How to calculate how many concrete blocks you need
- Multiply the wall length by its height in feet to get square feet.
- Subtract the area of doors, windows and other openings.
- Multiply by 1.125, the number of standard blocks in one square foot of wall.
- Add about 5 percent for cut and broken blocks, and round up.
- Divide the blocks by 12 to estimate 80 lb bags of mortar mix.
Worked example. A garage wall 40 ft long and 8 ft high is 320 ft². Take off a 3 × 7 ft door (21 ft²) and 299 ft² is left. 299 × 1.125 = 336.4, so 337 blocks before extra, and about 29 bags of mortar mix.
Concrete block sizes
Blocks are named by their nominal size, which includes a 3/8 inch mortar joint. A standard "8 × 8 × 16" block actually measures 7 5/8 × 7 5/8 × 15 5/8 inches; with joints, it fills an 8 × 16 inch space on the wall.
| Nominal size | Actual size | Blocks per ft² of wall |
|---|---|---|
| 8 × 8 × 16 in | 7 5/8 × 7 5/8 × 15 5/8 in | 1.125 |
| 6, 10 or 12 × 8 × 16 in | Same 7 5/8 × 15 5/8 in face | 1.125 |
Counting by courses
Each course is 8 inches tall, so a wall has (height in inches ÷ 8) courses, rounded up. Each course needs (length in inches ÷ 16) blocks, which is 0.75 blocks per foot. A 20 × 4 ft wall has 6 courses of 15 blocks: 90 blocks, the same answer as 80 ft² × 1.125. Counting courses helps when the top course needs half-height blocks or a cap.
How much mortar for concrete block?
Quikrete's data sheet puts an 80 lb bag of Mortar Mix at about 12 standard 8 × 8 × 16 inch blocks. Mortar is wasted as you work, so round up and keep a spare bag on hand. A block wall usually sits on a poured footing over compacted base; size the concrete with the concrete slab calculator and the base with the fill dirt calculator, and use the linear feet calculator to total rebar or cap stones along the top.
Questions about concrete blocks
How many concrete blocks are in a square foot?
A standard 8 × 8 × 16 inch block covers 0.89 ft² of wall with its mortar joint, so you need 1.125 blocks per square foot, or 112.5 per 100 ft².
How many blocks do I need for a 4 ft high wall?
Each foot of wall length takes 0.75 blocks per course, and a 4 ft wall has 6 courses, so about 4.5 blocks per foot of length before extra for cuts.
How much mortar do I need for 100 blocks?
Quikrete lists about 12 standard blocks per 80 lb bag of Mortar Mix, so about 9 bags for 100 blocks.
Does block width change the count?
No. 6, 8, 10 and 12 inch wide blocks share the same 8 × 16 inch face, so the count per square foot is the same. Width changes the weight and the mortar used.
Sources
- 1.125 blocks per square foot for standard 8 × 8 × 16 in block; actual size 7 5/8 × 7 5/8 × 15 5/8 in: Titan America, Block Estimating Guide
- An 80 lb bag of mortar mix lays about 12 standard blocks: Quikrete, Mortar Mix data sheet
Last reviewed November 9, 2026. Found a wrong figure? Tell us.