How Wide Is The Block?STEP 1
Wall & Block0 SQ FT
= 20 ft 0 in
Type it the way your tape reads it. This box understands 12ft 6in, 12' 6", 12 6 and 6 1/2, and it echoes back what it understood. For a rectangle of wall, add the four sides together and enter the total length.
= 8 ft 0 in
Block is sized so the unit plus a 3/8 in joint hits a round number. A standard unit measures 15 5/8 by 7 5/8 in, so with a 3/8 in joint it occupies a face of exactly 16 by 8 in. Change the joint here and the face size, and the block count, change with it.
Doors and windows
Block is laid around an opening, not over it, so an opening really is block you never buy. The deduction rounds down on purpose, because the jambs above and beside an opening still need cut units and every opening needs a lintel or a bond beam over it.
no doors
no windows
Core fill and rebar
This is the single biggest variable in a core fill estimate and it is not standardized. It depends on the block width, on whether the unit has two cores or three, and on the taper of the webs. The defaults here shift with the width you picked and are planning figures only. Your supplier can give you the net volume for the exact unit, and that number beats this one.
Whatever this page prints for steel is a planning quantity only, so you can price a delivery. Bar size, spacing, lap length, hooks, embedment into the footing and where the grout stops are set by your local building code and by the engineered drawings for your wall. Ask the building department before you order steel, and never take a spacing from a calculator, including this one.
Cubes, Mortar Yield & Bar Length
Cube counts differ by plant, unit size and pallet. Type the figure from your own supplier's ticket, or set it to 0 to hide the cube line.
Published yields for an 80 lb bag of preblended mortar run from roughly 10 to 15 standard blocks, and the spread is real: it depends on joint size, on whether you bed the full unit or just the face shells, on how much you temper and on how much lands on the ground. This tool prints a range for that reason. The bag you buy states its own coverage. Use that one.
Face shell thickness sets how much mortar the bed joint actually holds, because block is normally face shell bedded rather than fully bedded. It varies with the unit width and the manufacturer, and 1 1/4 in is a fair figure for a standard 8 in unit. Rebar is commonly stocked in 20 ft lengths, but check before you order, and remember that lap splices add length the calculator does not know about.
Wall Elevation1:SCALE
Drawn to your real length and height with your real coursing and running bond. Openings are placed for illustration, not to your actual layout.
Blocks To BuyLIVE
Blocks to buy
0 blocks
Waste allowance, shown as its own line
5% for a straight wall with square corners and no openings. 10% once there are openings, corners, returns or a pilaster. 15% for a cut up wall, a stepped footing or a job where half blocks and bond beam units have to be cut from full units on site. Block chips at the corners in handling, and a chipped block is a hidden block, not a face block.
Blocks & Courses#CMU
Mortar, Fill & Steel#MORTAR
Materials Ticket#CMU
Enter the wall size to build the ticket.
Order corner units, bond beam units and half blocks as their own line items when you place the order. Cutting them from full blocks on site is slow, wasteful and the reason most block jobs run short.
How This Is CalculatedSHOW WORK
Where 1.125 blocks per square foot comes from
A standard concrete masonry unit is nominal 8 by 8 by 16 in and actually 7 5/8 by 7 5/8 by 15 5/8 in. The missing 3/8 in on each dimension is the mortar joint. So with a 3/8 in joint the face the block occupies in the wall is exactly 16 in by 8 in, which is 128 square inches, or 128 / 144 = 0.889 sq ft. One divided by 0.889 is 1.125 blocks per square foot.
Run the anchor case through it: a 20 ft by 8 ft wall is 160 sq ft, and 160 x 1.125 = 180 blocks before waste. The coursing method gives the same answer, which is the point of the check this page prints. 8 ft of height is 96 in, divided by 8 in per course is 12 courses. 20 ft of length is 240 in, divided by 16 in per block is 15 blocks per course. 12 x 15 = 180.
Cinder block or concrete block?
They are the same thing in ordinary speech, but the correct term is concrete masonry unit, shortened to CMU. A true cinder block used coal cinders as the aggregate, which is why it was lighter and why the name stuck. What your yard sells today is a concrete unit made with sand, gravel or a lightweight aggregate, and it is what every code document, spec sheet and delivery ticket calls a CMU. Ask for cinder blocks and you will get them. Read the spec sheet for the unit you are actually buying.
Courses, blocks per course, and why both are rounded up
Courses are the wall height divided by the face height of one block plus its joint, which is 8 in for a standard unit and 4 in for a half high unit. Blocks per course is the wall length divided by the face length, 16 in for a full block. Both are rounded up, because a wall that comes out at 11.4 courses still needs a twelfth course, cut or otherwise, and a run that needs 15.2 blocks still needs a sixteenth block.
