How Many Cinder Blocks Do I Need?
To find how many cinder blocks you need, multiply the wall area in square feet by 1.125, since one standard 8x8x16 block with a 3/8 inch mortar joint covers 0.889 square feet of wall face. Here is the formula in plain terms, blocks per 100 square feet, how much mortar to buy, and a calculator that does the count for you.
Key takeaways
- A standard 8x8x16 block covers 0.889 sq ft, so you need about 1.125 blocks per square foot of wall.
- A 100 square foot wall takes roughly 113 blocks before waste, so plan for about 120 with a 5 to 10% allowance.
- Buy about 6 to 8 bags of 80 lb pre-mixed mortar per 100 blocks, or one bag per 30 to 35 blocks.
- Subtract door and window openings from the wall area before multiplying, and round blocks up to a whole number.
- Most CMU costs 1 to 3 dollars per block in 2026, with a pallet of 70 to 90 blocks running 115 to 225 dollars.
The formula: blocks per square foot
Cinder blocks are counted by wall area, not volume. The most common unit is the 8x8x16 concrete masonry unit, which presents an 8 inch high by 16 inch long face to the wall once it is laid.
That face is 128 square inches, but with a standard 3/8 inch mortar joint each block actually occupies an 8 by 16 inch grid space, which is 0.889 square feet. Divide 1 by 0.889 and you get the rule of thumb: 1.125 blocks per square foot.
Wall area (ft2) = Length(ft) x Height(ft)Blocks = Wall area x 1.125Always add 5 to 10% for breakage, cutting, and the odd block that arrives chipped. Round the final number up to a whole block, because you cannot buy a fraction.
How many blocks per 100 square feet
For a quick estimate, a 100 square foot wall of standard 8x8x16 blocks needs about 113 blocks before waste, or roughly 120 once you add a 5 to 10% allowance. That keeps the math simple: take your wall area, divide by 100, and multiply by 113.
The table below shows common wall sizes using 8 inch blocks. These figures assume a 3/8 inch mortar joint and no openings, so subtract any doors or windows from the area first.
| Wall (8x8x16) | Area (sq ft) | Blocks (+10%) |
|---|---|---|
| 10 x 8 ft | 80 | 99 |
| 20 x 8 ft | 160 | 198 |
| 10 x 12 ft | 120 | 149 |
| 25 x 8 ft | 200 | 248 |
To skip the arithmetic, enter your length and height in the cinder block calculator and it returns the block count, mortar bags, and an optional cost.
How much mortar do you need
Mortar is the second material to buy, and it is easy to under order. As a field estimate, plan on about 6 to 8 bags of 80 lb pre-mixed Type S or N mortar per 100 blocks laid with a standard 3/8 inch joint.
Put another way, one 80 lb bag lays roughly 12 to 14 standard blocks, so a simple rule is one bag per 30 to 35 blocks if you are working with smaller batches. Buy a spare bag or two, since running short mid course means matching color and texture later.
Block sizes and which one to count
Not every job uses the 8 inch block, and the width changes the count only slightly, since all standard CMU share the same 8 by 16 inch face. The 4 inch block is the same 1.125 per square foot, just thinner.
The 4 inch CMU (nominal 4x8x16) is used for non load bearing partitions, veneers, and decorative work where you do not need structural mass. The 6 inch unit suits fire rated and medium load walls, while 12 inch blocks handle foundations and retaining structures.
If you are sizing a structural wall, the standard concrete block sizes guide lays out the nominal and actual dimensions for each width so you order the right unit.
Example: blocks for a 10x12 wall
Say you are building a 10 foot long by 12 foot high wall with standard 8x8x16 blocks. The area is 10 times 12, which is 120 square feet.
Multiply 120 by 1.125 and you get 135 blocks, then add 10% for waste to reach about 149 blocks. For mortar, 135 blocks at roughly 7 bags per 100 means about 10 bags of 80 lb pre-mixed mortar.
If the wall has a 3 by 7 foot doorway, subtract that 21 square feet first: 99 square feet times 1.125 is 112 blocks, plus waste. Always deduct openings before you multiply.
Cinder blocks vs pouring concrete
A common question is whether to lay blocks or pour a solid wall. For low and short walls, block is usually 10 to 25% cheaper upfront because it needs no forms and no concrete truck, though it asks for more skilled labor.
In 2026 a standard CMU runs about 1 to 3 dollars per block, and a pallet of 70 to 90 blocks costs roughly 115 to 225 dollars. Poured walls win on tall heights and long term waterproofing, since they have no mortar joints to crack.
If your project is a garden or retaining wall, the cost trade off changes with height and drainage, which the retaining wall cost guide breaks down before you commit to a material.
Let the calculator do the count
Working out blocks, mortar, and waste by hand is fine for one straight wall, but it is easy to forget the openings or the 10% allowance. Enter your wall length, height, and block size, and the tool returns the block count, the number of mortar bags, and an optional material cost.
For an L shaped or stepped wall, split it into straight sections, run each one, and add the totals. That keeps the count accurate without forcing an odd shape into a single rectangle.
Frequently asked questions
How do you calculate the number of cinder blocks needed?
How many cinder blocks for a 10x12 wall?
What are 4 inch cinder blocks used for?
Is it cheaper to pour concrete or use cinder blocks?
How many bags of mortar do I need per 100 blocks?
References
- Concrete masonry unit (Wikipedia)
- ASTM C90: Loadbearing Concrete Masonry Units
- NCMA: Concrete Masonry Unit Shapes and Sizes
- QUIKRETE Mason Mix (Type S) data sheet