Concrete Block Calculator

Planning a CMU wall, retaining wall, or foundation? Punch in the dimensions, choose your block size, and we'll calculate exact block counts, mortar bags, and an estimated cost — with a built-in waste factor so you don't run a block short on the last course.

Concrete Block Calculator

Wall length (ft)

Wall height (ft)

Block size (W × H × L)

Mortar joint (in)

Waste factor (%)

Price per block ($)

Price per mortar bag ($)

Block Wall Materials

Wall square footage—

Blocks needed (with waste)—

Mortar bags (70 lb, Type S)—

Estimated material cost—

Industry rule of thumb: 1.125 standard 8 × 8 × 16 in blocks per square foot of wall with a 3/8″ mortar joint. Mortar coverage runs about 3 bags (70 lb Type S) per 100 standard blocks. Bump that to 4 bags per 100 for 12″ blocks and drop to 2 per 100 for half-thickness 4″ blocks.

How to Calculate Block Counts

A standard concrete masonry unit (CMU) measures 7-5/8″ × 7-5/8″ × 15-5/8″, but with a typical 3/8″ mortar joint, each block effectively occupies 8″ × 16″ of wall — exactly 128 sq in, or 0.889 sq ft. That works out to 1.125 blocks per square foot, the number used universally for takeoffs.

Half-blocks (sometimes called "halves" or "stretchers cut short") are needed at corners and openings. They're typically counted by adding 5–10% on top of the wall-area block count — the waste factor in this calculator covers that plus chipped or broken blocks delivered to the site.

Don't Forget the Footing

This calculator covers the wall itself. You'll also need a continuous concrete footing below the wall — typically 16–24″ wide and 8–12″ deep — and rebar in both the footing and the cells of the wall every 32–48″ for structural strength. Use the concrete calculator to size the footing pour.