Concrete Block Calculator

What this concrete block calculator does

This concrete block calculator tells you how many blocks a wall needs and what else goes with them: mortar, core fill, pallets and a materials cost. It works for concrete blocks, CMU and cinder blocks alike, because they share the same standard sizes.

Enter the wall length and height, take off the doors and windows, pick the block size and add a waste allowance. The calculator counts blocks from the wall area, rounds up to whole blocks, and draws the wall course by course so you can see the layout. A second tab shows the nominal and actual size of each block with a scale drawing. If you need the wall area first, use our square footage calculator. If you plan to paint the finished wall, our paint coverage calculator has a rough-surface rate for block.

How to use the block calculator

1. Choose a tab. Use “How many blocks do I need?” for a wall. Use “Block sizes” to look up dimensions.
2. Enter the wall. Give the length and height, then the doors and windows in one wall. For several matching walls, set the number of identical walls.
3. Pick the block. The 8 × 8 × 16 inch block is the default. Choose Custom if your block face is different.
4. Set the waste. The default is 5 percent. Raise it for walls with many cuts.
5. Add the extras if you want them. Choose a core fill spacing, check the blocks per pallet and per mortar bag, and add prices to see a cost.

Tap the small i beside any field for what to measure. The results update as you type.

Concrete block sizes and dimensions

Block sizes are given as thickness × height × length, and they are nominal. A nominal 8 × 8 × 16 inch block is really 7 5/8 × 7 5/8 × 15 5/8 inches. The missing 3/8 inch is the mortar joint, so every block lays out on an exact 8 by 16 inch module. That is why CMU block dimensions and concrete block dimensions are usually listed twice, nominal and actual.

Every unit with an 8 by 16 inch face covers 0.889 square feet, which is 1.125 blocks per square foot whatever the thickness. Half-height and half-length blocks have half that face, so they take 2.25 per square foot.

How big is a concrete block?

The standard concrete block is 8 inches thick, 8 inches high and 16 inches long nominal. Thickness sets the depth of the wall, not the block count, so the same wall needs the same number of 4 inch or 12 inch blocks.

How long is a cinder block?

A full cinder block is 16 inches long nominal and 15 5/8 inches actual. The half-length block is 8 inches nominal, and you use it at corners and wall ends to keep the pattern staggered.

Cinder block vs concrete block vs CMU

These three names describe nearly the same product. Original cinder blocks used coal cinders as aggregate, which made them lighter and weaker. Today’s units are concrete masonry units (CMU), made from cement, sand and gravel or lightweight aggregate. Most people still say cinder block, so a cinder block calculator and a CMU block calculator give the same count.

For ordering, three details matter more than the name. Hollow units, the common kind, have cores for rebar and fill, while solid units are used for caps and tight spots. Normal-weight blocks are denser and stronger, and lightweight blocks are easier to lift and cut. The nominal size sets the count, so confirm the type with your supplier.

How to calculate concrete blocks needed

1. Find the wall area. Length × height. For several walls, add them up.
2. Subtract openings. This calculator removes 21 square feet for a door and 15 for a window.
3. Find the face area of one block. For an 8 × 16 inch face, 8 × 16 ÷ 144 = 0.889 square feet.
4. Divide. Wall area ÷ block face area gives the block count. Divide by 0.889, or multiply by 1.125.
5. Add waste. Multiply by 1.05 to 1.10 and round up.

A second check is the course count. An 8 inch high block gives one course per 8 inches of wall, so a 96 inch wall is 12 courses. A 24 foot wall is 288 inches, or 18 blocks of 16 inches per course. Courses times blocks per course gives the count with no openings, which is the layout the diagram draws.

Worked example

Take a wall 24 feet long and 8 feet high with one door and one window, built from standard 8 × 8 × 16 inch blocks with 5 percent waste. These are the calculator defaults, so you can confirm the result.

