How Many Concrete Blocks for a Wall? 1.125 per Square Foot

Standard 8 × 8 × 16 in blocks lay at 1.125 per square foot, so a 20 × 4 ft wall (80 sq ft) takes 90 blocks, or 95 with 5% spare, and 8 bags of 80 lb mortar mix. In metric, 440 × 215 mm blocks lay at about 9.9 per m² and 390 × 190 mm blocks at 12.5 per m².

Blocks and mortar by wall size

8 × 8 × 16 in blocks with 5% spare, and 80 lb bags of mortar mix
Wall	Area	Blocks	Mortar bags
10 × 4 ft	40 sq ft	48	4
20 × 4 ft	80 sq ft	95	8
20 × 6 ft	120 sq ft	142	12
30 × 6 ft	180 sq ft	213	18
40 × 8 ft	320 sq ft	378	32

Mortar uses Quikrete's usage chart: about 12 standard blocks per 80 lb bag of Mortar Mix (9 per 60 lb bag).

Metric: blocks with 5% spare
Wall	Area	UK 440 × 215 mm	AU 390 × 190 mm
3 × 1 m	3 m²	32	40
5 × 1.2 m	6 m²	63	79
5 × 1.8 m	9 m²	94	119
10 × 1.8 m	18 m²	187	237

Why 1.125 per square foot?

An "8 × 8 × 16" block is a nominal size. The block itself is 7⅝ × 7⅝ × 15⅝ in, and the ⅜ in mortar joint makes up the rest, so each block plus joint covers exactly 16 × 8 in = 128 sq in. 144 ÷ 128 = 1.125.

Worked example: a 20 ft garden wall, 4 ft high

Area: 20 × 4 = 80 sq ft.
Blocks: 80 × 1.125 = 90. With 5% spare: 95.
Mortar: 95 ÷ 12 = 7.9, so 8 bags of 80 lb mix.

Before you build

Footings: block walls sit on a concrete footing. Work it out with the concrete calculator.
Height and loads: walls that hold back soil, carry a load or stand taller than about 4 ft (1.2 m) usually need reinforcement and may need a permit.
Openings: subtract doors and windows in the concrete block calculator.

Sources

Quikrete Mortar Mix data sheet (usage chart)