Skip to content

Concrete Block Calculator

How many blocks, how much mortar and how much core fill your wall needs, using the National Concrete Masonry Association (NCMA) estimating method.

Wall
sq ft
Width × height of each opening, added together. Leave 0 for a solid wall.
Block
Block
All of these have the same 8 × 16 in face, so the block count is the same. Width changes the core fill.
%
5% covers cuts and breakage on a simple wall (NCMA). Use 10% for many corners or openings.
Mortar and core fill
Mortar and core fill
Reinforced walls are grouted in the cells that hold rebar. Your plans or local code set the spacing.
Prices (examples: use your store's)
Prices (examples: use your store's)
$each
$per bag
Blocks to buy 142 blocks For 120 sq ft of wall, including 5% waste.

Wall

Wall area
120 sq ft
Blocks before waste
135
Courses (rows) high
6
Blocks per course
23

Mortar

Mortar mix, 80 lb
11 bags

Core fill (grout)

Material cost

Blocks
$284.00
Mortar
$99.00
Total
$383.00

How many concrete blocks do I need?

Multiply the wall length by its height to get square feet, subtract doors and windows, then multiply by 1.125. A standard block has an 8 × 16 in face (with mortar joints), which covers 8/9 of a square foot. Add 5% for cuts and breakage. A 30 ft × 4 ft wall is 120 sq ft, which is 135 blocks, or 142 blocks with waste.

Worked example: a 30 × 4 ft garden wall

A concrete block wall 30 feet long and 4 feet high: 6 courses of 8 inch blocks, about 23 blocks per course, laid in a running bond. One block = 8 × 16 in of wall 30 ft long → 23 blocks per course 4 ft → 6 courses
  1. Area: 30 ft × 4 ft = 120 sq ft. No openings to subtract.
  2. Blocks: 120 × 1.125 = 135 blocks. Add 5% waste: 135 × 1.05 = 141.75, rounded up to 142.
  3. Layout: 4 ft is 48 in, which is 6 courses of 8 in. Each course is 360 in ÷ 16 in = 22.5 blocks, so you lay 23 per course, with one cut in half.
  4. Mortar: one 80 lb bag of mortar mix lays about 13 blocks (Quikrete), so 135 ÷ 13 = 10.4, rounded up to 11 bags.
  5. Cost: at example prices of $2 a block and $9 a bag of mortar, the materials come to about $383.00. Prices vary by store and region, so enter yours in the calculator.

If the same wall were solid grouted (every cell filled), it would need 1.2 × 36.1 = 43.3 cu ft of grout, about 1.6 cubic yards. That is 66 bags, or one small ready-mix delivery, which is usually far cheaper.

Blocks, mortar and grout for common wall sizes

For 8 in block with a 5% waste allowance, mortar-mix bags at 13 blocks per bag, and grout for a solid-grouted wall.

Wall (length × height)AreaBlocksMortar bags (80 lb)Solid grout
Per 100 sq ft100 sq ft11991.34 cu yd
10 × 3 ft30 sq ft3630.40 cu yd
20 × 4 ft80 sq ft9571.07 cu yd
30 × 4 ft120 sq ft142111.60 cu yd
30 × 6 ft180 sq ft213162.41 cu yd
100 × 3 ft (crawlspace)300 sq ft355264.01 cu yd
50 × 8 ft400 sq ft473355.35 cu yd

Block counts match NCMA TEK 4-2A, which gives 119 units per 100 sq ft including about 5% waste. Grout uses TEK 4-2A Table 3 (36.1 cu ft per 100 sq ft for 8 in block, all cells filled, 3% waste included).

Block sizes: what "8 × 8 × 16" really means

Block sizes are nominal: they include a 3/8 in mortar joint. A standard 8 × 8 × 16 block actually measures about 7⅝ × 7⅝ × 15⅝ in, so once it is laid with mortar it takes up exactly 8 × 16 in of wall.

The first number is the width (wall thickness): 4, 6, 8, 10 or 12 in. Every width has the same 8 × 16 in face, so the number of blocks for a wall does not change with width. Width changes the weight, the strength and how much grout the cores take.

