Calculate how many cinder blocks you need for a wall, plus the mortar bags and total cost, from your wall size or an existing block count.
Cinder Block Formula
The number of cinder blocks for a wall is the net wall area divided by the face area of one block, increased by a waste allowance.
Blocks = (WA - O) / FA * (1 + W/100)
To find the wall area you can build from a set of blocks, rearrange the same relationship:
WA = B * FA / (1 + W/100)
To find mortar and cost for a known block count:
Bags = B / BPB
Cost = B * PB + Bags * PMB
- Blocks: number of cinder blocks needed, rounded up to a whole block.
- WA: gross wall area (length times height).
- O: area of door and window openings you subtract from the wall.
- FA: face area of one block (face length times face height).
- W: waste or breakage percent added to cover cuts and damaged blocks.
- B: known number of blocks (used in the wall-size and cost modes).
- BPB: blocks laid per bag of mortar.
- PB: price per block.
- PMB: price per mortar bag.
The calculator works in three modes. In the blocks-needed mode you enter the wall size, by length and height or by total area, and it returns the block count after subtracting openings and adding waste. In the wall-size mode you enter how many blocks you have and it returns the area and approximate length they cover. In the cost mode you enter a block count and it returns the mortar bags and the total material cost. The face area comes from the block size you pick, and a standard cinder block has a nominal face of 16 inches by 8 inches.
Block Sizes and Coverage
Cinder block coverage depends on the nominal face size, which includes the 3/8 inch mortar joint. The values below use nominal dimensions, which is how blocks are estimated.
| Nominal face size | Face area (sq ft) | Blocks per 100 sq ft |
|---|---|---|
| 16 in x 8 in (standard CMU) | 0.889 | 113 |
| 8 in x 8 in (half block) | 0.444 | 225 |
| 16 in x 4 in | 0.444 | 225 |
| 12 in x 4 in (patio block) | 0.333 | 300 |
Mortar and cost vary by job, but these figures are useful starting points for an estimate.
| Item | Typical value |
|---|---|
| Blocks laid per 70 lb bag of mortar | about 25 to 30 |
| Waste allowance for cuts and breakage | 5% to 10% |
| Price per standard cinder block | about $1 to $3 |
Example Problems
Example 1. You are building a wall 20 feet long and 8 feet high using standard 16 in by 8 in blocks, with one 3 ft by 7 ft door and a 10% waste allowance. The gross area is 20 times 8, or 160 square feet. The door removes 21 square feet, leaving 139 square feet. At 0.889 square feet per block that is 139 divided by 0.889, or about 156 blocks. Adding 10% gives about 172 blocks.
Example 2. You have 300 standard blocks and want the wall area they cover at 5% waste. The usable area is 300 times 0.889 divided by 1.05, which is about 254 square feet. At 8 feet high that is roughly 32 feet of wall length.
FAQ
How many cinder blocks are in a square foot? A standard 16 in by 8 in block covers 0.889 square feet of wall, so you need about 1.125 blocks per square foot, or roughly 113 blocks for every 100 square feet of wall before waste.
Is a cinder block the same as a concrete block? The terms are used interchangeably today. True cinder blocks were made with coal cinders, while modern units are concrete masonry units (CMU), but the nominal sizes and the estimating math are the same.
How much waste should I add? A 5% to 10% allowance is normal. Use the lower end for a simple straight wall and the higher end when you have many cuts around openings, corners, or curves.
