Concrete block calculator for CMU walls

Work from the area of the wall, not from courses and block lengths. A standard concrete block, with its mortar joint, takes up 8 × 16 in. of wall face. That is 128 sq in., or 8/9 of a square foot.

Blocks = net wall area × 1.125

Net area is the length times the height, less the doors and windows. Then add waste for broken and cut blocks. CMHA's TEK 4-2A, the concrete masonry industry's estimating note, works it out to 119 blocks per 100 sq ft with about 5% waste included. The calculator gives the same answer for a 100 sq ft wall: 112.5 blocks, plus 5%, rounded up to 119.

The count is the same for 4, 6, 8, 10 and 12 in. block, because they all share the 8 × 16 in. face. Width changes the weight, the grout and the price, not the number of blocks.

Corner blocks, bond beam blocks, lintels and pilaster units are counted separately. Count them from the plan and take that number off the plain block total so you do not buy both.

Mortar is the number people get wrong most often, because it depends on the mix, the mason and the job. The figures below assume face shell bedding, the normal way to lay hollow block: mortar on the two face shells, not across the webs. Because only the face shells are bedded, the amount does not change with block width.

Bagged mix costs more per block but saves batching and gives a consistent mortar, which suits small walls and repairs. Masonry cement and sand is the usual choice on bigger walls. CMHA adds a tip for sand: on jobs that use more than 3 tons, add half a ton for waste. A ton of damp loose sand is about 25 ft³.

If the blocks are fully bedded, as some engineers specify for the first course or for grouted walls, plan on more mortar than this.

Grout is the fluid concrete poured into the cores to lock in rebar and make the wall act as one piece. How much you need depends on the block width and how many cores get filled. The table is CMHA TEK 4-2A Table 3, in cubic feet of grout per 100 sq ft of wall. It assumes two-core block and already includes 3% waste.

Divide cubic feet by 27 for cubic yards when you order from a ready-mix plant or a grout pump. For bagged grout, an 80 lb bag of preblended grout yields about 0.66 ft³.

Bond beams. A bond beam is a course of U-shaped or knockout block filled with grout and a horizontal bar, often at the top of the wall. TEK 4-2A Table 5 gives the yield per 80 lb bag: 2.7 linear ft of 6 in. bond beam, 2.0 ft of 8 in. and 1.6 ft of 12 in. The calculator adds that to the core grout. TEK 4-2A gives no figure for 10 in. bond beams, so the calculator leaves those out and says so.

4 in. block is not grouted. CMHA does not recommend it: the cores are too small to place and consolidate grout properly.

A 40 ft garage wall, 8 ft high, 8 in. block, one 3 × 7 ft door. Net area is 320 − 21 = 299 sq ft. Blocks: 299 × 1.125 = 336.4, plus 5% = 354. Mortar mix at 13 blocks a bag: 28 bags. Grouted every 48 in. with a bond beam on top: 2.99 × 6.1 = 18.2 ft³ in the cores, plus 40 ft ÷ 2.0 ft a bag × 0.66 = 13.2 ft³ in the bond beam, 31.4 ft³ total. That is 1.16 cu yd, or 48 bags of preblended grout. Rebar every 48 in.: 11 vertical bars of 8 ft plus 40 ft in the bond beam, 128 ft before laps.

A 30 ft planter wall, 4 ft high, 6 in. block, grouted solid. 120 sq ft. Blocks: 135 plus 5% = 142. Masonry cement at 30 blocks a bag: 5 bags and 0.6 tons of sand. Grout: 1.2 × 25.6 = 30.7 ft³, 1.14 cu yd.

A 100 ft commercial wall, 10 ft high, 12 in. block, grouted every 24 in. with two bond beams. 1,000 sq ft. Blocks: 1,125 plus 5% = 1,182. Portland cement and lime mortar at 62 blocks a bag: 20 bags of cement, 10 bags of lime and 3.4 tons of sand. Grout: 10 × 19.7 = 197 ft³ in the cores plus 200 ft of bond beam ÷ 1.6 × 0.66 = 82.5 ft³, 279.5 ft³ total, or 10.35 cu yd. On a wall this size, order grout by the yard from a plant rather than by the bag.

The calculator counts one vertical bar per grouted cell at the rebar spacing you pick, plus one at the end, each as long as the wall is high. It adds one horizontal bar per bond beam course along the full length. It does not add lap splices, dowels into the footing or corner bars, because those come from the drawings: add them to the footage before you order.

• Courses. Wall height in inches ÷ 8. An 8 ft wall is 12 courses; a 4 ft wall is 6.
• Block weight. Ask your supplier for the weight of their units before you plan how many go on a pallet, a truck or a scaffold.
• Footing. The wall sits on a concrete footing that is figured separately, from the footing size on the drawings. For flatwork next to the wall, the concrete driveway cost calculator prices a slab.

Once you have the counts, the price is materials plus labor plus markup. Price each line from your own supplier: block by the unit or the pallet, mortar and grout by the bag or the yard, rebar by the foot or the 20 ft stick. Then add the labor to lay the wall, usually priced by the square foot or by the block, plus a pump or a mixer for grout if the job needs one.

Press "Save as an estimate in Koira" to carry the block, mortar, grout, rebar and wall area over as lines. Add your prices there and send it. To set the labor rate and the markup, use the labor rate calculator and the markup calculator. To see how Koira keeps estimates, jobs and invoices in one place, see estimates.