Block walls are laid out best when the length and the height are multiples of the unit. A wall in whole 16 in increments of length and whole 8 in increments of height means no cutting except at the corners, which is why block buildings are dimensioned that way in the first place. If your numbers are close to a module, moving the wall an inch or two is often cheaper than cutting a hundred blocks.
Mortar: two honest answers, so this tool gives a range
The geometric figure is the face of the cell minus the face of the block, times the depth the mortar actually covers. Block is normally face shell bedded, so the bed joint is only as deep as the two face shells rather than the full width of the unit. For a standard 8 in unit with 1 1/4 in face shells and 3/8 in joints that comes out near 1.3 cubic feet of mortar per 100 blocks.
Field figures are much higher. Published bag yields commonly run 10 to 15 blocks per 80 lb bag of preblended mortar, which is several times the geometric volume. The gap is mortar that falls off the trowel, beds that get furrowed and refilled, over full head joints, tempering and the last batch of the day. Both numbers are honest and the sources genuinely disagree, so this page prints a low and a high and lets you edit both. Buy the first pallet, lay 100 blocks, then work out your own rate and order the rest against that.
Core fill and grout
Filling the cores turns a hollow wall into something much heavier and much stronger, and it is what most reinforced block walls do at the reinforced cells. The volume depends on three things: the width of the block, how many cores you fill, and the shape of the unit, because a two core unit and a three core unit of the same nominal size do not hold the same volume.
A standard 8 in unit holds roughly a quarter of a cubic foot per block when every core is filled, so around 100 to 110 blocks per cubic yard of grout. Wider units hold proportionally more. The core volume figure is an input on this page rather than a constant, precisely because it varies. Ask your supplier for the net volume of the unit they are selling you.
Grout for masonry is not the same material as concrete: it is a fluid mix designed to flow into a narrow cell and bond to the steel. Ordering ready mix concrete for a core fill and finding it will not consolidate is an expensive afternoon. For the footing under the wall, see the concrete calculator. For volumes at any depth, see the cubic yard calculator.
Rebar: a planning quantity only
The steel figures on this page exist so you can price a delivery, and for nothing else. Bar size, vertical and horizontal spacing, lap splice length, hooks, dowels out of the footing, bond beam locations and how high the grout lift can go are all set by your local building code and by the engineered design for your wall. Seismic and high wind zones tighten every one of those. Nothing on this page substitutes for either.
Two practical notes the calculator cannot include for you. Lap splices add real length, so a wall taller than one stock bar needs more steel than height times count. And vertical bars usually start as dowels cast into the footing, which means the steel decision happens before the footing is poured, not when the block arrives.
Block Quick ReferenceSCREENSHOT ME
Nominal versus actual block size
| Nominal | Actual unit | Face with 3/8 in joint | Sq ft of face | Per sq ft |
|---|---|---|---|---|
| 8 x 8 x 16 in | 7 5/8 x 7 5/8 x 15 5/8 | 16 x 8 in | 0.889 | 1.125 |
| 4 x 8 x 16 in | 3 5/8 x 7 5/8 x 15 5/8 | 16 x 8 in | 0.889 | 1.125 |
| 6 x 8 x 16 in | 5 5/8 x 7 5/8 x 15 5/8 | 16 x 8 in | 0.889 | 1.125 |
| 10 x 8 x 16 in | 9 5/8 x 7 5/8 x 15 5/8 | 16 x 8 in | 0.889 | 1.125 |
| 12 x 8 x 16 in | 11 5/8 x 7 5/8 x 15 5/8 | 16 x 8 in | 0.889 | 1.125 |
| 8 x 4 x 16 in, half high | 7 5/8 x 3 5/8 x 15 5/8 | 16 x 4 in | 0.444 | 2.250 |
| 8 x 8 x 8 in, half block | 7 5/8 x 7 5/8 x 7 5/8 | 8 x 8 in | 0.444 | 2.250 |
Blocks for common wall sizes, 8 in high courses
| Wall | Sq ft | Courses | Per course | Blocks |
|---|---|---|---|---|
| 10 x 4 ft | 40 | 6 | 8 | 48 |
| 20 x 4 ft | 80 | 6 | 15 | 90 |
| 20 x 8 ft | 160 | 12 | 15 | 180 |
| 30 x 8 ft | 240 | 12 | 23 | 276 |
| 40 x 8 ft | 320 | 12 | 30 | 360 |
| 50 x 10 ft | 500 | 15 | 38 | 570 |
Core fill volume, all cores filled
| Block width | Cu ft per block | Blocks per cu yd | Cu yd per 100 blocks |
|---|---|---|---|
| 4 in | 0.09 | 300 | 0.33 |
| 6 in | 0.17 | 159 | 0.63 |
| 8 in | 0.25 | 108 | 0.93 |
| 10 in | 0.33 | 82 | 1.22 |
| 12 in | 0.42 | 64 | 1.56 |
Mortar, the honest spread
| Figure | Per 100 blocks | What it covers |
|---|---|---|
| Mortar in the joints, geometry | 1.3 cu ft | Face shell bedding, nothing wasted |
| Add 25% site waste | 1.6 cu ft | Board loss, tempering, last batch |
| 80 lb bags, best case | 7 bags | At 15 blocks per bag |
| 80 lb bags, worst case | 10 bags | At 10 blocks per bag |
Now Price The Job
- BrickBricks, cubes, mortar
- Retaining WallBlocks, base, drainage
- ConcreteFootings and slabs
- Cubic YardAny depth of fill
- GravelTons and yards of base
- Square FootageOdd shapes and curves
- PaversPavers, sand, base
- DrywallSheets, mud, tape
- InsulationBatts and blown R value
- PaintGallons per coat
- SidingSquares of siding
- FencePosts, rails, pickets
Concrete Block Calculator FAQ
How many blocks do I need for a 20 x 8 ft wall?
