Cinder Block Calculator

Cinder block calculator: wall area ÷ 0.888

This cinder block calculator counts the standard 8 × 16 inch blocks in a wall. The formula is blocks = (wall length × wall height) ÷ 0.888, rounded up to a whole block. That 0.888 sq ft is the face of one block measured to the center of its mortar joints. Enter wall length in the length box and wall height in the width box. Tick the 10% option for breakage and cuts.

Why 0.888 square feet per block

A standard block is sold as 8 × 8 × 16 inches, but its real size is 7⅝ × 7⅝ × 15⅝ inches. The missing ⅜ inch is the mortar joint. So each block, plus its share of mortar, fills 8 × 16 = 128 square inches of wall. Divide 128 by 144 and you get 0.888 sq ft. That is why the formula divides by 0.888, or multiplies by about 1.125.

The waste allowance multiplies the raw count by 1.10 before rounding up. The cost line is the final whole-block count times your price per block.

Worked examples

1. Default wall: 20 ft long × 8 ft high, $2.50 per block

• Area: 20 × 8 = 160 sq ft
• Blocks: 160 ÷ 0.888 = 180.2, rounded up to 181 blocks, costing $452.50
• With 10% on: 180.2 × 1.10 = 198.2, rounded up to 199 blocks, costing $497.50

2. Garden wall: 30 ft long × 4 ft high, 10% on

• Area: 30 × 4 = 120 sq ft
• Net: 120 ÷ 0.888 = 135.1, which rounds to 136 blocks
• With 10%: 148.6, rounded up to 149 blocks, costing $372.50

Courses help you sanity-check the count. A 4 ft wall is 6 courses of 8 in block. A 30 ft run holds 22.5 blocks per course, so 6 × 22.5 = 135, which matches the net figure.

Cinder block reference tables

Wall size	Area	Blocks, no waste	Blocks, 10% on
10 × 4 ft	40 sq ft	46	50
30 × 4 ft	120 sq ft	136	149
20 × 8 ft	160 sq ft	181	199
40 × 16 ft	640 sq ft	721	793

Nominal size (W × H × L)	Common use
4 × 8 × 16 in	Non-load-bearing partitions, veneer backup
6 × 8 × 16 in	Interior partitions, light walls
8 × 8 × 16 in	Most foundation, garage, and retaining walls
10 or 12 × 8 × 16 in	Taller or more heavily loaded walls

Every size in the table has the same 8 × 16 in face, so the count per square foot does not change with wall thickness. Thicker blocks only change weight, mortar, and grout.

How to measure a block wall

• Length: measure along the footing. Count each corner only once.
• Height: measure from the top of the footing to the finished top of the wall, including any cap course.
• Openings: subtract windows and doors that are bigger than a few square feet. Measure the rough opening, not the finished trim.
• Round heights to courses. Each course is 8 in. A 7 ft 4 in wall is 11 courses, which avoids a cut top course.

Planning lengths around the block module

Blocks are made in 8 inch steps, so walls work best when their length and height are multiples of 8 inches. A wall planned at 20 ft 8 in is exactly 31 blocks long. A wall at 20 ft 3 in leaves an awkward cut at one end of every course.

Openings follow the same idea. A door or window placed on the 8 inch grid avoids cutting dozens of blocks. Bond beams, lintels, and corners also come as special shapes, so mark them on a sketch before ordering. Your supplier can then split the total count into stretchers, corners, halves, and lintel units.

Common cinder block mistakes

• Using the actual block size. Dividing by the 7⅝ × 15⅝ in face instead of the nominal face overstates the count by about 7%.
• Forgetting corner, half, and bond-beam blocks. The total count is right, but some of those units should be special shapes. Ask your supplier to split the order.
• Leaving out mortar and grout. A block wall also needs mortar, and reinforced walls need grout in the cores. Size them with the mortar calculator and the core fill grout calculator.
• Skipping the footing. Block walls need a level, solid footing. The concrete footing calculator sizes it.
• Building tall retaining walls without design. Walls that hold back soil carry real loads; check local code, and have taller walls designed by an engineer.

For the same math with optional extras, see the concrete block calculator.