CMU Block Calculator
Work out blocks, courses, blocks per course, mortar bags and reinforcement for a concrete block wall, then add your own block, mortar and labor prices.
Blocks, Courses and Steel for a Block Wall
A standard block is nominally 8 by 8 by 16 inches, so its face covers 0.888 square feet and 100 square feet of wall takes 112.5 blocks. Enter the wall length and height, take out the openings, and this gives you the block count, the number of courses, the blocks in each course, the mortar bags and the reinforcement — vertical bar at your chosen spacing and joint wire in every other course. Prices are yours to enter. Core fill grout is deliberately left out: the void volume is printed on your block supplier's data sheet and it differs between units, and on a reinforced wall that is the last item worth guessing at.
Results
THE WALL
Core fill grout is deliberately not calculated here: the void volume differs between two-core and three-core units and between manufacturers, and it is printed on the unit's own data sheet. On a reinforced wall that is the most expensive single item, so it is worth reading rather than guessing.
Cost Breakdown
How to Estimate a Concrete Block Wall
A standard concrete block is nominally 8 by 8 by 16 inches, so its face covers 8/9 of a square foot — which is why the number every mason knows is 112.5 blocks per 100 square feet of wall. Everything else follows from the courses: the wall height divided by 8 inches gives you the number of courses, and there is no cutting a course in half the way you would rip a sheet. The calculator above does the blocks, the courses, the mortar and the steel; what it will not do is guess the grout for filling the cores, because that number is printed on your block supplier's data sheet and it differs between units.
1. Get the net wall area
Length times height, then take out the doors, windows and any opening you are not laying through. Unlike drywall, subtract every opening — a block you do not lay is a block you do not buy, and the offcut from a cut block is rarely usable elsewhere.
2. Multiply by 1.125 for the block count
A nominal 8 x 16 face with its mortar joint covers 0.888 square feet, so a square foot takes 1.125 blocks and 100 square feet takes 112.5. A 40 by 8 foot wall is 320 square feet, so 360 blocks before waste.
3. Work out the courses, and check they come out whole
Each course is 8 inches with its joint, so the wall height in inches divided by 8 is the course count. An 8 foot wall is 12 courses exactly. A wall height that does not divide by 8 means either a cut course or a change to the design, and it is much cheaper to find that out now.
4. Work out the blocks per course
The wall length in inches divided by 16. This is what tells you whether the corners and openings land on a joint or in the middle of a block, which is where the cutting time comes from.
5. Add waste — less than you would for brick
5% is normal on a straightforward wall. Block is big and regular, there is less breakage than with brick, and most of the loss is at corners and openings rather than in cutting.
6. Add the mortar by the bag
Around three 70 lb bags of mortar mix per 100 blocks laid. If you are mixing masonry cement with sand rather than buying pre-blended mix, that also brings in the sand, which is sold by the ton and is worth pricing separately.
7. Add the steel the wall actually needs
Vertical bar in the cores at the spacing your design or code calls for, running the full height plus a lap into the footing. Horizontal joint reinforcement — ladder or truss wire — in every other course is the usual residential practice, sold in 10 foot lengths and lapped, so allow for the overlap.
8. Get the grout figure from the block, not from a rule of thumb
If the cores are being filled, look up the void volume for the exact unit you are buying. Two-core and three-core blocks differ, and lightweight units differ again. On a reinforced wall the grout is often the biggest single material line, which is exactly why it should not be estimated from a number found on the internet.
