How to measure a block wall
Measure the length along the footing and the height from the top of the footing to the top of the wall. Block comes in courses of 8 inches, so a wall that is not a multiple of 8 inches means cutting a whole course lengthwise. Try to pick a height that lands on a full course, and do the same for length in steps of 8 or 16 inches.
Work out each wall on its own, then add them. Corners matter more than they seem, because where two walls meet, the units overlap. TEK 04-02A warns that returns and corners add units, and that its figures should be checked against your own judgment.
Take off doors and windows by area. Lintels and bond beams over them are special units, and you will need to count those from your plan, along with anything else that is not a plain stretcher.
A worked example
Here is a 20 by 5 foot wall in 8 inch block, 100 square feet of it, with every cell filled. These lines come from the same formula the calculator above runs.
20 by 5 ft wall, 8 in block, every cell filled
- Blocks: 100 × 1.125 = 113 before waste
- With 5% waste: 119 blocks
- Mortar: 119 ÷ 13, rounded up = 10 bags
- Grout: 36.1 cu ft, or 1.34 cu yd
- Or in 80 lb bags: 55 bags
That many grout bags is a lot of mixing, and at this volume a truck may be worth pricing. The bag count for mortar uses Quikrete's figure of up to 13 blocks. CMHA's table says 16, which would make 8 bags. We buy to the lower figure, since coming up short is the worse mistake. For steel in the filled cells, see the rebar calculator, and for a brick skin, the brick calculator.
Materials for common walls
All in 8 inch block, with no openings. The two grout columns show how much it matters how often the cells are filled, which your wall's design decides and which we cannot pick for you.
| Wall | Blocks | Mortar bags | Grout, every 48 in | Grout, every cell |
|---|---|---|---|---|
| Low garden wall, 8 × 4 | 38 | 3 | 0.07 | 0.43 |
| Long garden wall, 20 × 4 | 95 | 8 | 0.18 | 1.07 |
| Shed wall, 20 × 8 | 189 | 15 | 0.36 | 2.14 |
| Garage wall, 24 × 8 | 227 | 18 | 0.43 | 2.57 |
| Long wall, 40 × 8 | 378 | 30 | 0.72 | 4.28 |
| Boundary wall, 100 × 6 | 709 | 55 | 1.36 | 8.02 |
Mistakes that throw off a block order
- Double counting waste. The 119 per 100 square feet already includes 5 percent, and the grout table includes 3. Do not add more on top.
- Mixing up the mortar rules. The common rule of 3 bags per 100 block is for masonry cement with sand. Preblended bags are 6 to 8 per 100.
- Treating a count as a design. A wall that holds back soil or carries a load needs an engineer or a code table, not just a count.
- Forgetting bond beams. Horizontal bond beams need extra grout not in the table, about 2 linear feet per 80 lb bag in an 8 inch wall.
- Using the wrong face size. Half-high and half-length units are sold too. The count here is for 8 by 16 inch faces.
Questions
How many concrete blocks are required for 100 sq ft?
You need 119 standard 8 by 16 inch blocks for 100 square feet of wall, including 5 percent for waste and breakage. That is the figure printed in CMHA's TEK 04-02A. Without waste it is 112.5.
How much mortar for a block wall?
Allow one 80 pound bag of preblended mortar for every 13 blocks, the most Quikrete says a bag will lay. CMHA's table gives 16 per bag, so 100 blocks takes between 7 and 8 bags. Masonry cement mixed with sand is a different product and is measured differently.
How much concrete to fill 8x8x16 block?
Filling every cell in an 8 inch wall takes 36.1 cubic feet of grout per 100 square feet, or about 1.34 cubic yards, by CMHA's table. Filling a cell every 48 inches takes only 6.1 cubic feet. Those figures include 3 percent waste.
How many blocks per square foot?
There are 1.125 standard blocks per square foot, because each 8 by 16 inch face covers 8/9 of a square foot. Half-high and half-length units differ. Add waste separately, and do not add it to a figure that already includes it.
Do I add waste for grout?
No, because CMHA's grout table already includes 3 percent. Adding more would double count it. You do need extra for horizontal bond beams, which the table leaves out.
Can I build a retaining wall from this?
You can count the block, but a retaining wall needs a design first, because soil pressure can push over a wall that looks fine. Start with the retaining wall calculator, and ask your building department when a permit or an engineer is required.
This block count is a planning estimate from published masonry tables, and returns, corners, lintels and bond beams all add to it. A structural or retaining wall needs a design, not just a count. Confirm the unit size with your supplier and the mortar and grout yields on the bag label before you order.
Sources and review
- TEK 04-02A, estimating concrete masonry materials, Concrete Masonry and Hardscapes Association, no date shown, read November 2026.
- Mortar Mix 1102 data sheet, Quikrete, 2020.
- Rule of thumb figures, General Shale Brick, 2024.
Checked in November 2026. Spotted a wrong number? Report it and it goes in the corrections log.
How this is calculated
Every figure below is read from the code that produces the answer, so what is written here is what the calculator ran. The constants were last checked against their sources in November 2026.
