Block Wall Calculator

About the Block Wall Calculator

Building a wall from concrete blocks is a course-by-course job. Each course is a row of blocks 8 inches high, and in the usual running bond pattern each course is offset by half a block from the one below, so the vertical joints never line up. That offset means a straight wall with open ends needs half blocks at both ends of every other course. Knowing the number of courses, the full and half blocks, the caps for the top and the footing before you start makes ordering and laying far simpler.

This block wall calculator lays out a wall of standard 8 × 8 × 16 inch blocks. Enter the length and height and it gives the number of courses, the blocks in each course, the full and half blocks in total, cap blocks for the top, the mortar needed in bags of pre-mixed mortar, and the concrete for a standard footing.

How to Use the Block Wall Calculator

Enter the wall length and wall height in feet.

Choose whether to put cap blocks on top.

The Formulas

  courses            = ⌈height × 12 ÷ 8⌉
  blocks per course  = ⌈length × 12 ÷ 16⌉
  odd courses        = ⌈courses ÷ 2⌉ courses of full blocks
  even courses       = the rest, each with (blocks − 1) full and 2 halves
  caps               = blocks per course
  mortar             = 8.5 ft³ per 100 blocks; 80 lb bag ≈ 0.67 ft³
  footing            = length × 16 in × 8 in

Step-by-Step Example

A wall 20 ft long and 4 ft high, with caps.

  courses        48 ÷ 8 = 6
  per course     240 ÷ 16 = 15
  courses 1,3,5  15 full blocks each = 45
  courses 2,4,6  14 full + 2 halves each = 42 full, 6 halves
  total          87 full and 6 half blocks, 15 caps
  mortar         90 ÷ 100 × 8.5 = 7.65 ft³ → 12 bags
  footing        20 × 1.33 × 0.67 ÷ 27 = 0.66 yd³

Block Sizes

Running Bond and Corners

In running bond, each block sits over the joint between two blocks below. On a straight wall with free ends, alternate courses therefore start with a half block. Half blocks can be bought ready-made, or full blocks can be cut with a masonry saw or split with a chisel. At a corner, blocks from the two walls interlock: one course runs through on one wall, the next through on the other, so corners usually need fewer half blocks and more care with layout. Corner blocks with a finished end are also available for walls that will be seen.

Mortar

Mortar bonds the blocks and fills the joints. A commonly used figure is about 8.5 cubic feet of mortar per 100 standard blocks, which allows for the bed joints under each course and the head joints between blocks. Pre-mixed mortar in 80-pound bags yields roughly two-thirds of a cubic foot, so 100 blocks need about 13 bags. For larger walls, mixing your own from masonry cement and sand is cheaper; the mortar calculator covers that.

The Footing

A block wall needs a solid concrete footing, wider than the wall, to spread its weight into the soil. A common guide for garden and low walls is a footing twice the block width and as deep as the block is wide: 16 inches wide and 8 inches deep for 8-inch blocks. In cold climates the bottom of the footing must be below the frost line, and retaining walls need footings designed for the soil they hold back.

Caps and Coping

Cap blocks are thin solid blocks, often 2 to 4 inches thick, laid on top to close the hollow cores. They stop rain getting into the wall, where it can freeze and crack the blocks, and give a clean, finished edge. They are usually set in mortar or construction adhesive and can overhang slightly to shed water.

Laying a Straight Wall

Drainage Behind Walls

Even a low wall that holds back a little soil needs drainage. Water building up behind it pushes on the blocks and can crack or tip the wall. Leave weep holes in the bottom course, or lay a perforated pipe in gravel behind the wall, and backfill with free-draining stone.

Understanding Your Result

The headline gives the courses, blocks per course, and total full and half blocks.

The courses line explains the running bond pattern.

The caps line gives the cap blocks.

The mortar and footing lines give the materials for joints and foundation.

When Should You Use This Calculator?

Use it when planning a garden wall, planter or low retaining wall.

Use it to order full, half and cap blocks separately.

Use it to plan the footing before the blocks arrive.

Common Mistakes

Counting blocks by area only. Running bond needs half blocks.

Ignoring the 8-inch module. Odd sizes mean cutting blocks.

Building on soil without a footing. Walls crack and lean.

Leaving cores open on top. Water freezes inside.