Concrete Block Wall Calculator

About the Concrete Block Wall Calculator

A hollow concrete block wall gets much of its strength from what goes inside it. Steel reinforcing bars set vertically through the hollow cores, and horizontally in a bond beam, turn a stack of blocks into a wall that can resist wind, soil pressure and earthquakes. The bars only work when the cores around them are filled solid with grout, a fluid concrete mix. Knowing how much grout and rebar a wall needs is essential for ordering and for planning the pour.

This concrete block wall calculator estimates the grout and rebar for a reinforced wall of standard 8-inch concrete masonry units. Choose whether to grout only the cells with rebar plus a bond beam at the top, or to grout every core solid. It gives the grout in cubic feet and cubic yards, the number of vertical bars, the total length of rebar to order, and the bagged mix needed if you are mixing by hand.

How to Use the Concrete Block Wall Calculator

Enter the wall length and wall height in feet.

Choose the vertical rebar spacing and the grouting method.

Enter the extra bar length per vertical for the lap with footing dowels.

The Formulas

  vertical bars   = ⌈length × 12 ÷ spacing⌉ + 1
  cells only      = bars × height × 0.173 ft³ + length × 0.173 ft³ (bond beam)
  solid grouted   = length × height × 0.26 ft³
  rebar           = bars × (height + lap) + 2 × length × 1.1
  20 ft bars      = ⌈rebar ÷ 20⌉
  bagged mix      = ⌈grout ft³ ÷ 0.6⌉ × 80 lb bags

The figures 0.26 cubic feet of grout per square foot of wall, and 0.173 cubic feet per foot of height for a single grouted cell or per foot of bond beam, are typical for 8-inch blocks.

Step-by-Step Example

A 20 × 8 ft wall with bars every 48 inches and 2 ft laps.

  bars          ⌈240 ÷ 48⌉ + 1 = 6
  grout cells   6 × 8 × 0.173 = 8.3 ft³
  bond beam     20 × 0.173 = 3.5 ft³
  total         11.8 ft³ = 0.44 yd³
  solid grout   20 × 8 × 0.26 = 41.6 ft³ = 1.54 yd³
  rebar         6 × 10 + 2 × 20 × 1.1 = 104 ft → six 20 ft bars

Partially or Solidly Grouted?

Most garden walls, fences and many house walls are partially grouted: only the cores with steel are filled, plus bond beams. This saves grout and keeps the wall lighter. Solid grouting fills every core. It is used for retaining walls holding back high soil, walls that need extra mass for sound or fire resistance, and in high seismic areas where codes require it. It uses about three and a half times as much grout as the partial example above. Your plans or local code decide which is required.

Rebar Placement

Vertical bars are placed at the spacing shown on the plans, and also at each corner, wall end and beside each door or window opening. They usually lap with short dowels cast into the footing, with a lap length of about 40 to 48 bar diameters, which is why each vertical bar is longer than the wall is high. At the top, a bond beam course of U-shaped or knock-out blocks carries horizontal bars around the whole wall. Additional bond beams may be required at floor levels and over openings, and ladder-type joint reinforcement is often laid in the mortar every other course.

Grout Versus Mortar and Concrete

Grout is not the same as mortar. Mortar is a stiff paste that bonds the blocks; grout is a fluid mix, usually cement, sand and small pea gravel, made runny enough to flow down the cores and around the steel without leaving voids. It is placed in lifts and consolidated with a rod or small vibrator. Ready-mix suppliers deliver grout by the yard and a pump makes large walls quick. For small walls, bagged grout or a thinned concrete mix can be used where the code allows.

Cleanouts and Pour Heights

Before grouting tall walls, mortar droppings must be cleared from the bottom of each core so grout can bond to the footing. Cleanout openings in the bottom course allow this. Codes limit how high grout can be poured in one lift, often 5 feet, and how high the wall can be built before grouting, so plan the work in stages for tall walls.

Ordering Grout

Ready-mix plants supply grout by the cubic yard, often with a minimum load, and charge extra for small loads. Add about 10% to the calculated volume for waste, spillage and cores that take more than expected. A grout pump or concrete pump with a small hose makes filling tall walls much faster and cleaner than buckets, and is worth hiring for anything more than a small garden wall.

Inspection

Many building departments inspect reinforced masonry before the grout is placed, to check bar sizes, spacing, laps and cleanouts. Schedule the inspection before ordering grout, so the pour is not held up.

Understanding Your Result

The headline gives the grout in cubic yards.

The working line shows how the grout volume was worked out.

The rebar line gives the vertical and bond-beam bars and the 20 ft bars to order.

The bagged mix line gives the bags if mixing by hand.

When Should You Use This Calculator?

Use it when ordering grout and rebar for a reinforced block wall.

Use it to compare partial and solid grouting.

Use it to check an estimate for a block foundation or retaining wall.

Common Mistakes

Using mortar to fill cores. Grout must flow around the steel.

Forgetting bars at corners and openings. Plans usually require them.

Leaving out the lap length. Vertical bars must overlap the dowels.

Pouring too high in one lift. Follow the code's pour height.