How many courses of block for a 6 ft wall?

9: a standard block is 8 in high with its joint, so courses are the height in inches divided by 8. 4 ft is 6 courses, 8 ft is 12. A height that is not a multiple of 8 in rounds up to the next course, or the footing is set lower to make it come out.

How many half blocks do I need?

Two per alternate course in running bond, so 8 for the 9 courses of a 6 ft wall and 6 for a 4 ft wall, at each end of every second course. Buy them as half blocks or cut whole ones with a masonry saw; the 5% waste covers the cuts. A wall length that is not a multiple of 16 in also needs a cut block in every course.

How big should the footing be for a block wall?

For a freestanding garden wall the usual rule is twice the block width and at least 8 in thick: 16 x 8 in for an 8 in block, which is 19.6 cubic feet (0.72 cubic yards, 33 bags of 80 lb) for 20 ft with 10% waste. The bottom goes below the frost line on undisturbed soil. Retaining walls and building foundations are sized by the code or an engineer, not by this rule.

How far apart does rebar go in a block wall?

Where the plan or the local code says: 32 in and 48 in are the common spacings for fence walls, 16 or 24 in for retaining walls and walls in wind or seismic zones, every cell for the strongest walls. For the 20 x 6 ft wall that is 6 bars at 48 in, 9 at 32 in, 16 at 16 in, each with a matching dowel out of the footing.

Do I need a bond beam?

A bond beam ties the top of the wall together: the top course is laid with bond beam blocks (the webs knocked down), two horizontal bars go in, and the course is grouted solid. Most codes ask for one at the top of a reinforced wall and at storey lines. On the 20 ft wall it is 40 ft of bar and the grout for the 15 blocks of the course, already counted in the 19 bags.

How much mortar per course?

15 blocks per course at 13 blocks per 80 lb bag is a bag and a bit per course, 12 bags for the 9 courses with 10% extra. The first course on the footing is laid on a full bed of mortar rather than the face shells; the extra covers it.

Building a block wall: courses, rebar, grout and mortar for a 20 ft wall

A 20 ft wall of standard 8 x 8 x 16 in block, 6 ft high, is 9 courses of 15 blocks: 135 blocks, 142 with 5% waste, 8 of them half blocks for the running bond, 12 bags of 80 lb mortar, 9 vertical bars every 32 in in 19 bags of grout with a bond beam on top, on a 16 x 8 in footing of 0.72 cubic yards. This guide walks the job in order and gives each number for 4, 6 and 8 ft walls.

Open the concrete block calculator

The job in order

Footing. Dig to below the frost line, pour a strip 16 in wide and 8 in thick (twice the block width, 8 in minimum) on undisturbed ground, and set the dowels while it is wet: one at every future bar position, 9 for the 20 ft wall at 32 in, bent into the footing and standing up at least 24 in. The concrete calculator does other footing sizes.
First course. Snap a line, lay the corner blocks first, then the course between them on a full bed of mortar, levelled both ways; every course after it is bedded on the face shells only. Blocks per course is the length in inches divided by 16, rounded up: 15 for 20 ft.
Running bond. Each course is offset half a block, so the second, fourth and every even course starts and ends with a half block: 8 halves in a 6 ft wall. Keep the cells that will take bars clean of mortar droppings.
Bars and grout. Drop the vertical bars into the cells over the dowels, lap them, and grout those cells in lifts of about 4 ft, rodding each lift. Bars every 32 in fill one cell in four; the cell fill guide has the grout for every spacing.
Bond beam. Lay the top course in bond beam blocks, set two horizontal bars on chairs, and grout the whole course. Cap blocks or a mortar wash finish the top.

Materials for a 20 ft wall by height

Standard 8 x 8 x 16 in block, bars every 32 in with a bond beam, 80 lb bags of mortar (13 blocks each, plus 10%) and of grout (0.6 cubic feet each), 5% waste on blocks and 10% on grout. The footing is 16 x 8 in with 10% waste.

20 ft wall, bars every 32 in, bond beam on top
Height	Courses	Blocks (+5%)	Half blocks	Mortar bags	Vertical bars	Grout bags	Footing
4 ft	6	95	6	8	9 × 4 ft	14	0.72 yd³
6 ft	9	142	8	12	9 × 6 ft	19	0.72 yd³
8 ft	12	189	12	16	9 × 8 ft	23	0.72 yd³

The same 6 ft wall at other lengths

6 ft wall, bars every 32 in, bond beam on top
Length	Blocks per course	Blocks (+5%)	Mortar bags	Vertical bars	Grout bags	Footing
10 ft	8	76	7	5	10	0.36 yd³
20 ft	15	142	12	9	19	0.72 yd³
30 ft	23	218	18	13	28	1.09 yd³
40 ft	30	284	23	16	37	1.45 yd³

What the bar spacing changes

The blocks and the mortar do not change with the spacing; the dowels, the bars and the grout do. Cells are 8 in apart, so the share of the wall that is grouted is 8 divided by the spacing, plus the bond beam course.

