Concrete Block (CMU) Calculator

A standard CMU measures 7 5/8 × 15 5/8 inches, but with a 3/8-inch mortar joint it occupies a nominal 8 × 16 inch space on the wall face. That is 128 square inches, or 8/9 of a square foot (0.889 ft²). Divide 1 by 0.889 and you get 1.125 blocks per square foot. The figure is identical for 4, 6, 8, 10, and 12-inch thick block, because thickness changes weight, price, and footing width — not face coverage. Only a different face changes the count: an 8 × 8 inch half block or a 4 × 16 inch soap each cover half as much, so both run 2.25 per square foot.

Budget roughly one 80 lb bag of Type S mortar per 28 standard blocks, laid face-shell bedded with 3/8-inch joints. The 378-block example wall works out to 14 bags. That allowance already assumes normal droppings and board waste; the first course, corners, control joints, and any bond beam get full-bed joints and eat noticeably more. Buy one or two spare bags — dry mortar keeps for months in a sealed bag on a pallet, and running out mid-course leaves you with a cold joint you cannot tool properly.

Treat this as a planning estimate only. Any block wall that retains soil, carries a roof or floor load, or stands more than roughly 3 to 4 feet freestanding is a structural element. Most jurisdictions require a permit plus an engineered footing, specified vertical rebar, and grouted cores. The International Residential Code and your local amendments govern footing depth below the frost line, reinforcement spacing, and wind or seismic bracing. Have a licensed engineer or your building department review the design before you order block — the material count is the easy part of the job.

On a typical reinforced residential wall, vertical #4 or #5 bar sits in grouted cores at 32 to 48 inches on center, with extra bar at corners, wall ends, and each side of an opening, plus joint reinforcement or a bond beam at the top and often at mid-height. Grout is a different product from mortar — it is a soupy, high-slump mix. A grouted 8-inch core takes roughly 0.2 cubic feet per vertical foot, so an 80 lb bag of core-fill grout (about 0.6 ft³) fills around 3 feet of core. Your engineer stamps the actual spacing.

No — the estimate covers block and mortar only. Add footing concrete, gravel base, rebar, grout, cap block, and any parge or veneer separately. A common residential footing for an 8-inch wall is 16 inches wide by 8 inches deep, which is about 0.033 cubic yards per linear foot, so a 40-foot wall needs roughly 1.3 yards of concrete. Labor is also excluded: block laying commonly runs about $8 to $14 per square foot installed, varying with region, wall height, and how much reinforcement the drawings call for.