Blocks, mortar and weight for a wall

Concrete Block Calculator

A 20 ft long, 4 ft high wall of 8×8×16 standard blocks takes 90 blocks, or 95 with 5% waste, in 6 courses, with 7 to 8 bags of 80 lb mortar. The blocks weigh about 3,600 lb. Enter your wall below.

Sources checked Oct 15, 2026

Your estimate

95 × 8×8×16 standard blocks

90 to lay plus 5% waste, in 6 courses of 15.

Wall elevation, running bondLive sketch

Each course is 8 in high and every other course starts with a half block. The wall is drawn at 4 ft, whole courses. A sketch of the count, not a structural drawing.

Wall face20 ft × 4 ft
80 sq ft
Blocks to lay80 sq ft × 1.125 blocks a sq ft, rounded up
90
Courses15 blocks a course; laid in whole courses the wall is 4 ft high
6
Weight of the blocks90 × 40 lb (Basalite); cells empty, mortar not counted
3,600 lb

Mortar: QUIKRETE Mortar Mix 1102, Nov 2020 sheet90 ÷ 13 blocks a bag (80 lb, Type N), rounded up
7 bags
Mortar: QUIKRETE Mortar Mix 1102, older undated sheet90 ÷ 12 blocks a bag (80 lb, Type N), rounded up
8 bags
Mortar: SAKRETE Mortar Mix Type S, US90 ÷ 12 blocks a bag (80 lb, Type S), rounded up
8 bags

Filling the cells with grout or concrete is not counted: no cell volume is in our data.

How to calculate concrete blocks for a wall

Multiply wall length by height. The example is 20 ft × 4 ft: 80 sq ft. Subtract door and window openings (none here).
Divide by the face of one block. An 8×8×16 block is laid as 8 × 16 in, or 1.125 blocks per square foot (144 ÷ 128): 80 × 1.125 = 90 blocks.
Add your waste allowance for cut and broken blocks. At 5% that is 95 blocks to buy.
Check the layout. The wall is 6 courses of 8 in, 15 blocks long (20 ft ÷ 16 in). Alternate courses start with a half block.
Count mortar from the blocks laid, using the blocks a bag lays: 90 ÷ 12 = 7.5 bags, or 8 bags rounded up; some labels say a bag lays more, which gives 7.
Multiply blocks by the unit weight for the weight of the wall: 90 × 40 lb = 3,600 lb, cells empty and mortar not included.

This is a count of materials for a straight wall, not a design. It does not set a footing, the steel, or whether the cells are filled; your building department and the wall’s designer do. For footing concrete see the concrete calculator and the concrete slab calculator, and for bars the rebar calculator.

Concrete block wall calculator: blocks for common walls

8×8×16 blocks at 4 ft high, and at 20 ft long. Blocks to lay, before waste.
Wall	Face	Blocks	Courses × per course	Mortar bags
10 ft × 4 ft	40 sq ft	45	6 × 8	4
20 ft × 4 ft	80 sq ft	90	6 × 15	7 to 8
30 ft × 4 ft	120 sq ft	135	6 × 23	11 to 12
40 ft × 4 ft	160 sq ft	180	6 × 30	14 to 15
20 ft × 2 ft	40 sq ft	45	3 × 15	4
20 ft × 4 ft	80 sq ft	90	6 × 15	7 to 8
20 ft × 6 ft	120 sq ft	135	9 × 15	11 to 12
20 ft × 8 ft	160 sq ft	180	12 × 15	14 to 15

Block sizes and concrete block weight

An 8×8×16 block is named for its nominal size. As made it is 7.625 × 7.625 × 15.625 in, a 0.375 in mortar joint short of the nominal size, so a laid block takes up 8 × 16 in of wall. Angelus Block, CMU Basics: Dimensions and Sizes

Block weight from one maker’s cut sheet (Basalite). It does not say whether the blocks are normal or lightweight concrete.
Block	Face laid	Blocks per sq ft	Weight each
8×8×16 standard	8 × 16 in	1.125	40 lb
6×8×16 standard	8 × 16 in	1.125	27 lb
8×4×16 solid	4 × 16 in	2.25	28 lb
8×8×8 half block	8 × 8 in	n/a	22 lb

Every weight on this page comes from that one cut sheet, so it is one maker’s block, not an average. Another maker’s, or a lightweight mix, can be different; ask your supplier before you plan a lift or a load. Basalite building blocks cut sheet, WA (sizes, weights, pallets)

How much mortar for concrete blocks

Bag labels print how many blocks one 80 lb bag lays. They do not agree, so the calculator shows each:

Blocks one 80 lb bag of mortar lays (8×8×16)

13 blocks (Type N, QUIKRETE Mortar Mix 1102, Nov 2020 sheet)QUIKRETE Mortar Mix No. 1102 data sheet (Nov 2020)
12 blocks (Type N, QUIKRETE Mortar Mix 1102, older undated sheet)QUIKRETE Mortar Mix No. 1102, older data sheet (undated)
12 blocks (Type S, SAKRETE Mortar Mix Type S, US)SAKRETE Mortar Mix Type S spec sheet, US (Rev. 05/18)

The form shows a bag count for each label, and the lead sentence gives the range. For 90 blocks that is 7 to 8 bags. If you buy once, buy the higher count; mix type, joint thickness and how much you waste change what a bag lays.