A 20 ft by 8 ft wall is 160 sq ft, and standard 8 x 8 x 16 in units cover 1.125 blocks per square foot, so 180 blocks before waste and 189 with a 5% allowance.
The coursing check agrees: 96 in of height divided by 8 in per course is 12 courses, and 240 in of length divided by 16 in per block is 15 blocks per course. 12 x 15 = 180. Add corner units, bond beam units and half blocks as separate line items on the order.
How many concrete blocks are in a square foot?
1.125 blocks per square foot for standard 8 x 8 x 16 in units with a 3/8 in mortar joint, or put the other way round, one block covers 0.889 sq ft of wall. Multiply your wall area by 1.125, or divide it by 0.889, and you get the same number.
Block width does not change this. A 4 in, 6 in, 8 in, 10 in and 12 in unit all present the same 16 by 8 in face to the wall. Width changes the weight, the price and the core fill volume, not the count. Half high units at 4 in tall and half blocks at 8 in long both work out at 2.25 per square foot.
Is a cinder block the same as a concrete block?
In everyday speech, yes. The correct term is concrete masonry unit, or CMU, and that is what the spec sheets, the code documents and the delivery ticket will say. The name cinder block comes from units made with coal cinders as the aggregate, which made them lighter.
What is sold today is a concrete unit made with sand, gravel or a lightweight aggregate. Both names get you the same pallet at the yard. Use CMU when you are reading a spec, and read the spec for the unit you are actually buying, because strength, weight and net volume all vary between makers.
What is the actual size of an 8 x 8 x 16 block?
7 5/8 by 7 5/8 by 15 5/8 in. Every dimension is 3/8 in short of the nominal size, and that missing 3/8 in is the mortar joint. Lay the unit with a 3/8 in joint and it occupies exactly 16 by 8 in of wall face, which is why block dimensions work out to round numbers on a drawing.
This is also why the joint size matters to your order. Work a 1/2 in joint and each unit occupies a slightly larger face, so you need slightly fewer blocks and slightly more mortar. The joint selector on this page changes both figures together.
How many bags of mortar do I need per 100 blocks?
Published figures run from about 7 to 10 bags of 80 lb preblended mortar per 100 standard blocks, which is the same as saying 10 to 15 blocks per bag. That is a wide spread and it is genuine, which is why this calculator prints a range instead of pretending to know.
The pure geometry is much lower, near 1.3 cubic feet of mortar per 100 blocks for face shell bedding with 3/8 in joints. The difference between the geometry and the bag count is mortar that never reaches the wall. Buy one pallet, lay 100 blocks, then measure your own rate and order the rest against it.
How much grout does it take to fill block cores?
A standard 8 in unit holds roughly 0.25 cubic feet per block with every core filled, which works out near 100 to 110 blocks per cubic yard. Wider units hold more: figure around 0.42 cubic feet for a 12 in unit and around 0.17 for a 6 in unit.
These are planning figures and this is the least certain number on the page, because core volume changes with the maker, with two core versus three core units and with the taper of the webs. Most walls do not fill every core anyway, only the cells that carry reinforcement, so the spacing selector matters as much as the volume. Ask your supplier for the net volume of the exact unit and type it in.
How far apart should the rebar be in a block wall?
That is a code question, and this calculator will not answer it. Vertical spacing, bar size, lap splice length, hooks, footing dowels and bond beam locations come from your local building code and from the engineered design for your specific wall, and seismic and high wind zones tighten all of them.
What this page gives you is a quantity once you already know the spacing, so you can price a steel delivery. Enter the spacing from your drawings, not from a web page. And note that lap splices add length the calculator cannot see, so order a little more bar than the linear feet shown.