Concrete Block Calculator: Cinder Blocks, Mortar and Fill

A 20 × 4 ft wall needs 90 standard 8 × 8 × 16 in blocks before waste, or 95 with 5% extra. Mortar is 8 bags of 80 lb mix at up to 13 blocks per bag. Subtract doors and windows. Fill cores only if you are grouting every cell.

Structural and retaining walls over local limits need an engineer and a permit.

Enter wall length and wall height in feet. Open More options for feet + inches.
Add another wall for a second run. Tap Subtract on an extra wall to take out a door or window.
Leave block size on 8 × 8 × 16 in unless you are laying 4, 6, 10, or 12 in widths. The face is still 8 × 16, so the block count does not change.
Leave waste at 5% (CMHA TEK 04-02A). Drag Custom waste if you are cutting a lot of units.
Optionally turn on Fill all cores for grout volume from TEK 04-02A. 4 in block has no grout figure.

Blocks = wall length × wall height × 1.125, then add waste and round up. 1.125 is 144 ÷ 128: a nominal 8 × 16 in face with a 3/8 in mortar joint. Mortar bags = blocks to buy ÷ 13, rounded up. Grout cubic yards = (wall sq ft / 100) × the TEK 04-02A table value ÷ 27 when cores are filled.

How many cinder blocks do I need?

Multiply wall length by wall height for square feet, then multiply by 1.125. A 20 × 4 ft wall is 80 square feet × 1.125 = 90 blocks. Add 5% for waste and breakage and round up: 95 blocks. Subtract door and window openings first. The 8 × 16 in face (with a 3/8 in joint) is the same for 4, 6, 8, 10, and 12 in widths; only grout volume changes with width.

Worked example: 20 × 4 ft wall

Worked example

A 20 ft long, 4 ft high wall, 8 × 8 × 16 in block, no openings.

20 × 4 = 80 square feet. 80 × 1.125 = 90 blocks. With 5% extra, 90 × 1.05 = 94.5, which rounds up to 95. Mortar: 95 ÷ 13 = 7.31, which rounds up to 8 bags of 80 lb mix.

Those are the default numbers on this calculator. Subtract a 3 × 7 ft door and the net wall is 59 square feet: 59 × 1.125 = 66.38, which rounds up to 67 blocks before waste.

Blocks for common wall sizes

Wall (ft)	Area	Blocks	With 5% waste
10 × 3	30 sq ft	34	36
20 × 4	80 sq ft	90	95
30 × 6	180 sq ft	203	213
40 × 8	320 sq ft	360	378

Counts use 1.125 blocks per square foot, then 5% extra, then round up. Openings are not subtracted in this table.

Questions

How much does a cinder block weigh?

We do not ship a single block weight. ASTM C90 (via CMHA) classes the concrete as lightweight (under 105 lb per cubic foot), medium weight (105 to under 125), or normal weight (125 or more). The same 8 × 8 × 16 in hollow unit can sit in any class, so the pallet ticket varies. Ask your supplier for the unit weight on the load.

What are the dimensions of a cinder block?

Nominal 8 × 8 × 16 in is the module, including a 3/8 in mortar joint. The specified (actual) unit is typically 7-5/8 in wide, 7-5/8 in high, and 15-5/8 in long (CMHA Concrete Masonry Unit Shapes, Sizes, Properties, and Specifications; ASTM C90). Other common nominal widths are 4, 6, 10, and 12 in, still with an 8 × 16 in face.

How many cinder blocks are on a pallet?

Belgard lists a standard medium-weight 8 × 8 × 16 in CMU at 90 pieces per pallet (manufacturer, SiteOne product 30164944-90W). Other widths and producers use a different count. Confirm the pallet before you order.

How much do cinder blocks cost?

HomeGuide (cost guide, Sarah Noel, January 21, 2026) lists standard hollow 8 × 8 × 16 in CMU at about $1.25 to $2.50 per block. Type your quote under Estimate cost.

How much weight can a cinder block hold?

Do not size a wall from a single-block load. ASTM C90 (2014 onward, CMHA FAQ) requires a minimum net-area compressive strength of 2,000 psi averaged on three units (1,800 psi each). That is a unit test, not the capacity of a built wall. Structural and retaining walls need an engineer and TMS 402/602 design.

Can you use cinder blocks for a fire pit?

Cooperative Extension (Ask Extension, July 2016) advises that ordinary cinder blocks are usually not a good choice to line a fire pit: they do not take fire well.

Concrete block calculator

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