A concrete block calculator estimates the blocks and mortar bags a wall needs. It multiplies net wall area by 1.125 blocks per square foot for standard 8x8x16 CMU, then adds waste. A 10 by 8 ft wall takes 90 blocks, 99 with 10 percent waste, and about 7 bags of mortar.
Blocks to buy (with waste)
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How to Use the Concrete Block Calculator
- Set Wall size by to length x height and enter the wall length and height in feet, or switch to Area and type the wall area. Subtract any openings such as doors or gates.
- Choose the block size. Standard 8x8x16 CMU is the default; pick half-high or Custom to enter a different face size. Set a waste percent for cuts and breakage.
- Pick the mortar product to load its blocks-per-bag yield, then read the blocks to buy and the number of 80 lb mortar bags.
| Result | What it means |
|---|---|
| Blocks to buy (with waste) | Net blocks plus your waste allowance, rounded up to whole blocks. |
| Blocks (no waste) | The bare wall count before any waste, from area times blocks per square foot. |
| Mortar bags (80 lb) | Net blocks divided by the blocks-per-bag yield, rounded up to whole bags. |
| Mortar bags for waste count | The same, but sized for the waste-inflated block count if you prefer to buy mortar to match. |
| Blocks per sq ft (basis) | The exact figure used, derived from your block face size, so you can check it. |
What Is a Concrete Block Calculator?
A concrete block calculator estimates how many concrete masonry units (CMU, often called cinder blocks) a wall needs, plus the bags of mortar to lay them. It turns wall size and block size into a block count and a mortar order.
It counts the blocks in the wall face and the mortar that joins them. It does not estimate grout poured inside the block cores; for that, use the CMU grout fill calculator. For a clay-brick wall instead of block, see bricks per square foot.
Block quantity depends almost entirely on the face area of one unit. A standard block shows a 16 by 8 inch face, so it covers a fixed share of the wall, which is what makes the estimate reliable.
How Does the Concrete Block Calculator Work?
The calculator finds the net wall area, multiplies by the blocks each square foot holds, then adds waste. Mortar is a second step from the block count.
blocks = (length x height - openings) x blocks_per_sq_ft x (1 + waste) and mortar bags = ceil(blocks / blocks_per_bag)The blocks-per-square-foot figure is derived, not a magic constant. One standard CMU presents a nominal 16 by 8 inch face, which is 128 square inches. A square foot is 144 square inches, so 144 / 128 = 1.125 blocks per square foot. This nominal face already includes the 3/8 inch mortar joint, so the joints are built into the count.
- Net area. Length times height, in square feet, minus any openings you subtract.
- Blocks per sq ft. 144 divided by the block face area in square inches. Standard 8x8x16 gives 1.125; a half-high 16 by 4 inch face gives 2.25.
- Blocks. Net area times blocks per square foot, then multiplied by one plus the waste fraction and rounded up.
- Mortar. Blocks divided by the blocks laid per 80 lb bag, rounded up to whole bags.
| Mortar product | Type | Blocks per 80 lb bag | Data sheet |
|---|---|---|---|
| Quikrete Mason Mix (No. 1136) | Type S | up to 13 (8x8x16) | Revised Nov 2020 |
| Quikrete Mortar Mix (No. 1102) | Type N | up to 13 (8x8x16) | Revised Nov 2020 |
Both Quikrete mixes state the same yield, so the mortar count is the same whether you lay with Type S or Type N. The type you choose depends on the wall, not the bag count.
Concrete Block Calculation Example
Take a wall 10 ft long and 8 ft high with no openings, built from standard 8x8x16 block, with a 10 percent waste allowance and Quikrete Mason Mix.
- Net area: 10 x 8 =
80 sq ft, no openings to subtract. - Blocks (no waste): 80 x 1.125 =
90 blocks. - Blocks with 10 percent waste: 90 x 1.10 = 99, so
99 blocksto buy. - Mortar: 90 / 13 = 6.9, rounded up to
7 bagsof 80 lb mix for the net count, or 8 bags if you buy mortar for the 99-block count.
