Quick answer
Blocks per square foot = 144 ÷ ((block length + joint) × (block height + joint)). A 30 × 8 ft wall (240 ft²) with 15.625 × 7.625 inch blocks and 3/8 inch joints needs 284 blocks including waste, and about 10.56 ft³ of mortar.
Enter wall length and height, block size, mortar joint and waste allowance. The calculator returns units, net area and mortar. Use this concrete block calculator to check your own figures.
Related tools: Concrete Block Fill Calculator: Grout or concrete needed to fill block cores. Concrete Calculator: Cubic yards of concrete for a slab, plus bag counts. Brick Calculator: Bricks and mortar volume for a wall.
How to use the Concrete Block Calculator
- Enter wall length (ft) (above 0).
- Enter wall height (ft) (above 0).
- Optionally, enter openings (optional) (ft²) (0 or more).
- Enter block length (in) (above 0). It starts at 15.625.
- Enter block height (in) (above 0). It starts at 7.625.
- Enter block thickness (in) (above 0). It starts at 7.625.
- Enter mortar joint (in) (0 or more). It starts at 0.375.
- Enter waste allowance (%) (between 0 and 100). It starts at 5.
- Press Calculate to see the result, and use Reset to start again.
| Input | What to enter | Allowed values |
|---|---|---|
| Wall length | A number, in ft | > 0 |
| Wall height | A number, in ft | > 0 |
| Openings (optional) (optional) | A number, in ft² | ≥ 0 |
| Block length | A number, in in | > 0, starts at 15.625 |
| Block height | A number, in in | > 0, starts at 7.625 |
| Block thickness | A number, in in | > 0, starts at 7.625 |
| Mortar joint | A number, in in | ≥ 0, starts at 0.375 |
| Waste allowance | A number, in % | ≥ 0, ≤ 100, starts at 5 |
Formula and method
Formula
blocks per ft² = 144 ÷ ((length + joint) × (height + joint)); mortar = wall volume − block volume
IEEE-754 double precision; shown to the decimals indicated.
Defaults are an actual 8 × 8 × 16 in nominal block (15⅝ × 7⅝ × 7⅝ in). Plan geometry only; confirm quantities with your supplier and local code before ordering.
Worked examples
These examples are verified: each one is an automated test case that the live calculator must reproduce exactly.
Example 1
Inputs: Wall length = 30; Wall height = 8; Block length = 15.625; Block height = 7.625; Block thickness = 7.625; Mortar joint = 0.375; Waste allowance = 5.
| Result | Value |
|---|---|
| Units needed | 284 |
| Net wall area | 240 ft² |
| Mortar (cubic feet) | 10.56 |
Example 2
Inputs: Wall length = 24; Wall height = 6; Block length = 15.625; Block height = 7.625; Block thickness = 11.625; Mortar joint = 0.375; Waste allowance = 10; Openings (optional) = 20.
| Result | Value |
|---|---|
| Units needed | 154 |
| Net wall area | 124 ft² |
| Mortar (cubic feet) | 8.31 |
The first line is the headline result (units needed). Below it the concrete Block Calculator lists net wall area and mortar (cubic feet), which describe the same case from other angles. IEEE-754 double precision; shown to the decimals indicated.
Where 1.125 comes from
Each block with its joint covers 16 × 8 = 128 in², so 144 ÷ 128 = 1.125 blocks per ft². For 240 ft² that is 270 blocks; a 5% waste allowance brings it to 284.
Extras to order
Cores that are grouted or reinforced need extra grout, and corners, lintels and half-blocks change the count. Subtract openings from the wall area for doors and windows.
How the result changes with wall length
Holding every other input at the first worked example, this table changes only wall length. It shows how sensitive the answer is to that one value; each row was produced by running this calculator.
| Wall length | Units needed | Net wall area | Mortar (cubic feet) |
|---|---|---|---|
| 15 | 142 | 120 ft² | 5.28 |
| 30 | 284 | 240 ft² | 10.56 |
| 60 | 567 | 480 ft² | 21.11 |
| 120 | 1134 | 960 ft² | 42.22 |
What happens with invalid input
These messages are produced by the calculator itself, so you know what to fix.
- Wall length: entering "0" shows "Wall length must be greater than 0." beside that field and no result.
- Openings (optional): entering "200" shows "Openings cannot cover the whole wall." beside that field and no result.
