How to use the concrete block calculator
- Pick the block size. Most walls use standard 8×8×16 blocks (also called CMU or cinder blocks).
- Enter the wall length and height above the footing.
- Add any openings, such as a door or window, so they are taken off.
- Read your total with breakage included, plus the number of rows and bags of mortar.
The block formula
Net wall area = length × height − openings
Blocks = net area × blocks per sq ft × (1 + waste), rounded up
Blocks per sq ft = 144 ÷ (nominal length × nominal height in inches)
For a standard block, 144 ÷ (16 × 8) = 1.125 blocks per square foot. Nominal sizes include the 3/8 in mortar joint, so you do not need to add joints separately.
Worked example
A garden wall 20 ft long and 8 ft high
- Wall area = 20 × 8 = 160 sq ft.
- Blocks = 160 × 1.125 = 180.
- Add 5% for breakage: 180 × 1.05 = 189 blocks.
- Rows: 8 ft = 96 in ÷ 8 in = 12 rows of 15 blocks.
- Mortar at about 11 sq ft per 80 lb bag: 160 ÷ 11 = 14.5, so 15 bags.
That wall is taller than a typical garden wall, which is why the calculator flags it for engineering.
Block sizes
| Nominal size (T × H × L) | Actual size | Blocks per sq ft | Blocks per 100 sq ft |
|---|---|---|---|
| 8 × 8 × 16 in | 7 5/8 × 7 5/8 × 15 5/8 in | 1.125 | 112.5 |
| 6 × 8 × 16 in | 5 5/8 × 7 5/8 × 15 5/8 in | 1.125 | 112.5 |
| 12 × 8 × 16 in | 11 5/8 × 7 5/8 × 15 5/8 in | 1.125 | 112.5 |
| 4 × 8 × 16 in (half height) | 3 5/8 × 3 5/8 × 15 5/8 in | 2.25 | 225 |
| 8 × 8 × 8 in (half block) | 7 5/8 × 7 5/8 × 7 5/8 in | 2.25 | 225 |
Thickness does not change the count, only the weight and strength. A standard hollow 8 in block weighs roughly 35 to 40 lb, so plan how you will move them.
Common mistakes to avoid
- Using actual size instead of nominal. Dividing by 15 5/8 × 7 5/8 forgets the joint and over-orders by about 5%.
- Measuring from the ground, not the footing. Count from where the first row sits.
- No allowance for breakage. Blocks chip in delivery and cutting. Keep at least 5% extra.
- Skipping the footing. Even a garden wall needs a solid, level footing. Use the concrete calculator to size the pour.
Frequently asked questions
How many concrete blocks are in a square foot?
A standard 8×8×16 block has a nominal face of 8 × 16 inches including the mortar joint, which is 0.889 sq ft. That works out at 1.125 blocks per square foot, or 112.5 blocks per 100 sq ft of wall.
How many blocks do I need for a 20 ft wall 8 ft high?
The wall is 160 sq ft. At 1.125 blocks per sq ft that is 180 blocks, or 189 with 5% extra for breakage. It is 12 rows of 15 blocks.
How much mortar do I need for concrete blocks?
One 80 lb bag of mortar mix lays up to about 13 standard blocks according to manufacturers, which is roughly 11 to 12 sq ft of wall. Allow a little more than that if you are new to block laying.
What does 8×8×16 mean?
Those are nominal sizes in inches: 8 in thick, 8 in high and 16 in long. The block itself is 3/8 in smaller in each direction (7 5/8 × 7 5/8 × 15 5/8) so that, with a 3/8 in mortar joint, it fills exactly 8 × 16 in of wall.
Do I need to count blocks for corners?
Corners use the same number of blocks overall, but you may want corner blocks or a few extra half blocks. The waste allowance usually covers this.