Corner blocks, bond-beam blocks and pilaster blocks count toward the same total. NCMA recommends working out how many of those specials you need, then taking them off the total for standard blocks.

Mortar: bagged mix or mixing your own

  • Mortar mix (80 lb bags) is pre-blended cement, lime and sand. Just add water. Quikrete says one bag lays up to 13 standard blocks, and NCMA's rule of thumb is 16. We use 13 so you don't run short.
  • Masonry cement (70 lb bags) plus sand costs less for big walls. NCMA says one bag lays about 30 blocks, and you need about 1 ton of masonry sand for every 8 bags (add half a ton for waste if you need more than 3 tons).
  • Which type? Type S is the usual choice below ground and for retaining or load-bearing walls. Type N is the usual choice above ground for walls that carry no load. Your plans or local code have the final say.

Core fill (grout): when and how much

Hollow block walls get their strength from vertical rebar set in grouted cells. A wall can be solid grouted (every cell filled) or grouted only at the rebar, for example every 32 or 48 in. The calculator uses NCMA's grout table for each wall width and spacing, which already includes 3% waste.

A bag of preblended grout yields only about 0.66 cu ft, so bags make sense for a few cells. Once you need about a cubic yard or more, order ready-mix grout, which is usually far cheaper. Don't grout 4 in block: NCMA advises against it because the cores are too small to fill properly.

Before you order

  • Measure the wall's finished size and subtract every door and window opening.
  • Use 10% waste instead of 5% if the wall has many corners, openings or cut blocks.
  • Ask for the pallet count and pallet price. Yards often sell full pallets cheaper per block, and the number of blocks on a pallet varies by plant and block size.
  • Plan the footing. Block walls sit on a concrete footing. The concrete calculator works out its volume.
  • Over 4 ft, or holding back soil? You will usually need an engineered design and a permit. Check with your building department first.

Frequently asked questions

How many concrete blocks are in a square foot?
Standard 8 × 16 in blocks: 1.125 per square foot, because each covers 8/9 of a square foot. That is 112.5 blocks per 100 sq ft, or about 119 with 5% waste. Half-high (4 × 16 in) blocks take twice as many.
How many blocks are in one course of a wall?
Divide the wall length in inches by 16. A 20 ft wall is 240 in ÷ 16 = 15 blocks per course. Each course adds 8 in of height, so a 4 ft wall has 6 courses.
How many bags of mortar do I need for 100 blocks?
About 8 bags of 80 lb mortar mix (100 ÷ 13, rounded up), using Quikrete's yield. If you mix masonry cement with sand, it is about 4 bags of 70 lb masonry cement and half a ton of sand.
How much concrete does it take to fill a block wall?
For 8 in block with every cell filled, about 36 cu ft of grout per 100 sq ft of wall (NCMA), or roughly 0.32 cu ft per block. Grouting only every 32 in takes about a quarter of that. Use grout, not regular concrete mix: grout is made fluid enough to fill the cells completely.
Are cinder blocks and concrete blocks the same thing?
In everyday use, yes. "Cinder block" is the old name for blocks made with coal cinders as the aggregate. Almost every block sold today is a concrete masonry unit (CMU) made with sand and gravel or lightweight aggregate. This calculator works the same for both.
What is the actual size of an 8 × 8 × 16 block?
About 7⅝ × 7⅝ × 15⅝ in. The 3/8 in mortar joint brings it up to the nominal 8 × 8 × 16 in when laid.
Does the cost estimate include labor?
No. It covers blocks, mortar and grout at the prices you enter. It doesn't include rebar, the footing, delivery, tool rental or a mason's labor.

Related calculators

Concrete CalculatorCubic yards and 40, 60 or 80 lb bags for slabs, footings, posts and steps.Retaining Wall CalculatorWall blocks, caps, base gravel and drainage stone for a block retaining wall.Rebar CalculatorRebar grid for a slab: bars, linear feet, 20 ft sticks and weight.Concrete Cost CalculatorReady-mix vs bagged concrete: total cost, short-load fees and which is cheaper.

Sources and method

Last checked October 12, 2026. How we build and test every calculator: methodology.