Concrete Block Calculator

Estimate blocks and mortar bags for a wall

Wall length (feet)

Wall height (feet)

Waste factor (%)

Price per block ($, optional)

Frequently Asked Questions

How many blocks do I need per square foot? ▼

A standard 8×16-inch concrete or cinder block (CMU), including a standard 3/8-inch mortar joint, covers 128 square inches, or about 0.89 square feet of wall face — so you need roughly 1.125 blocks per square foot of wall.

How much mortar do I need? ▼

A common rule of thumb is about 3 bags of mortar mix (80 lb bags) per 100 blocks for standard 3/8-inch joints, which works out to roughly 1 bag per 33 blocks. Actual usage varies with joint thickness and mason technique.

Should I include a waste factor? ▼

Yes — most contractors add 5-10% extra to account for cuts, breakage, and layout adjustments. This calculator lets you set your own waste percentage.

Does this calculator account for openings like doors and windows? ▼

No — enter the gross wall area and then separately subtract the area of any door or window openings from your total before ordering material, since this calculator doesn't automatically account for openings.

How many concrete blocks you need

A standard CMU block measures 7-5/8 × 15-5/8 inches; with a 3/8-inch mortar joint it fills a nominal 8 × 16 inch space, or 0.889 sq ft of wall. That works out to 1.125 blocks per square foot.

Blocks = wall area ÷ 0.889 × (1 + waste %), and mortar is estimated at about one 80 lb bag per 33 blocks.

Example

A wall 20 ft long and 8 ft high is 160 sq ft, which takes 180 blocks. With 10% waste, order 198 blocks and about 6 bags of mortar mix. At $2.50 a block, the blocks cost about $495.

Quick reference

Wall size	Area	Blocks (no waste)
10 × 4 ft	40 sq ft	45
20 × 4 ft	80 sq ft	90
20 × 8 ft	160 sq ft	180
40 × 8 ft	320 sq ft	360

Before you order

Subtract openings: take door and window areas off the wall area first.
Count special blocks: corners, ends and bond beam blocks are bought separately but replace standard blocks one for one.
Plan the footing: block walls need a concrete footing, typically twice as wide as the wall and below the frost line.
Reinforcement: load-bearing and retaining walls usually need vertical rebar and grouted cores, set by plans and local code.

Grout for filled cores

If cores are filled, an 8-inch wall fully grouted needs roughly 0.25 to 0.3 cubic feet of grout per block. Filling only the reinforced cells, often every 32 to 48 inches, uses much less.

Block sizes

The 8 × 8 × 16 block is the most common for foundations and walls. 6-inch blocks are used for non-load-bearing partitions, and 10- or 12-inch blocks for taller or retaining walls. The face size, and therefore the block count per square foot, is the same for all of them.

Plan the footing with the concrete foundation calculator and steel with the rebar calculator. For a short landscape wall, compare with the retaining wall calculator and the brick calculator.