Concrete Block Calculator — Blocks, Mortar and Fill
Enter the wall area and get the block count, the mortar to lay them and the grout if you are filling the cores. Block is sold by the piece, mortar by the bag and grout by the yard — three different units out of one measurement.
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We'll email the quantities above, what you typed in, and a link that reopens this calculator already filled in — so you can adjust it on your phone in the aisle.
How to work it out
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Measure the wall face
Length times height in feet, the same as any other wall. Block thickness does not change the count — a 6 inch and an 8 inch block have the same 8 × 16 in. face, so a square foot of wall takes the same number of either.
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Work out the blocks
A standard block is 7 5/8 × 15 5/8 in., and with a 3/8 in. mortar joint it occupies exactly 8 × 16 in. of wall — 128 square inches. A square foot is 144, so it takes 1.125 blocks to cover one. That is the neatest number in masonry and it is why block is sized the way it is.
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Add the mortar
One 80 lb bag of mortar mix lays about a dozen blocks. That is the figure to plan with; it drops if your joints are sloppy and rises if you are neat, and no calculator can tell which you are.
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Decide about the cores
A hollow wall is fine for a shed, a planter or anything low. Anything retaining soil, carrying a roof, or over about four feet free-standing usually wants rebar in the cores and grout around it. That is a structural decision, not an estimating one — get it specified before you order.
Blocks per square foot
Face size including a 3/8 in. mortar joint.
| Block | Actual size | With joint | Per sq ft |
|---|---|---|---|
| 8 × 8 × 16 in. | 7 5/8 × 15 5/8 in. | 8 × 16 in. — 128 sq in. | 1.125 |
| 6 × 8 × 16 in. | 7 5/8 × 15 5/8 in. | 8 × 16 in. — 128 sq in. | 1.125 |
| 8 × 8 × 24 in. | 7 5/8 × 23 5/8 in. | 8 × 24 in. — 192 sq in. | 0.750 |
| 8 × 4 × 16 in. | 3 5/8 × 15 5/8 in. | 4 × 16 in. — 64 sq in. | 2.250 |
The first number in a block size is its thickness, which does not affect the count. A 6 in. and an 8 in. block cover the same wall area.
Blocks and mortar by wall size
Standard 8 × 8 × 16 block, 5% waste, no openings deducted.
| Wall | Area | Blocks | 80 lb bags |
|---|---|---|---|
| 10 × 4 ft | 40 sq ft | 48 | 4 |
| 20 × 8 ft | 160 sq ft | 189 | 16 |
| 24 × 8 ft | 192 sq ft | 227 | 19 |
| 30 × 8 ft | 240 sq ft | 284 | 24 |
| 40 × 8 ft | 320 sq ft | 378 | 32 |
| 50 × 10 ft | 500 sq ft | 591 | 50 |
One 80 lb bag of mortar lays about 12 blocks.
How many blocks in a square foot?
1.125 — nine blocks for every eight square feet. A concrete block is made 3/8 of an inch under its nominal size in both directions precisely so that block plus mortar joint comes to a round 8 by 16 inches. That is 128 square inches, and 144 divided by 128 is 1.125.
The thickness does not come into it. A 4, 6, 8 or 12 inch block all have the same 8 × 16 face, so a wall of any of them takes the same number per square foot. What changes is the weight, the price and what the wall can hold up.
Cinder block or concrete block?
People use the words interchangeably and mostly it does not matter, but they are not the same thing. A true cinder block is made with coal cinders as the aggregate, is lighter, and has not been widely produced in decades. What you buy today is a concrete masonry unit — CMU — made with sand and gravel.
If someone is selling you genuine cinder block, it came out of a building. It is weaker than modern CMU and should not be used structurally.
How much grout to fill the cores?
Roughly 1.1 cubic yards per 100 square feet of wall if you are filling every core of an 8 inch block. A hollow block is a bit over half solid, so the voids in 100 square feet of 8 inch wall come to about 30 cubic feet.
