How to use
- Pick US or metric units. Enter the wall length and height, or the total area if you already know it.
- Add the doors and windows. Each row is one size of opening, and "How many" lets you count several of the same size.
- Pick the block face. Most walls use the standard 8 × 16 inch block. Choose "Custom" if your block is a different size.
- Leave the extra at 5% or change it. If you plan to fill the cores, turn on grout.
- Select "Calculate" to see the block count and the mortar and grout estimates.
How the block count is calculated
Blocks are counted by wall area, not by length. The nominal face of a standard block is 8 inches high and 16 inches long. That is 8 × 16 = 128 square inches, and a square foot is 144 square inches, so 144 ÷ 128 = 1.125 blocks per square foot. The Concrete Masonry & Hardscapes Association (CMHA, the group formerly known as NCMA) gives the same figure as 8/9 square foot of wall per block.
Example: a wall 20 ft long and 6 ft high is 120 square feet. 120 × 1.125 = 135 blocks. Now take out a window 3 ft wide and 4 ft high. The wall is 108 square feet, and 108 × 1.125 = 121.5, which rounds up to 122 blocks. Add 5% for breakage and cuts: 121.5 × 1.05 = 127.575, which rounds up to 128 blocks.
The calculator rounds up once, at the end, so a half block in the exact result becomes a whole block you buy.
Nominal size and actual size
A block sold as "8 × 8 × 16" is not 8 × 8 × 16 inches. CMHA's tech note on block shapes and sizes says the specified dimensions are typically 3/8 inch less than the nominal ones, so that the 4 or 8 inch module still works once you add a 3/8 inch mortar joint. Count with the nominal size. It already includes the joint.
| Nominal size (thickness × height × length) | Actual size, about | Face area in the wall |
|---|---|---|
| 8 × 8 × 16 in | 7-5/8 × 7-5/8 × 15-5/8 in | 128 sq in (1.125 blocks per sq ft) |
| 6 × 8 × 16 in | 5-5/8 × 7-5/8 × 15-5/8 in | 128 sq in (1.125 blocks per sq ft) |
| 4 × 8 × 16 in | 3-5/8 × 7-5/8 × 15-5/8 in | 128 sq in (1.125 blocks per sq ft) |
| 8 × 4 × 16 in (half-high) | 7-5/8 × 3-5/8 × 15-5/8 in | 64 sq in (2.25 blocks per sq ft) |
| 8 × 8 × 8 in (half-length) | 7-5/8 × 7-5/8 × 7-5/8 in | 64 sq in (2.25 blocks per sq ft) |
The 8 × 8 × 16 actual size is the one CMHA shows. The other actual sizes subtract 3/8 inch from the nominal size, which is the rule it gives. CMHA also says block dimensions can vary by up to 1/8 inch from the specified size. Your supplier's blocks may differ a little, and some producers make other sizes.
Quick wall chart
These counts use the standard 8 × 16 inch face and no openings.
| Wall (length × height) | Area | Blocks, no extra | Blocks, with 5% extra |
|---|---|---|---|
| 10 ft × 4 ft | 40 sq ft | 45 | 48 |
| 20 ft × 6 ft | 120 sq ft | 135 | 142 |
| 30 ft × 8 ft | 240 sq ft | 270 | 284 |
Mortar
For standard blocks laid with mortar on the face shells, CMHA's estimating tech note gives these rules of thumb. One 70 lb bag of masonry cement lays about 30 blocks. Eight bags go with about 1 ton of sand, which works out to about 240 blocks per ton of sand. Those figures already include a nominal amount for waste. The calculator rounds bags up. For the 122-block wall in the example, that is 5 bags (122 ÷ 30 = 4.07) and about 0.51 ton of sand.
CMHA warns that mortar is one of the most commonly misestimated materials. Mason skill, how thick the joints end up, and whether you mix on site or buy pre-bagged mortar can all change real use. If you use a different mortar product, follow the yield on its bag.