The 7-bag figure uses Quikrete's "up to 13" yield, which is a best-case lab number. On a real wall with full bed joints, plan for roughly 10 to 15 percent more mortar than that.
Factors That Change Your Block and Mortar Count
A few inputs move the answer, and block size moves it most.
Block Face Size
Blocks per square foot come straight from the face area. A standard 16 by 8 inch face gives 1.125 per square foot; a half-high 16 by 4 inch face doubles that to 2.25, so a half-high wall needs twice the blocks and about twice the mortar.
Openings
Doors, windows and gates come off the wall area before counting. On a wall with a big gate this is a real saving, so measure and subtract them.
Waste Allowance
Cuts at corners, ends and openings break blocks. Five to ten percent is typical, more on a wall with many cuts. This is your own assumption, not a published figure.
Mortar Joint and Workmanship
The 1.125 figure assumes the standard 3/8 inch joint. Thicker joints or full bed joints use more mortar than the "up to 13" bag yield suggests.
Block Walls vs Brick and Poured Concrete
Block, brick and poured concrete each suit different walls, and each is estimated differently.
| Wall type | Estimated by | Best for |
|---|---|---|
| Concrete block (CMU) | Blocks per square foot of face, plus mortar bags | Foundations, retaining and structural walls, garden walls |
| Clay brick | Bricks per square foot (about 6.9 for standard), plus mortar | Facing and veneer, decorative walls |
| Poured concrete | Cubic yards of concrete | Monolithic walls and slabs |
For a brick wall, use the bricks per square foot calculator. For poured walls, footings or a slab, the concrete calculator works in volume. If the block cores will be filled with grout and rebar, add that with the CMU grout fill calculator.
When to Use This Concrete Block Calculator
Ordering Blocks for a Wall
Enter the wall size, pick the block, add waste, and read how many blocks and mortar bags to buy.
Pricing a Garden or Retaining Wall
Subtract gates and openings, then use the block and bag counts to get a quick material cost from your supplier.
Checking a Contractor Quote
Run the same wall here. A large gap usually means a different block size, joint thickness or waste allowance than the quote assumed.
Planning Mortar and Reinforcement
Use the bag count for mortar, and if the wall is reinforced, size the steel with the rebar spacing and quantity calculator and the core grout with the CMU grout fill tool.
Common Concrete Block Estimating Mistakes
1. Trusting "up to 13" Blocks per Bag
That is a maximum lab figure. Real full-bed joints use more mortar, so order roughly 10 to 15 percent more than the bare bag count.
2. Forgetting the Waste Allowance
Corners, ends and openings all need cut blocks. Buying the exact net count almost always leaves you short.
3. Using the Wrong Block Face
Half-high and custom blocks cover less wall each, so they need more units. Match the block size to what you are actually laying.
4. Not Subtracting Openings
Counting the whole rectangle over-orders on walls with large doors or gates. Subtract their area first.
5. Confusing Mortar with Core Grout
Mortar joins the blocks; grout fills the cores of reinforced walls. They are separate orders and separate calculators.
6. Skipping the Engineer on Structural Walls
Load-bearing and retaining walls need an engineer's specification and local code review. This tool estimates quantity, not design.
Accuracy and Limitations
The block count is exact arithmetic on the standard nominal module, and the mortar count uses the manufacturer's published yield. Real jobs still vary with workmanship.
What it calculates accurately
- Blocks per square foot derived from the standard nominal face (1.125 for 8x8x16)
- Net wall area after openings, with your waste allowance
- Mortar bags from Quikrete's verified 13-blocks-per-80-lb-bag yield
- Half-high and custom blocks, derived the same transparent way
What it does not account for
- Extra mortar for full bed joints beyond the "up to 13" maximum (plan 10 to 15 percent more)
- A cubic-foot mortar yield, which Quikrete does not publish for these mixes
- Structural design, reinforcement, bond beams or core grout
- Non-standard bond patterns, curved walls or special-shape units
The 1.125 blocks-per-square-foot figure is a derived standard, computed from the CMHA nominal module; CMHA does not print the number itself. Always confirm the final quantity and the wall design against the manufacturer's current data sheet, your project engineer's specification and local building code.