The arithmetic
Blocks
Blocks = Wall area × 1.125 × 1.05, rounded up
- 1.125
- Blocks per square foot of 8 × 16 in face
- 1.05
- 5 percent for waste and breakage
Worked through: 100 × 1.125 × 1.05 = 118.1, so 119 blocks
Grout
Grout = Wall area ÷ 100 × Table figure
- Table figure
- Cubic feet per 100 sq ft, by thickness and spacing, including 3 percent waste
Worked through: 100 ÷ 100 × 36.1 = 36.1 cu ft
The constants, and where they come from
BLOCKS_PER_SQ_FT
Published figureStandard 8 × 16 in face units per square foot of wall
Applies to: Nominal 8 × 16 in block with mortar joints
Source: TEK 04-02A, estimating concrete masonry materials, Concrete Masonry and Hardscapes Association, no date on the page, read 2026-11.
Also from: Rule of thumb figures, General Shale Brick, published 2024-07, read 2026-11.
TEK 04-02A gives a face of 8/9 sq ft, and General Shale prints 1.125 per sq ft. NCMA has merged into CMHA, so the older name no longer applies.
WASTE_PERCENT
Published figureAllowance for waste and breakage, in percent
Applies to: Concrete masonry units on site
Source: TEK 04-02A, estimating concrete masonry materials, Concrete Masonry and Hardscapes Association, no date on the page, read 2026-11.
TEK prints 119 units per 100 sq ft with this allowance. Rounding up gives the same figure. Returns and corners add more.
BLOCKS_PER_80_LB_BAG
Published figureStandard blocks laid by an 80 lb bag of preblended mortar, the maker's figure
Applies to: Quikrete Mortar Mix. It is stated as up to 13
Source: Mortar Mix 1102 data sheet, Quikrete, published 2020-11, read 2026-11. This source is about 6 years old, and it is the one the figure rests on.
Used for buying because it is lower than the association's figure, and running out halfway up is the worse mistake.
BLOCKS_PER_80_LB_BAG_ASSOCIATION
Published figureConventional units per 80 lb bag of preblended mortar, the association's figure
Applies to: Face shell mortar bedding, with nominal waste included
Source: TEK 04-02A, estimating concrete masonry materials, Concrete Masonry and Hardscapes Association, no date on the page, read 2026-11.
TEK Table 2. Shown beside the maker's figure so you can see the range.
GROUT_CU_FT_PER_100
Published figureGrout per 100 sq ft of wall, by wall thickness and grouted cell spacing, in cubic feet
- 6
- 8 25.6, 16 12.8, 24 8.6, 32 6.4, 48 4.3
- 8
- 8 36.1, 16 18.1, 24 12.1, 32 9.1, 48 6.1
- 10
- 8 47, 16 23.5, 24 15.7, 32 11.8, 48 7.9
- 12
- 8 58.9, 16 29.5, 24 19.7, 32 14.8, 48 9.9
Applies to: Two-core hollow units. Bond beams need more, and are not in this table
Source: TEK 04-02A, estimating concrete masonry materials, Concrete Masonry and Hardscapes Association, no date on the page, read 2026-11.
TEK Table 3. It already includes 3 percent waste, so none is added. TEK does not recommend grouting 4 inch units.
GROUT_BAG_CU_FT
Published figureYield of an 80 lb bag of preblended grout, in cubic feet
Applies to: Preblended grout
Source: TEK 04-02A, estimating concrete masonry materials, Concrete Masonry and Hardscapes Association, no date on the page, read 2026-11.
TEK Table 4 states about 0.66 cubic feet. Check the yield on your own bag.
CU_FT_PER_CU_YD
DefinitionalCubic feet in one cubic yard
Applies to: Definitional, everywhere
True by definition, so there is nothing to cite. The derivation is below.
Three feet in a yard, and a volume is three lengths multiplied, so 3 × 3 × 3 = 27.
What the labels mean
- Published figure:
- taken directly from the source named beside it.
- Definitional:
- true by definition, like the number of inches in a foot, so there is nothing to cite.
If a figure here looks wrong, it may be. Tell us what you found and we will check it against the source and publish a correction if it needs one. How constants are chosen and how often they are reviewed is set out in our methodology.
How to use the concrete block calculator
- 1
Measure the wall's length and height
- 2
Choose the thickness of block
- 3
Choose how often the cells are filled, from your design
- 4
Read the blocks, mortar and grout
- 5
Take off any openings in the further section
What this calculator assumes
Every estimate here rests on the assumptions below. Where your job differs from one of them, the figure will differ too.
- Standard 8 × 16 inch face blocks with mortar joints. Other faces change the count
- Corners, returns and lintels are not counted separately, and they add units
- Mortar is preblended in 80 lb bags, with face shell bedding
- Core fill assumes two-core hollow units, with no bond beams
Pro tips
- Count the block courses, since a wall height that is not a multiple of 8 in means cut block
- Round wall lengths to whole or half units when you can, so you cut less
- Order bond beam and corner units separately from your plan