Choosing between the Concrete Block Calculator and similar calculators
Each row shows what a related calculator on this site asks for and what it returns, so you can pick the one that matches your question.
| Calculator | You enter | It returns |
|---|---|---|
| [Concrete Block Fill Calculator](/construction/concrete-block-fill-calculator/) | blocks with filled cores, core volume per block and waste allowance | Fill needed |
| [Concrete Calculator](/construction/concrete-calculator/) | length, width and depth / thickness | Cubic yards |
| [Brick Calculator](/construction/brick-quantity-calculator/) | wall length, wall height and brick length | Units needed |
| [Concrete Steps Calculator](/construction/concrete-steps-calculator/) | number of steps, riser height and tread depth | Concrete volume |
| [Concrete Volume Calculator](/construction/concrete-volume-calculator/) | length, width and thickness | Concrete needed |
Background: construction estimates
Material quantities are geometry plus a margin. Calculate the exact volume or area, add an allowance for waste, spillage, cuts and uneven ground (calculator pages commonly mention 5 to 10 percent, a rule of thumb that varies by material and job) and round up to whole units. One cubic yard is exactly 27 cubic feet.
Suppliers often have minimum order sizes and may charge for small loads, so confirm quantities, product yields and local code with your supplier or a qualified estimator before ordering.
Assumptions and limitations
- Cores filled with grout need extra volume
Quantities are estimates. Add allowances for waste and site conditions, and follow local codes and a qualified professional's instructions for structural or safety-critical work.
If a required value is empty, not a number, or outside its allowed range, the form marks that field with a message and shows no result instead of a misleading number. The automated checks deliberately try invalid entries for wall length and openings (optional) to confirm this.
Frequently asked questions
What is a standard block?
A nominal 8 × 8 × 16 inch block measures 7⅝ × 7⅝ × 15⅝ inches.
How much mortar per block?
Roughly 0.04 ft³ of mortar per block, as the mortar figure shows.
Do I add waste?
Yes, typically 5 to 10%.
What does the Concrete Block Calculator calculate?
Concrete blocks and mortar volume for a wall. It returns Units needed, Net wall area and Mortar (cubic feet).
What formula does the Concrete Block Calculator use?
blocks per ft² = 144 ÷ ((length + joint) × (height + joint)); mortar = wall volume − block volume. Defaults are an actual 8 × 8 × 16 in nominal block (15⅝ × 7⅝ × 7⅝ in).
What do I need to enter in the concrete Block Calculator?
You enter Wall length (ft), Wall height (ft), Block length (in), Block height (in), Block thickness (in), Mortar joint (in) and Waste allowance (%). Optional: Openings (optional) (ft²).
How is the result rounded or limited?
IEEE-754 double precision; shown to the decimals indicated. Cores filled with grout need extra volume.
What is an example of a concrete Block result?
With wall length 30, wall height 8, block length 15.625, block height 7.625 and block thickness 7.625, the units needed is 284.
Can I rely on this calculator for a decision?
Quantities are estimates. Add allowances for waste and site conditions, and follow local codes and a qualified professional's instructions for structural or safety-critical work. For this calculator in particular: Cores filled with grout need extra volume.
Related calculators
- Concrete Block Fill Calculator — Grout or concrete needed to fill block cores.
- Concrete Calculator — Cubic yards of concrete for a slab, plus bag counts.
- Brick Calculator — Bricks and mortar volume for a wall.
- Concrete Steps Calculator — Concrete volume for solid stacked steps.
- Concrete Volume Calculator — Cubic yards of concrete and bags needed for a slab, footing or pad.
- Concrete Weight Calculator — Weight of a volume of concrete from its density.
- Asphalt Tonnage Calculator — Tons of hot-mix asphalt for a paved area.
- Asphalt Sealer Calculator — Gallons of driveway sealer for an area.
- Trim and Moulding Calculator — Linear feet of baseboard or trim for a room.
Sources
- OpenStax, Prealgebra 2e (Rice University): perimeter, area, volume, circumference and the Pythagorean theorem
- NIST Special Publication 811, Guide for the Use of the International System of Units (SI), Appendix B conversion factors (international foot 0.3048 m, inch 25.4 mm, US gallon 3.785411784 L)
Content last reviewed 2026-11-04. Formulas and worked examples are checked by automated tests against independently computed values.