Grout for filled cores
Some walls have grout poured into the hollow cores. The right spacing is a design decision, not something this calculator can pick for you. If you already know your spacing, the grout option uses CMHA's table for hollow single-wythe walls. It assumes two-core units and 3% waste. These are values for an 8 inch thick block:
| Cores grouted every | Grout per 100 sq ft of wall |
|---|---|
| 8 in (every core) | 36.1 cu ft |
| 16 in | 18.1 cu ft |
| 24 in | 12.1 cu ft |
| 48 in | 6.1 cu ft |
Example: the 108 sq ft wall above, 8 inch block, cores grouted every 16 inches. 108 ÷ 100 × 18.1 = 19.5 cubic feet, or about 0.72 cubic yard. Whether you need grout, rebar, or a footing depends on the wall and your local code. Check with your building department and a qualified professional.
Common mistakes
- Using the actual block size. A block that measures 7-5/8 × 15-5/8 inches takes up 8 × 16 inches in the wall once you add the joint. Count with the nominal size, or enter the actual size plus the joint under "Custom".
- Forgetting the openings. Doors and windows can take a big bite out of the count. Subtract them, but remember that blocks above and beside an opening still need cuts.
- Not planning for corners and half blocks. Corners, ends, and bond beam courses can use special units. Order them from your supplier and take them out of the main count.
- Counting by length. A 20 ft wall takes 15 blocks per course. A wall nine courses high needs 135, not 15. Count the rows and the blocks in each one, or just use the wall area.
- Ordering the exact number. The 5% extra is a planning allowance. Increase it if your wall has many corners and cuts.
Sources
Face size, nominal and specified dimensions, 3/8 inch joint, and 1/8 inch tolerance: CMHA Tech Note CMU-TEC-001-23, "Concrete Masonry Unit Shapes, Sizes, Properties, and Specifications," section 3.1. Blocks per area, mortar yield, and grout volumes: CMHA Tech Note TEK 04-02A, "Estimating Concrete Masonry Materials" (Tables 1 to 3). Both are from the Concrete Masonry & Hardscapes Association, formerly the National Concrete Masonry Association (NCMA).
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Frequently asked questions
How many concrete blocks do I need per square foot?
A standard block has a nominal face of 8 by 16 inches. That is 128 square inches, so 144 divided by 128 gives 1.125 blocks per square foot of wall. Multiply your wall area, after taking out doors and windows, by 1.125 and round up.
How many cinder blocks do I need for a wall?
Multiply the wall length by the wall height to get square feet, subtract the openings, then multiply by 1.125. A wall 20 feet long and 6 feet high is 120 square feet, which takes 135 blocks before you add any extra. Cinder block and concrete block are two names people use for the same kind of hollow wall block, and the face sizes are the same.
Do I use the nominal size or the actual size of the block?
Use the nominal size, 8 by 16 inches for a standard block. The actual block is about 3/8 inch smaller in each direction, 7-5/8 by 15-5/8 inches, so that a 3/8 inch mortar joint brings it back to the 8 by 16 inch module. If you use the actual size and forget the joint, you will over-count.
Does the block thickness change how many blocks I need?
No. The count depends only on the face, which is the height and length you see in the wall. A 4 inch, 8 inch, or 12 inch thick block with an 8 by 16 inch face covers the same wall area. Thickness matters for grout and for weight.
How much mortar do I need for concrete blocks?
The CMHA tech note on estimating masonry says one 70 lb bag of masonry cement lays about 30 hollow blocks with face shell bedding, and about 1 ton of sand goes with every 8 bags. This calculator uses those figures for 8 by 16 inch blocks. Treat them as a rule of thumb, since mason skill, joint tooling, and weather all change real use.
How much extra should I order?
This calculator adds 5% by default for broken blocks and cuts. That is a planning allowance, not a standard for every job. The CMHA estimating guide also includes about 5% for waste in its units-per-100-square-feet figure. Raise it if you have many corners, or if the wall has lots of short runs.
Does this calculator work for structural walls and foundations?
It only counts blocks, and estimates mortar and grout. It does not size reinforcing steel, footings, or loads. For any wall that holds back soil, carries a roof or floor, or needs rebar, check your local building code and talk to a qualified professional.