How Many Concrete Blocks Do I Need? CMU, Mortar & Grout Guide

A concrete block wall estimate has four moving parts: the blocks themselves, the mortar that beds them, the grout that fills the cores, and the rebar in bond beams. Miss any one and the bid is wrong. Here is how each quantity is derived from the same wall dimensions.

Block count: wall area ÷ 0.8889

A standard block face is 8 × 16 inches regardless of whether the block is 6, 8, or 12 inches wide. That face covers 0.8889 square feet, so every 100 square feet of wall needs about 112.5 blocks before waste. Multiply wall length × height, divide by 0.8889, then add a cutting and breakage allowance 5% is typical for straight runs, more for walls with many openings or corners.

Mortar: 1 bag per 15 blocks

Type S mortar for an 80-lb bag covers roughly 15 standard blocks (manufacturer data sheets range from about 12 to 20 depending on joint fullness and block texture). Compute bags from the net block count broken blocks use no mortar then apply a mortar waste allowance on top, typically around 10% for joints, cuts, and spills.

Core-fill grout: spacing math that matters

Block cores sit on 8-inch centers, so the 'O.C.' spacing of grouted cores divides neatly: grouting every 24 inches fills every 3rd core, every 32 inches fills every 4th, and every 48 inches fills every 6th. A solid-grouted 8×8×16 block holds about 0.32 cubic feet of grout; scale down for 6-inch block and up for 12-inch.

This is the quantity most hand estimates get wrong usually by half because the cores look small until you multiply by hundreds of blocks. Divide total cubic feet by 27 for cubic yards when ordering from the ready-mix plant.

Bond beams: rebar in 20-ft sticks

Bond-beam courses are grouted solid along their full length with two continuous bars. Count the bars in 20-foot sticks, rounded up, and include a lap allowance (about 10%) because sticks must overlap at splices. Each bond-beam block also takes its own grout roughly 0.12 cubic feet per 8-inch knockout block.

Checklist before you price

• Wall area measured, openings noted
• Block count with waste, from net area ÷ 0.8889
• Mortar bags from net blocks ÷ 15, plus waste
• Grout yards from core spacing and block width
• Bond-beam sticks with lap allowance