How much mortar do I need per 100 block?

Roughly 12–14 cu ft of mortar per 100 block including joints and waste, which is about 25–28 bags of 80 lb mortar mix. If you mix your own from masonry cement and sand you will buy fewer bags of cement, since you are adding the sand separately.

How many courses will my wall have?

One course for every 8 in of height. An 8 ft wall is 12 courses; a 4 ft garden wall is 6. Half-height cap or bond-beam courses can shift this, so confirm against your actual block.

Do I need to grout the cores?

Only where the design calls for it — usually cells with vertical reinforcing, or a bond beam. Grouting is estimated separately because it depends on which cells you fill; use a grout calculator for that quantity.

CMU Block Wall Calculator

Enter a wall's length and height and get standard 8×8×16 block count, courses, blocks per course, and mortar bags.

Inputs

Wall size

ft ×

Wall height

Mortar waste — bed + head joints

Results update as you type — no submit

Blocks to buy

180 block

Courses12 courses

Blocks per course15 / course

Mortar (80-lb bags)50 × 80 lb bags

Wall area160 sq ft

Formula

blocks = 160 sq ft × 1.125 /ft² = 180 · courses = 8′ ÷ 8″ = 12
mortar = 180 × 0.125 cu ft +10% → ceil ÷ 0.5 = 50 bags

Diagram — wall elevationNTS

MeasureShedmeasureshed.com/calculator/cmu-block-wall-calculator

CMU block wall — material listUNITS: FT · SCALE: NTS

Inputs

Wall size20 × 8 ft

Block8×8×16 nominal

Mortar waste+10%

Results

Blocks (8×8×16)180

Courses12

Blocks per course15

80-lb mortar bags50

Wall area160 sq ft

Wall elevationNTS

Shopping list

☐ 180 × 8×8×16 CMU block

☐ 50 × 80 lb mortar mix

Formula

blocks = 160 sq ft × 1.125 /ft² = 180

courses = 8′ ÷ 8″ = 12 · per course = 15

mortar bags = ceil(180 × 0.125 × 1.1 ÷ 0.5) = 50

Estimates for planning only — verify before purchaseSHEET 1 OF 1

How it works

A standard concrete block is called an 8×8×16, and those are nominal dimensions — they already include the 3/8-inch mortar joint. So one block presents a face that is 16 inches long by 8 inches high, which works out to exactly 1.125 blocks for every square foot of wall. Multiply your wall area by 1.125 and round up, and you have the block count.

The same nominal sizing makes the rest of the layout fall out cleanly. Every 8 inches of wall height is one course, and every 16 inches of length is one block in that course. The calculator reports both, so you can sanity-check the count and see how the wall stacks before you order.

Mortar is the fuzzy part. A common estimating figure is about one-eighth of a cubic foot of mortar per block once you include bed joints, head joints, and normal waste. Divide that total by the yield of a bag of mortar mix — about half a cubic foot for an 80-pound bag — to get bags. Buying finished mortar mix takes more bags than site-mixing masonry cement with sand, so treat the bag count as a safe upper bound.

Worked example

Wall 20 ft × 8 ft → 160 sq ft
blocks = 160 × 1.125 = 180 · courses = 8 ft ÷ 8″ = 12 · per course = 15
mortar = 180 × 0.125 cu ft = 22.5 cu ft → +10% ÷ 0.5 = 50 × 80 lb bags

Questions from the jobsite

About 1.125 standard 8×8×16 blocks per square foot of wall, because the nominal 16″ × 8″ face covers 128 sq in and there are 144 sq in in a square foot. That is the same as 112.5 blocks per 100 sq ft.

SOURCES — NCMA nominal 8×8×16 module (1.125 block/ft²); mortar per QUIKRETE Mortar Mix No. 1102

Estimates for planning only — verify before purchase