Filling every fourth core — which is what you do when there is rebar every 32 inches — takes about a third of that. Set the dropdown and the calculator gives you both the cubic yards for a ready-mix order and the bag count if you are mixing it yourself.
Block walls need a footing
A block wall weighs 35 to 55 lb per square foot of face depending on thickness, and every bit of it lands on whatever is underneath. A footing below the frost line, wider than the wall, with rebar in it, is not optional for anything permanent.
Work the footing out with the footing calculator and pour it first. The block goes up after it has cured, not the same weekend.
What the first course decides
Every course above copies the first one. Get the bottom row dead level in both directions and square at the corners, and the wall builds itself. Get it out by a quarter inch and you will be chasing that quarter inch all the way up, thinning and thickening joints to hide it.
Lay the first course in a full mortar bed on the footing and take as long over it as it needs. It is the only course worth being slow about.
How this is worked out
No black box — here is the arithmetic the calculator runs, so you can check it against your own numbers.
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wall area = length × height, summed over every sectionThe face of the wall. Thickness does not change the block count. -
blocks = (area − openings) × (1 + waste) × 1.125144 sq in. in a square foot ÷ 128 sq in. of block face with joint = 1.125. -
mortar bags = blocks ÷ 12One 80 lb bag of mortar lays about a dozen standard blocks. -
core fill = area ÷ 100 × 1.1 cu yd, fully groutedA hollow 8 in. block wall is about 30 cu ft of void per 100 sq ft of face. -
bagged concrete = cubic yards × 45An 80 lb bag yields 0.6 cu ft, and 27 ÷ 0.6 = 45 bags to a cubic yard.
Common questions
How many concrete blocks per square foot?
1.125 — that is nine blocks for every eight square feet of wall. A block with its mortar joint takes up 8 × 16 in., which is 128 square inches, and 144 ÷ 128 = 1.125.
How many blocks for a 20 × 8 ft wall?
About 189 with 5% waste. The bare wall is 160 sq ft, which is 180 blocks, and the waste allowance takes it to 189. You will also want about 16 bags of mortar.
How many bags of mortar per 100 blocks?
Roughly nine 80 lb bags, or twelve 60 lb bags. One 80 lb bag lays about a dozen blocks. Buy an extra bag or two — mortar that has been mixed cannot wait for a store run.
Does an 8 inch block cover more wall than a 6 inch?
No. Both have the same 8 × 16 in. face, so both cover the same area and you need the same number. The difference is thickness: how much weight the wall carries, how much it weighs itself, and what it costs.
Do I need to fill the cores with concrete?
Not for a low garden wall or a shed base. Yes for anything retaining soil, carrying a roof, or standing much above four feet — and then it needs rebar in the filled cores too. Where the line falls is a question for your local code office, not a calculator.
How much does a concrete block weigh?
An 8 × 8 × 16 standard block is 30 to 38 lb. Lightweight versions run about 28. A pallet is usually 90 to 108 blocks, so plan on well over a ton and think about how it is getting from the truck to the wall.
Can I lay block on a gravel base instead of concrete?
For a dry-stacked garden edging, yes. For a mortared wall, no — mortar is rigid and will crack the moment the base moves, and gravel always moves. Mortared block needs a concrete footing below the frost line.
What is the difference between mortar and grout here?
Mortar goes between the blocks and is mixed stiff so it holds its shape under weight. Grout goes inside the cores and is mixed loose so it flows around the rebar and fills every void. Same family, different jobs, and you cannot swap them.
Where these figures come from
Every figure below is traceable. How we check the numbers explains how they are verified, and the coverage reference collects them all in one place.
- Concrete Masonry & Hardscapes Association — CMU dimensions and modular coursing
- Manufacturer product data — mortar yields and block weights
- International Residential Code, Chapter 6 — masonry wall and footing requirements
Think a number here is wrong? Tell us — we check every report, correct the page, and date the change. Last checked September 15, 2026.