Before you buy a single block, you need two numbers and a tape measure. Most block-count mistakes are not arithmetic. They are measuring the wrong rectangle, guessing the height, or forgetting that the garage door opening takes a big bite out of the wall. Get the measuring right and the count takes thirty seconds.
Read the wall with a tape
Start at one end of the wall and hook the tape on the corner. Pull it along the base to the other corner and write down the length in feet. Then measure up from the top of the footing to the top plate, because the block starts on concrete and ends under the roof framing. Multiply length by height to get the wall area in square feet. If the wall jogs or has a gable triangle, measure those pieces separately and add them on.
Subtract the openings
Do not count block where a door, window or garage door goes. Measure each opening the same way, length times height, and subtract it from the gross wall area. A two-car garage door alone is around 18 by 8 ft, which is 144 sq ft of block that does not exist. Forgetting that one opening is how people end up with 200 extra blocks in the driveway.
Pick the block size
The first number is how thick the wall is, and the face stays 16 inches long either way.
| Block size | Face area per piece | Blocks per square foot |
|---|---|---|
| 8x16 | 0.89 sq ft | about 1.12 |
| 6x16 | 0.67 sq ft | about 1.49 |
| 4x16 | 0.45 sq ft | about 2.22 |
An 8x16 block is the standard foundation and wall unit. A 6x16 is thinner and lighter, used on interior and garden walls. A 4x16 is the thinnest, for screen walls and panels that carry no load. The thinner the block, the more pieces you lay per square foot.
Walk a 20 by 8 wall
Say a garage wall runs 20 ft long and 8 ft high before you deduct the door. That is 160 sq ft of gross wall. Divide by 0.89 sq ft per 8x16 block and you get about 180 blocks. Add 5 percent waste for broken units and cut half-blocks at the ends, and the order lands near 190 blocks. Mortar runs at one bag per 100 blocks, so 190 blocks means two bags. If the garage door eats 144 sq ft out of that wall, redo the subtraction before you order.
Cinder block versus concrete block
People say both names and they usually mean the same grey rectangle. Cinder block uses cinders or ash as the light aggregate. Concrete block uses sand and gravel, which makes it denser and heavier. On the stringer, the face size is identical, so the count and the mortar do not change. Pick based on what the local yard stocks, not on the label.
Stack the load
Blocks move on pallets of roughly 100 to 120 pieces weighing about a ton. A wall of 190 blocks is two pallets plus two mortar bags, so budget for a delivery or a trailer. Have the blocks on site before the mason arrives, because a crew that waits on material bills you while they stand around.
A block wall goes up straight when the length and height are taped, the openings are subtracted, and the block count covers the 5 percent waste. Measure twice, mix once.
Concrete Block Calculator
Learn how to read a wall with a tape, compare 8x16, 6x16 and 4x16 block faces, and order mortar at one bag per 100 blocks.
How to use
- Measure the wall length in feet along the base
- Measure the wall height from footing to top plate
- Subtract doors and windows from the wall area
- Choose 8x16, 6x16 or 4x16 block size
- Add 5 percent waste for broken units
- Read the block count and mortar bag count
Supported formats and limits
- Estimate only and assumes a solid block face
- Standard 8x16 block covers 0.89 sq ft each
- Mortar runs about one bag per 100 blocks
- Openings are deducted by hand before ordering
Examples
20 by 8 ft wall with 8x16 blocks needs about 190 blocks and 2 bags of mortar.Common errors
- Wall length and height must be positive
- Opening area cannot exceed the wall area
- Waste factor must sit under 20 percent
Privacy and browser support
All calculations run locally in your browser. No measurements are uploaded.
FAQ
How do I measure a block wall before ordering
Run a tape along the base for length, then from the footing to the top for height. Multiply the two, then subtract every door and window opening.
How many 8x16 blocks go on a square foot
One 8x16 block covers 0.89 sq ft with the mortar joint, so a wall runs about 1.12 blocks per sq ft.
What is the difference between 8x16, 6x16 and 4x16 block
The number is the wall thickness. 8x16 covers 0.89 sq ft, 6x16 covers 0.67, and 4x16 covers 0.45 sq ft per piece.
How many mortar bags do I mix
Plan one bag of mortar per 100 blocks. A wall of 190 blocks takes two bags, which is the practical minimum.
Is a cinder block the same as a concrete block
They lay the same and count the same. Cinder block uses cinders as aggregate, while concrete block uses sand and gravel. The face size and the math do not change.
Do I fill the blocks solid with concrete
Load-bearing walls and walls with rebar get filled. Garden and screen walls stay hollow, which saves concrete and weight.