How We Calculate Blocks and Mortar
Frequently Asked Questions About Concrete Block Estimating
How many concrete blocks do I need per square foot?
About 1.125 standard 8x8x16 blocks per square foot of wall face. That comes from 144 square inches in a square foot divided by the 16 by 8 inch (128 square inch) block face, which already includes the 3/8 inch mortar joint.
How many blocks are in a 10 by 8 foot wall?
A 10 by 8 ft wall is 80 square feet, so 80 x 1.125 = 90 standard blocks with no waste. Adding 10 percent waste for cuts gives 99 blocks to buy.
How many blocks does one bag of mortar lay?
Quikrete Mason Mix (No. 1136) and Mortar Mix (No. 1102) each state up to 13 standard 8x8x16 blocks per 80 lb bag. That is a maximum, so real full-bed jobs lay fewer per bag and need more bags.
How many bags of mortar do I need for 100 blocks?
At 13 blocks per bag, 100 / 13 is about 7.7, so roughly 8 bags of 80 lb mix as a floor. Because the 13 figure is a maximum, plan for about 10 to 15 percent more on a real wall.
What is the formula for a concrete block wall?
Blocks equal net wall area times blocks per square foot times one plus the waste fraction. Mortar bags equal the block count divided by the blocks laid per bag, rounded up to whole bags.
What is the difference between Type S and Type N mortar?
Type S (Mason Mix) is higher strength and suits structural and below-grade work; Type N (Mortar Mix) suits above-grade non-structural walls. Both Quikrete products lay the same 13 blocks per 80 lb bag, so the bag count is identical.
Should I use the net or the waste-inflated block count for mortar?
Use the net block count for the base mortar estimate, since mortar is spread on blocks actually laid. Because the yield is a maximum and some mortar is wasted, buying for the waste-inflated count or adding 10 to 15 percent is the safer order.
How do I estimate half-high or non-standard blocks?
Pick the block size or choose Custom and enter the face length and height in inches. The calculator derives blocks per square foot as 144 divided by the face area, so a half-high 16 by 4 inch face gives 2.25 per square foot.
Does this calculator cover grout for filling the cores?
No. This tool counts the blocks and the mortar in the joints. Grout poured into reinforced cores is a separate order; use the CMU grout fill calculator for that volume.
Is my data saved?
No. The calculation runs in your browser and nothing is sent to our servers. Anything you choose to Save stays in this browser only.
Sources
- CMHA CMU-TEC-001-23, Concrete Masonry Unit Shapes, Sizes, Properties, and Specifications (nominal vs actual dimensions and the 3/8 in joint module that the 1.125 blocks/sq ft figure is derived from) (Concrete Masonry and Hardscapes Association, rev -23, read 2026-10-04).
- Quikrete Mason Mix No. 1136 (Type S) data sheet: up to 13 standard 8x8x16 blocks per 80 lb bag (Quikrete, revised November 2020, read 2026-10-04).
- Quikrete Mortar Mix No. 1102 (Type N) data sheet: up to 13 standard 8x8x16 blocks per 80 lb bag (Quikrete, revised November 2020, read 2026-10-04).
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Explore all construction calculatorsThis calculator gives an educational estimate and is not affiliated with or endorsed by the Concrete Masonry and Hardscapes Association (CMHA) or Quikrete. Quikrete is a trademark of its owner and is named here only to identify the products. Block quantities per square foot are derived from the standard nominal CMU module; mortar yields are read from the manufacturer's current data sheet. The waste allowance is your own planning assumption. For any structural or load-bearing wall, follow the project engineer's specifications and your local building code for the final quantity and layout. Spotted an error? Let us know.
Author
Shakeel Muzaffar is the Founder and Editor-in-Chief of MultiCalculators.com, bringing over 15 years of experience in digital publishing, product strategy, and online tool development. He leads the platform's editorial vision, ensuring every calculator meets strict standards for accuracy, usability, and real-world value. Shakeel personally oversees content quality, formula verification workflows, and the platform's commitment to publishing tools that are genuinely useful for students, professionals, and everyday users worldwide.
