How to use the concrete block calculator
- Enter the wall length and wall height in the unit you measured, and the total area of any doors or windows to subtract.
- Pick the block thickness (4, 6, 8, 10 or 12 inch) and the block height: standard 8 inch courses or half-height 4 inch blocks.
- Keep the 5% waste or change it. If you plan to fill cores with grout or concrete, enter the share of cores you will fill.
- Read the blocks to buy, the mortar bags and the core fill volume, then copy the shopping list or add a price per block.
Formula
Block face with joint = 16 in x course height / 144, in square feet (0.8889 for an 8 inch course)
Blocks = net wall area / block face, then x (1 + waste percent / 100), rounded up
Courses = wall height in inches / course height, rounded up
Mortar per block = (16 x course height - 15.625 x (course height - 0.375)) x (thickness - 0.375), in cubic inches
Mortar bags = blocks x mortar per block x (1 + extra percent / 100) / 1,728 / bag yield, rounded up
Core fill = blocks x block volume x hollow share x share of cores filled / 1,728, in cubic feet
Block sizes are nominal. An 8 x 8 x 16 inch block is really 7 5/8 x 7 5/8 x 15 5/8 inches, because 3/8 inch is left for the mortar joint. That is why one block covers exactly 16 by 8 inches of wall. The hollow share and mortar bag yield are editable example values.
Worked example: a 20 foot long, 4 foot high garden wall
The wall area is 20 x 4 = 80 sq ft. One 8 x 8 x 16 inch block covers 16 x 8 / 144 = 0.8889 sq ft, so the wall needs 80 / 0.8889 = 90 blocks. That is 6 courses of 15 blocks.
With 5% waste you need 94.5 blocks, so buy 95 blocks.
The joint around each block holds (128 - 119.14) x 7.625 = 67.55 cubic inches of mortar, or 0.0391 cubic foot. For 90 blocks that is 3.52 cubic feet, or 4.40 cubic feet with 25% extra. At 0.6 cubic foot per bag that is 7.33, so 8 bags.
If you fill every core and the block is 45% hollow, the fill is 90 x 408.8 cubic inches = 21.29 cubic feet, or 0.789 cubic yard.
Concrete block, cinder block and CMU
People use cinder block, concrete block and CMU (concrete masonry unit) for the same kind of hollow block. Modern blocks are made from concrete and aggregate rather than cinders, but the sizes are the same, so this tool works for all of them. The most common size is the 8 x 8 x 16 inch block, with 4, 6, 10 and 12 inch thicknesses made for different wall types.
Besides the standard stretcher block, a real wall also uses corner blocks, half blocks, bond beam blocks and caps. This tool counts them as ordinary blocks, so ask your supplier how they are packaged and add the special shapes to your order.
Mortar and core fill
Mortar is laid between blocks, and bags are sold by weight with a printed yield in cubic feet. The tool assumes the joint is filled across the whole thickness of the block, which uses more mortar than face-shell bedding, so treat the bag count as a cautious figure and allow a little for droppings.
Core fill is concrete or grout poured into the hollow cells, usually together with steel reinforcement. The share of cores filled depends on the wall design, so enter the percentage from your plans. The hollow share of the block is an example value that you can change after checking the product data.
When a block wall needs engineering
A garden wall that holds nothing back is a simple project. A retaining wall, a foundation or any wall that carries a building needs reinforcement, drainage and a footing sized for the load, and many places require a permit. This calculator counts blocks and mortar and does not check any of those, so ask a qualified professional for structural work.
Assumptions and limits
- Blocks are the standard 16 inch length. The thickness and course height are the nominal sizes you pick.
- The mortar joint is 3/8 inch and the mortar fills the full block thickness. Face-shell bedding uses less.
- The hollow share of 45%, the 25% mortar allowance and the 0.6 cubic foot bag yield are example values that you can edit.
- Special shapes, lintels, rebar, bond beams, caps and footings are not counted.
- The wall is a plain rectangle. Stepped walls and curved walls need a separate calculation for each section.
Frequently asked questions
How many cinder blocks do I need for a wall?
Divide the wall area in square feet by 0.8889, the face area of an 8 x 8 x 16 inch block including its joint. Add a waste allowance and round up. A wall of 80 square feet needs 90 blocks before waste.
How many blocks fit in one square foot of wall?
A standard 8 inch tall, 16 inch long block covers 0.8889 square feet, so about 1.125 blocks cover one square foot of wall.
How much mortar do I need for 100 concrete blocks?
With full bedding and 3/8 inch joints, 100 blocks of 8 inch thickness use about 3.9 cubic feet of mortar before any extra. That is roughly 6.5 bags at a typical 0.6 cubic foot per bag.
What is a course of block?
A course is one horizontal row of blocks. An 8 inch block makes an 8 inch course, so a wall 4 feet high has 6 courses.
Do I need to fill the block cores?
It depends on the wall design. Walls with reinforcement or structural loads usually have some or all cores filled, while a low garden wall may need none. Follow your plans and local rules.