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Cinder Block Wall Calculator

Enter wall length and height to estimate blocks, mortar, and rebar.

How it is calculated

Length and height are entered in feet. The tool takes the wall area A = length × height (sq ft) and assumes a standard 16" × 8" block, about 1.125 blocks per square foot: blocks = ⌈A × 1.125⌉. It then adds 5% for breakage and cuts, rounding up again. Mortar is estimated at 3 bags per 100 blocks (before the 5% allowance). Rebar is one vertical bar every 32" (2.667 ft) of length plus one at the end, each as tall as the wall, plus a horizontal bar along the length on every 4th course, counting 8" courses. Everything is rounded up to whole units. The figures are a planning estimate: openings such as doors or windows are not subtracted, so reduce the area yourself.

Worked example

A wall 20 ft long and 4 ft high has an area of 20 × 4 = 80 sq ft. Blocks: 80 × 1.125 = 90, and with 5% extra 94.5, rounded up to 95 blocks. Mortar: 90 ÷ 100 × 3 = 2.7, so 3 bags. Rebar: ⌊20 ÷ 2.667⌋ + 1 = 8 vertical bars × 4 ft = 32 ft; the wall has 6 courses, so 1 horizontal run of 20 ft; total 52 ft.

Try this example →

Common mistakes

  • Entering metres or centimetres. The tool reads feet, so a 6 m wall typed as 6 is treated as a 6 ft wall and the order is far too small.
  • Not subtracting door and window openings. The tool counts the full length × height, so a wall with openings will be over-estimated.
  • Ordering exactly the blocks shown for a wall with many corners, cut courses or a stepped footing. The 5% allowance only covers ordinary breakage.
  • Treating the rebar figure as a structural design. It uses a fixed 32" vertical spacing and a horizontal bar every 4th course; your local code or engineer may require different spacing, lap lengths or grout fill.

About this calculator

You enter the wall's length and height in feet. Block count comes from the wall's face area times 1.125 blocks per square foot (a standard 8"×16" block covers about 0.89 sq ft including a 3/8" mortar joint), then adding a fixed 5% waste allowance. Mortar is estimated at 3 bags per 100 blocks, and rebar is estimated as vertical bars every 32 inches along the wall's length, run the full wall height, plus a horizontal bar along the wall at every fourth 8" course.

Masons, contractors, and DIY builders use this before ordering materials for a concrete block (CMU) wall — a garden wall, foundation, or fence — to avoid both under-ordering (a mid-project supply run) and over-ordering (wasted budget), and to get a rough rebar length for structural reinforcement; always check the spacing against your local building code.

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Comments (1)

  • Editorial team

    The area here is a plain length times height, so doors, windows and gable cuts are not subtracted. For a 20 by 8 ft wall the tool gives 160.00 sq ft and 189 blocks including the 5% allowance, but a 3 by 7 ft door would still be counted as wall. Measure openings separately and deduct them yourself, then rerun the numbers. What kind of project are you planning: garden wall, retaining wall or garage?

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