• Wall area: 24 × 8 = 192 square feet.
• Openings: 21 + 15 = 36 square feet, so the wall to build is 156 square feet.
• Blocks: 156 × 1.125 = 175.5, which is 176 whole blocks.
• With 5 percent waste: 175.5 × 1.05 = 184.3, rounded up to 185 blocks.
• Pallets: 185 ÷ 90 = 2.06, so 3 pallets, or 2 pallets and 5 loose blocks if your supplier sells singles.
• Mortar: 176 ÷ 30 = 5.9, so 6 bags of 80 lb mix.

The wall is 12 courses high and 18 blocks long. Eighteen times twelve is 216, and the openings take that down to about 176 whole blocks.

How many blocks for common walls

This table uses standard 8 × 8 × 16 inch blocks with no openings and 5 percent waste. Subtract any doors and windows from the area before you use it as a guide.

How many concrete blocks on a pallet?

Suppliers load blocks in layers, so the count per pallet depends on how many fit in one layer. A pallet of standard 8 inch blocks commonly holds about 90. Thinner blocks fit more per layer and thicker ones fewer, which gives typical counts of about 180 for 4 inch, 120 for 6 inch, 72 for 10 inch and 60 for 12 inch blocks. Half-height and half-length blocks usually run about 180 per pallet.

Cement block and cinder block pallets follow the same counts. Treat them as typical, not guaranteed. Manufacturers differ, so confirm the count and any pallet deposit before you order. You can edit blocks per pallet in the calculator.

Concrete block fill calculator

Block cores are hollow, and walls that need strength are filled with grout or concrete around vertical rebar. A concrete block fill calculator needs two things: how much one block holds and how many cores you fill.

A hollow 8 inch block holds roughly 0.24 cubic feet, and thicker blocks hold more. If you fill every core, that is about one cubic yard of fill per 100 square feet of 8 inch wall. If rebar and fill run every 32 inches, only about a quarter of the cores are filled. The calculator uses that share, adds a 10 percent allowance for spillage, and converts the result to cubic yards and 80 lb bags at about 0.6 cubic feet each.

Using the worked example with fill every 32 inches, 176 blocks × 0.24 × one quarter × 1.10 gives about 11.6 cubic feet, or 0.43 cubic yard. Filling every core gives about 46.5 cubic feet, or 1.72 cubic yards. The same method works for cinder blocks. Fill spacing and rebar come from your wall design or local code, so use the figure for ordering only.

Mortar for a block wall

A common planning figure is one 80 lb bag of mortar mix for every 30 blocks laid. Full bedding, wider joints and less experienced hands all use more. Mix small batches you can lay before the mortar stiffens. The bag states the working time.

Concrete block wall cost

Block price depends on size, weight class, finish and your region, so look up your supplier’s price rather than trusting an average. Enter the price per block, per mortar bag and per fill bag in the optional section. The calculator multiplies each by its quantity and adds them for a materials total. It leaves out labor, delivery, the footing, rebar and any sealing or paint.

For an example only, if a block cost $2.50 and a mortar bag $9, the 185-block wall above would need $462.50 of blocks and $54 of mortar, or $516.50 in materials. Those prices are made up to show the math. Use your own quotes for a real budget.

Planning tips and safety

• Design to the module. Walls laid out in multiples of 8 inches high and 16 inches long need the fewest cuts. A wall 24 feet long uses whole blocks, while one 24 feet 4 inches long needs cuts.
• Check the footing. A block wall sits on a footing sized to the load and the soil. Retaining and load-bearing walls need a proper design and often a permit.
• Mind the dust. Cutting block releases silica dust. Wet-cut or use dust control and a respirator. See the OSHA silica guidance.
• Check the product standard. Load-bearing units are made to ASTM C90, and the National Concrete Masonry Association publishes design and detailing guidance.

Check your block data sheet and your local code before you order. The figures in this guide are planning estimates.

Concrete block FAQs

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