Worked Example: a 40 x 8 ft Garden Wall
40 feet long, 8 feet high, no openings, standard 8 x 8 x 16 block, 5% waste, no vertical bar, joint wire every other course.
| Item | Working | Result |
|---|---|---|
| Wall area | 40 x 8 | 320 sq ft |
| With 5% waste | 320 x 1.05 | 336 sq ft |
| Blocks | 336 x 1.125 | 378 blocks |
| Courses | 8 ft x 12 ÷ 8 in | 12 courses |
| Blocks per course | 40 ft x 12 ÷ 16 in | 30 per course |
| Check | 30 x 12 | 360 laid, plus the waste allowance |
| Mortar | 378 ÷ 100 x 3 bags | 12 x 70 lb bags |
| Joint wire | 6 courses x 40 ft, plus lap | 26 x 10 ft pieces |
Worked Example: a Reinforced Foundation Wall
The same 40 by 8 foot wall, but structural: vertical bar at 32 inch centres and all cores filled. This is where a block estimate stops resembling a brick one.
| Item | Working | Result |
|---|---|---|
| Blocks | As above | 378 blocks |
| Vertical bars | 40 ft x 12 ÷ 32 in, plus one | 16 bars |
| Bar length each | 8 ft wall + 1 ft lap | 9 ft |
| Total bar | 16 x 9 | 144 linear ft |
| Bond beam | Top course, full length | 40 ft plus laps |
| Core fill grout | Void volume x cores filled | From the unit data sheet |
| Extra labor | Placing steel, grouting, vibrating | A separate line, not a rate on area |
Block Sizes and the Numbers That Follow
Nominal dimensions include the mortar joint; actual block is 3/8 inch smaller in each direction. Estimate on nominal, cut on actual.
| Unit | Face coverage | Blocks per 100 sq ft |
|---|---|---|
| 8 x 8 x 16 standard | 0.888 sq ft | 112.5 |
| 4 x 8 x 16 (4 in thick) | 0.888 sq ft | 112.5 (same face, thinner wall) |
| 12 x 8 x 16 | 0.888 sq ft | 112.5 (same face, 12 in thick) |
| 8 x 4 x 16 half-high | 0.444 sq ft | 225 |
| Course height | 8 in nominal | 1.5 courses per foot of height |
| Blocks per foot of length | 0.75 | One block every 16 in |
| Mortar | 3 x 70 lb bags per 100 blocks | Pre-blended mix. |
| Joint wire | Every other course | 10 ft lengths, lapped. |
What a Block Wall Costs to Build
Block and mortar come straight out of the calculator above. What sits around them is the footing, the steel and the lifting, and on a retaining or load-bearing wall those three are most of the price. A dry-stack garden wall and an engineered retaining wall of the same length are different jobs with the same block count.
The footing
A block wall needs a designed footing, and a retaining wall needs reinforcement tied from that footing into the wall itself. Excavation, concrete and steel below ground regularly cost more than the visible wall above.
Reinforcement and grouting
Vertical bar in grouted cells, horizontal joint reinforcement and bond beams all add material and slow the laying. Core-fill grout is deliberately left out of the calculator because void volume varies by unit and belongs on the manufacturer's data sheet, but it is a real and substantial cost on any reinforced wall.
Block weight
A standard 8-inch block is heavy enough that a day of laying is limited by the labourer as much as by the mason. Where the pallets land on delivery matters: blocks stacked forty feet from the wall get handled twice.
Drainage on anything holding back ground
Gravel backfill, filter fabric and a drain line behind a retaining wall are not optional and they are not visible in the finished job. Leaving them out is how a wall ends up leaning.
The finish
Bare block, painted, rendered or faced with stone are four very different prices for the same structure, and the finish decides how much care the blockwork itself needs.
The Lines Behind a Block Wall Price
| Line item | What drives it |
|---|---|
| Block | Block count from wall area and unit size, plus breakage |
| Footing | Excavation, concrete, reinforcement, spoil removal |
| Mortar | Bags and sand from the block count and joint size |
| Steel and grout | Vertical bar spacing, bond beams, joint reinforcement, cells filled |
| Drainage and backfill | Gravel, fabric and a drain line on any retaining wall |
| Laying labour | Blocks per day, adjusted for height, corners and openings |
| Access and lifting | Where the blocks land on delivery, and any platform above waist height |
| Finish | Paint, render or facing, priced per square foot of face |
What Decides Whether a Block Wall Job Pays
The block count is the easy part and it is rarely where a block job goes wrong. These are the items that move the number.