That is 7.7 bags to 100 blocks at 13 a bag and 8.3 at 12. Our data has these mortar counts for 8×8×16 blocks only, so 6×8×16 and 8×4×16 walls show no mortar number rather than a guess.

Concrete block wall weight

The form multiplies blocks by the unit weight: 90 × 40 lb = 3,600 lb, or 45 lb per square foot of face. It leaves out mortar and anything in the cells. A second source gives a much higher figure for a different basis:

8 in hollow block wall, no grout, per square foot of wall

45 lb (Basalite 8×8×16 at 40 lb × 1.125 blocks a sq ft; blocks only)Basalite building blocks cut sheet, WA (sizes, weights, pallets)
34 lb (CMHA assembly with mortar at 125 lb/cu ft unit density; 24 to 39 lb across 85 to 145 lb/cu ft)CMHA CMU-TEC-002-23, Weights and Section Properties of Concrete Masonry Assemblies (2023)

The form uses the maker’s block weight, the heavier figure, so a weight estimate errs on the heavy side. The CMHA table is a wall with its mortar at a stated concrete density; the Basalite sheet weighs one block and does not say its density, so the two cannot be reconciled from our data. For a load or a foundation, get the weight from your block supplier.

For the 20 ft × 4 ft example the two give 3,600 lb and 2,720 lb.

Block foundation and retaining wall blocks

For a block foundation wall, enter its length and height and subtract the openings to count the blocks and mortar. The footing under it, the steel in it and any grout in the cells are a design question; this page does not size them, and our data has no cell-fill volume. Use the concrete calculator for a footing’s volume and the rebar calculator for bars.

These are mortared masonry blocks. A garden or landscape wall of stacked segmental units, with setback, base stone and drainage, is a different product: use the retaining wall calculator.

Numbers this page uses

Blocks per sq ft, 8×8×16 nominal: 1.125Angelus Block, CMU Basics: Dimensions and Sizes
Block weight, 8×8×16 standard: 40 lbBasalite building blocks cut sheet, WA (sizes, weights, pallets)
Blocks per 80 lb bag, QUIKRETE Mortar Mix 1102, Nov 2020 sheet: 13 (Type N)QUIKRETE Mortar Mix No. 1102 data sheet (Nov 2020)
Blocks per 80 lb bag, QUIKRETE Mortar Mix 1102, older undated sheet: 12 (Type N)QUIKRETE Mortar Mix No. 1102, older data sheet (undated)
Blocks per 80 lb bag, SAKRETE Mortar Mix Type S, US: 12 (Type S)SAKRETE Mortar Mix Type S spec sheet, US (Rev. 05/18)
Waste allowance: 5%Our default; change it above

Concrete block calculator questions

How many concrete blocks do I need for a wall?

Divide the wall’s face area by the face of one block. An 8×8×16 block is laid as 8 × 16 in, 1.125 blocks per square foot. A 20 ft × 4 ft wall is 80 sq ft, so 90 blocks, or 95 with 5% waste.

How many 8x8x16 blocks are in a square foot?

1.125, counting the nominal 8 × 16 in face with its mortar joint (144 ÷ 128). The block itself is a joint short of that, 7-5/8 × 15-5/8 in.

How much mortar does a concrete block wall need?

90 blocks take 7 to 8 bags of 80 lb mortar mix, depending on the label: QUIKRETE’s November 2020 sheet says a bag lays up to 13 blocks, while its older sheet and SAKRETE say 12. Both are for 8x8x16 blocks; the page shows each.

How many bags of mortar per 100 blocks?

7.7 bags at 13 blocks a bag, 8.3 bags at 12. Round up to whole bags, and keep a spare for uneven joints.

How much does a concrete block weigh?

Basalite lists its 8x8x16 standard block at 40 lb, so 90 blocks weigh about 3,600 lb before mortar. That is one maker’s block, and the sheet does not say whether it is normal or lightweight concrete; ask your supplier for yours.

How do I calculate a block foundation?

Count the wall the same way, using the foundation wall’s length and height, and subtract door and window openings. This page does not size a footing, fill the cells or call for steel. For footing concrete and steel use the concrete calculator in cubic yards and the rebar calculator.

Is this the same as the retaining wall calculator?

No. Concrete masonry blocks are laid with mortar in a running bond. Segmental retaining wall units stack dry with setback and drainage stone. For those use the retaining wall block calculator.

A planning aid, not an engineering calculation: your local code and building inspector have the final word. Terms