20 x 6 ft wall, bond beam on top
Bar spacing	Cells grouted	Vertical bars and dowels	Grout	Grout bags
16 in	50% + top course	16	18.2 cu ft	31
24 in	33% + top course	11	13.3 cu ft	23
32 in	25% + top course	9	10.9 cu ft	19
48 in	17% + top course	6	8.5 cu ft	15

Metric: 6 m wall of 215 mm hollow block

A 440 x 215 x 215 mm hollow block wall 1.8 m high is 8 courses (225 mm each) of 14 blocks (450 mm each): 112 blocks, 118 with waste, 8 of them halves, 11 bags of 25 kg mortar at 12 blocks a bag plus 10%. With bars every 900 mm (one cell in four, cells at 225 mm) and a bond beam it takes 8 bars and 32 bags of 25 kg concrete for 381 litres of grout. The footing rule is the same: 430 mm wide, 200 mm thick at least, below frost depth.

Before you start

Check the permit line: many places allow a fence wall up to 4 ft or 6 ft without one and want a permit and an engineer's drawing above that, for any retaining wall, and for anything that carries a roof.
Order from the concrete block calculator with your own length, height and openings; it takes doors and windows off in whole blocks and prices the list. The cost guide has what a mason charges to lay it.
Codes give mortar about two and a half hours from mixing: mix a bag at a time. Grout the cells the same day the bars go in, in lifts, and never let a lift sit half full overnight.

Frequently asked questions

How many courses of block for a 6 ft wall?: 9: a standard block is 8 in high with its joint, so courses are the height in inches divided by 8. 4 ft is 6 courses, 8 ft is 12. A height that is not a multiple of 8 in rounds up to the next course, or the footing is set lower to make it come out.
How many half blocks do I need?: Two per alternate course in running bond, so 8 for the 9 courses of a 6 ft wall and 6 for a 4 ft wall, at each end of every second course. Buy them as half blocks or cut whole ones with a masonry saw; the 5% waste covers the cuts. A wall length that is not a multiple of 16 in also needs a cut block in every course.
How big should the footing be for a block wall?: For a freestanding garden wall the usual rule is twice the block width and at least 8 in thick: 16 x 8 in for an 8 in block, which is 19.6 cubic feet (0.72 cubic yards, 33 bags of 80 lb) for 20 ft with 10% waste. The bottom goes below the frost line on undisturbed soil. Retaining walls and building foundations are sized by the code or an engineer, not by this rule.
How far apart does rebar go in a block wall?: Where the plan or the local code says: 32 in and 48 in are the common spacings for fence walls, 16 or 24 in for retaining walls and walls in wind or seismic zones, every cell for the strongest walls. For the 20 x 6 ft wall that is 6 bars at 48 in, 9 at 32 in, 16 at 16 in, each with a matching dowel out of the footing.
Do I need a bond beam?: A bond beam ties the top of the wall together: the top course is laid with bond beam blocks (the webs knocked down), two horizontal bars go in, and the course is grouted solid. Most codes ask for one at the top of a reinforced wall and at storey lines. On the 20 ft wall it is 40 ft of bar and the grout for the 15 blocks of the course, already counted in the 19 bags.
How much mortar per course?: 15 blocks per course at 13 blocks per 80 lb bag is a bag and a bit per course, 12 bags for the 9 courses with 10% extra. The first course on the footing is laid on a full bed of mortar rather than the face shells; the extra covers it.

How many concrete blocks do I need for a wall?
8x8x16 blocks per square foot and for walls from 10x4 to 40x8 ft with 5% waste, mortar volume, and the metric 440 x 215 mm block per square metre.
How much concrete to fill cinder blocks? (per block, per 100, per wall)
Grout per 8 in block (0.22 cu ft, three blocks per 80 lb bag), per 100 blocks and for 10 to 40 ft walls with every cell filled or bars every 16 to 48 in, plus metric.
How much does a cinder block wall cost? (materials per wall and per square foot)
Blocks, mortar and grout priced for walls from 10x4 to 40x8 ft at 2026 shelf prices, materials per square foot, what grouting adds, and HomeGuide's installed range.
How much does a cinder block garage cost? (blocks, mortar, grout and slab for 1, 2 and 3 car)
Blocks, mortar and grout for 1 to 3 car block garages, 12x20 to 36x24 ft, 8 ft walls, doors and windows taken off, plus the 4 in slab, at 2026 prices.

Last updated 2026-09-15. Figures are planning estimates from typical product data: check the yield, size or coverage printed on what you buy and round up.