The footing is a separate estimate
A block wall sits on a poured footing, and that is concrete by the cubic yard plus its own steel and forms. Quoting the wall without the footing is the most common omission on this kind of job.
Corners and openings are where the hours go
A long straight run goes up fast. Corners have to be built and levelled first, and openings need lintels or bond beams. Two walls with the same square footage and different corner counts are not the same job.
Whether the wall is seen
A foundation wall that gets backfilled is not finished to the same standard as a garden wall someone will look at every day. Struck joints, consistent colour and clean faces are real time, and worth pricing separately from the laying.
Getting the block to the wall
A standard block is heavy and there are hundreds of them. Delivery placement, a pallet that can be reached, and whether they have to be carried through a garden all matter more on block than on almost any other material.
Weather and lift heights
Mortar does not want to be laid in freezing weather, and you cannot build a full-height wall in one day without the lower courses taking the load. Both affect scheduling rather than materials, and both belong in the price.
Cleaning and sealing
Efflorescence, mortar smears and the decision to seal or paint are all after the wall is up. They are the parts a customer notices, and the parts most often left out of the quote.
Where Estimates Go Wrong
Estimating block off brick figures
Brick and block are counted, coursed and mortared differently. A brick figure applied to block gives a number that is wrong in both directions at once.
Designing a wall height that does not divide by 8 inches
Every course is 8 inches. A height that does not come out whole means cutting a course along its length, which is slow, looks wrong and weakens the bond.
Guessing the core fill grout
Void volume varies by unit and manufacturer, and on a reinforced wall grout can be the largest material line. It is published — read it.
Leaving the footing out of the quote
The concrete, the steel, the forms and the excavation for the footing are frequently more than the block wall standing on top of it.
Forgetting the horizontal wire
Individually cheap, and it goes into half the courses across the entire wall. Missing it from the estimate is a small error repeated a lot of times.
Adding brick-sized waste
10 to 15% is a brick allowance. Block is bigger, more regular and breaks less; 5% is usually right, and over-ordering block means paying to store and return pallets.
CMU Block — Common Questions
What does a block wall cost?
Block and mortar come from the calculator above. The footing, the reinforcement and the grout usually cost more than the visible wall, and on anything holding back ground the gravel backfill and drain line behind it are part of the job rather than an extra. Bare, painted, rendered or stone-faced are four different prices for the same structure.
How many concrete blocks do I need per 100 square feet?
112.5 standard 8 x 8 x 16 blocks. The nominal face including its mortar joint covers 0.888 square feet, so each square foot takes 1.125 blocks.
How many blocks are in a course?
The wall length in inches divided by 16. A 40 foot wall is 480 inches, so 30 blocks per course. Height works the same way with 8 inch courses: an 8 foot wall is 12 courses.
How much mortar do I need for concrete block?
Around three 70 lb bags of pre-blended mortar mix per 100 blocks laid. If you are mixing masonry cement with sand instead, price the sand separately — it is sold by the ton and it is easy to leave out.
How much waste should I allow on block?
About 5% on a straightforward wall, less than you would allow for brick. Block is larger, more uniform and breaks less, and most of the loss happens at corners and openings.
How much grout do I need to fill the cores?
Look it up for the exact block you are buying rather than using a general figure. Void volume differs between two-core and three-core units and between manufacturers, and on a reinforced wall the grout is often the biggest single material cost — which is the worst place to be approximate.
What is the actual size of an 8 inch block?
7 5/8 by 7 5/8 by 15 5/8 inches. The nominal 8 x 8 x 16 includes a 3/8 inch mortar joint, which is why the estimating arithmetic works on nominal sizes and the cutting works on actual ones.
Do I need rebar in a block wall?
A structural wall does, and the spacing comes from the design or the local code rather than from a rule of thumb. A low garden wall may need none. Where bar is used it runs the full height with a lap into the footing, and those cores get grouted.
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