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.

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.

How This Calculator Works

The calculator sizes the wall by face area, not volume. It multiplies wall length by wall height to get square feet, then divides by the face area of one block including its mortar joint. A standard CMU shows a 16×8-inch face, but with a 3/8-inch joint the block effectively occupies (16 + 0.375) × (8 + 0.375) ÷ 144 = about 0.95 square feet. That is why the rule of thumb “about 1.125 blocks per square foot” works out to roughly 1.05 once the joint is counted.

Raw block count is square footage divided by that per-block face area. The tool then multiplies by your waste factor (5% default) and rounds up, because you cannot buy a fraction of a block and you will crack a few. Mortar is figured from a yield table: roughly 2, 3, or 4 bags of mortar per 100 blocks for 4-, 8-, and 12-inch widths, since wider blocks have deeper bed joints that swallow more mud. Cost is simply blocks × block price plus mortar bags × mortar price.

A Worked Example

Take a 20 ft long, 8 ft tall garden wall in standard 8×8×16 block. Face area is 20 × 8 = 160 sq ft. Divide by 0.95 sq ft per block = 168 blocks before waste. Add the 5% waste factor and round up: 177 blocks. At 3 bags of mortar per 100 blocks that is ceil(1.77 × 3) = 6 bags. With blocks near $2.10 each and mortar near $9 a bag in 2026, the material runs about 177 × $2.10 + 6 × $9 = $425.70.

Estimator's tip: Order your block and mortar in the same delivery and stage them within a few feet of the wall — a mason laying 8x8x16 averages around 100 to 120 blocks a day, and double-handling a pallet kills that pace. I always add a full extra bag of mortar beyond the calculator's number on hot, dry days, because mud sets faster and you re-temper more, burning through it quicker than the per-100 yield assumes.

What Affects Your Block Count

• Mortar joint thickness — a 1/2-inch joint instead of 3/8 lowers your block count but raises mortar use.
• Block size: 12-inch units cover the same wall in the same count but need more mortar and cost more each.
• Waste factor — cut blocks at corners, openings, and the top course mean 5–10% is realistic.
• Openings for doors, windows, or vents that you can deduct from gross wall area.
• Whether you fill cells with grout and rebar, which is separate material this tool does not include.

Frequently Asked Questions

How many concrete blocks do I need per square foot?

About 1.05 standard 8x8x16 blocks per square foot of wall once a 3/8-inch mortar joint is included. Multiply your wall's square footage by 1.05, then add waste.

How much mortar do I need for concrete block?

Plan on roughly 3 bags of mortar per 100 standard blocks, 2 bags for thin 4-inch block, and 4 bags for 12-inch block. The calculator applies this automatically based on the size you choose.

What waste factor should I use for block?

Five percent is fine for a simple straight wall. Bump it to 8-10% for walls with many corners, openings, or curves where you will cut and discard more units.

Does this include the footing or grout fill?

No. The count covers blocks and bed/head-joint mortar only. Concrete for the footing and any grout, rebar, or cap block are separate line items.