sheet: one 80 lb bag lays up to 13 standard 8 × 8 × 16 in blocks.
Worked example: a 20 ft wall, 8 ft high
- Wall area: 20 × 8 = 160 ft².
- Blocks: 160 × 1.125 = 180 blocks. Check: 12 courses × 15 blocks per course = 180.
- Add 5% extra: 180 × 1.05 = 189, so buy 189 blocks.
- Mortar: 189 ÷ 13 = 14.5, so buy 15 bags of 80 lb mortar mix.
A 24 ft stem wall 4 ft high (6 courses) works out to 108 blocks, or 114 with 5% extra, and 9 bags of mortar.
Blocks per 100 square feet
| Nominal face | Actual face (approx.) | Blocks per ft² | Blocks per 100 ft² |
|---|---|---|---|
| 8 × 16 in | 7⅝ × 15⅝ in | 1.125 | 112.5 |
| 8 × 8 in | 7⅝ × 7⅝ in | 2.25 | 225 |
| 4 × 16 in | 3⅝ × 15⅝ in | 2.25 | 225 |
The block’s thickness (6, 8, 10 or 12 in nominal) doesn’t change the count for a given wall face; it changes the weight you’re lifting and the mortar in each bed joint.
Tips for block walls
- Plan to the 8 in module. CMHA describes nominal sizes as the module for planning layouts around doors and windows. Wall lengths and opening positions in 8 in steps avoid cut blocks.
- Count special units separately. Corner, bond beam, lintel and cap blocks replace some standard blocks; order them by count from your plans.
- Core fill and rebar are a design question. Your structural design or local code sets which cores get grout and steel. The calculator doesn’t size grout, but the chart below gives typical volumes.
- Pour the footing first. Use the concrete calculator for the footing under the wall.
Grout fill for block cores
Typical grout (core fill) for standard two-cell blocks, 8 in high and 16 in long, from Westbrook Concrete Block’s grout yield chart. These figures have no allowance for waste, so add some.
| Wall thickness | Cores filled | yd³ per 100 blocks | yd³ per 100 ft² of wall |
|---|---|---|---|
| 6 in | All cells | 0.83 | 0.93 |
| 8 in | All cells | 1.00 | 1.12 |
| 8 in | Every 16 in (rebar cells) | 0.58 | 0.65 |
| 10 in | All cells | 1.23 | 1.30 |
| 12 in | All cells | 1.54 | 1.73 |
Example: the 20 × 8 ft wall above (160 ft², 8 in block) fully grouted takes about 160 × 1.12 ÷ 100 = 1.79 cubic yards. Grouting only the cells at 16 in centers takes about 1.04 cubic yards. To convert to bags, use the yield printed on your grout or concrete bag, or the concrete bag calculator.
Questions
How many blocks are in a square foot?
1.125 standard 8 × 16 in blocks, which is 112.5 blocks per 100 ft².
How many blocks do I need for a 10 ft wall, 8 ft high?
80 ft² × 1.125 = 90 blocks, before any extra for breakage.
How many blocks does one bag of mortar lay?
QUIKRETE states that an 80 lb bag of its Mortar Mix lays up to 13 standard 8 × 8 × 16 in blocks. Joint thickness and waste on the board can lower that.
Is a cinder block the same as a concrete block?
In everyday use, yes: stores and contractors use both names for the same 8 × 8 × 16 in block. Older cinder blocks were made with coal cinders as the aggregate; most blocks sold today are concrete masonry units made with sand and gravel, and they are counted the same way.
Should I subtract doors and windows?
Subtract large openings such as doors and garage openings. Small ones can stay in the total as spare blocks, since the blocks around an opening are often cut.
How do I calculate concrete blocks for a wall?
Multiply wall length by height in feet, then by 1.125 (standard 8 × 16 in blocks per sq ft). Add about 5% for cuts and breakage.
How much concrete does it take to fill cinder blocks?
About 1 cubic yard of grout for every 100 standard 8 in blocks with every core filled (roughly 0.27 ft³ per block), per Westbrook Concrete Block’s yield chart. Filling only the rebar cells at 16 in centers cuts that to about 0.58 cubic yards per 100 blocks.
Does block thickness change how many blocks I need?
No. The count depends only on the face size, so 6, 8, 10 and 12 in blocks with an 8 × 16 in face all need 1.125 per square foot. Thickness changes the weight, the grout per core and the footing width. For a step-by-step wall example, see how many concrete blocks for a wall.
Sources
- CMU-TEC-001-23: Concrete Masonry Unit Shapes, Sizes, Properties, and Specifications, Concrete Masonry & Hardscapes Association, Dec 12, 2024.
Used for: Nominal vs. specified block sizes and 3/8 in mortar joints. - Mortar Mix Product No. 1102 data sheet, QUIKRETE.
Used for: One 80 lb bag lays up to 13 standard 8 × 8 × 16 in blocks. - TEK 4-2A: Estimating Concrete Masonry Materials, Concrete Masonry & Hardscapes Association.
Used for: Face area per unit, 119 units per 100 ft² with 5% waste. - Grout Calculation for Concrete Block Construction (FineCoreFill grout yields), Westbrook Concrete Block Co.
Used for: Grout volume per 100 blocks and per 100 ft² of wall.