Concrete Block Calculator
Enter the length and height of the wall. You get the number of standard 8x8x16 concrete blocks, the bags of mortar mix to lay them, and the grout it takes to fill the cells.
How many concrete blocks do I need?
Multiply the length of the wall by its height in feet, then multiply by 1.125 blocks per square foot of wall and add 5 % for breakage and cuts. A wall 20 ft long and 6 ft high is 120 sq ft, which is 135 blocks, or 142 blocks with 5 % extra, laid with 11 bags of 80 lb mortar mix.
Blocks you need
120 sq ft of wall takes 135 blocks, 142 with 5 % extra, and 11 bags of mortar mix.
- Wall area
- 120 sq ft9 courses
- Mortar mix
- 11 bags80 lb, 13 blocks per bag
- Cell fill
- 43.32 cu ft1.6 cu yd, or 64 bags of core-fill grout
The numbers above are a worked example for a wall 20 ft long and 6 ft high, fully grouted, with 5 % extra blocks. Allow this page to run scripts in your browser to use your own dimensions, or follow the formula further down the page and do it by hand.
How the concrete block calculator works, step by step
A concrete masonry unit, or CMU, sold as an 8x8x16 block actually measures 7 5/8 in x 7 5/8 in x 15 5/8 in. The missing 3/8 in is the mortar joint. Laid up, every block fills exactly 8 in x 16 in of wall face, which is 128 square inches, or 0.889 sq ft. One square foot of wall therefore takes 1 / 0.889 = 1.125 blocks, whatever the thickness of the block.
blocks to buy = area x 1.125 x (1 + extra / 100), rounded up
bags of core-fill grout = grout (cu ft) / 0.68, rounded up
Worked example: a wall 20 ft long and 6 ft high, fully grouted
- Area: 20 x 6 = 120 sq ft
- Blocks: 120 x 1.125 = 135 blocks
- With 5 % extra: 135 x 1.05 = 141.75, so 142 blocks to buy
- Courses: 72 in / 8 in = 9 courses of 15 blocks
- Mortar: 135 / 13 = 10.4, so 11 bags of 80 lb mortar mix
- Grout: 120 x 0.361 = 43.32 cu ft, which is 1.6 cu yd, or 64 bags at 0.68 cu ft
Concrete blocks for common wall sizes
The table counts standard 8x8x16 blocks with 5 % extra and the 80 lb bags of mortar mix to lay them. A wall 10 ft long and 10 ft high, 100 sq ft, takes 113 blocks and 119 with the extra, which is the 119 units per 100 sq ft, waste included, that the concrete masonry industry tables give for this unit.
| Wall | Area | Blocks | With 5 % | Mortar bags | Full fill |
|---|---|---|---|---|---|
| 10 x 4 ft | 40 sq ft | 45 | 48 | 4 | 14.44 cu ft |
| 20 x 4 ft | 80 sq ft | 90 | 95 | 7 | 28.88 cu ft |
| 10 x 10 ft | 100 sq ft | 113 | 119 | 9 | 36.1 cu ft |
| 20 x 6 ft | 120 sq ft | 135 | 142 | 11 | 43.32 cu ft |
| 30 x 8 ft | 240 sq ft | 270 | 284 | 21 | 86.64 cu ft |
| 40 x 8 ft | 320 sq ft | 360 | 378 | 28 | 115.52 cu ft |
Mortar: how many blocks per bag
The manufacturer data sheet for a standard pre-blended mortar mix gives up to 13 standard 8x8x16 blocks per 80 lb bag, or 37 bricks. That figure assumes a 3/8 in joint and a mason who does not drop much on the ground, so it is a ceiling rather than a promise. The calculator divides the blocks actually laid, not the spares, by 13 and rounds up. On a first wall, buy one or two bags more than the result: mortar left in the tub after it has stiffened is not reusable, and the cost of a spare bag is trivial next to a trip to the yard.
For a large wall, masons often mix their own from masonry cement and sand instead of pre-blended bags. The quantity of mortar in the wall is the same; only the way it is bought changes.
Cell fill: how much grout to fill concrete block
The hollow cores of the blocks are filled with grout wherever the wall is reinforced: at corners, at the ends of walls, beside openings and at the spacing your plans give for the vertical rebar. Retaining walls and below-grade walls are often grouted solid. The concrete masonry industry tables give the grout volume per 100 sq ft of wall, two-core units and 3 % waste included:
| Block width | Every cell | Per sq ft of wall |
|---|---|---|
| 6 in | 25.6 cu ft | 0.256 cu ft |
| 8 in | 36.1 cu ft | 0.361 cu ft |
| 10 in | 47.0 cu ft | 0.470 cu ft |
| 12 in | 58.9 cu ft | 0.589 cu ft |
An 8 in wall grouted every other cell, at 16 in on center, takes 18.1 cu ft per 100 sq ft, half the fully grouted figure. Per block, a fully grouted 8 in wall works out to about 0.32 cu ft of grout.
For a small wall, bagged core-fill grout is the simple route: the manufacturer gives a yield of about 0.68 cu ft per bag. Past a cubic yard of fill, a ready-mix plant can deliver grout by the yard and a grout pump places it far faster than a bucket. Grout is not mortar and not ordinary concrete: it is a fluid mix made to flow down the cores and around the rebar, and it is consolidated as it goes in.
Cases the simple formula does not cover
Corners, ends and half blocks
The area method counts every corner and every half block as a share of a full block. On a wall with many short returns, pilasters or openings, the count runs a few blocks short, and the 5 % extra is what covers it. Corner blocks and half blocks are sold separately: count them along the plan, one corner unit per course at each corner, and buy them as their own line.
Bond beams and lintels
The top course of many walls is a bond beam, a course of U-shaped units filled with grout around horizontal rebar. The block count stays the same, but that course is grouted along its whole length, so add its grout on top of the cell fill. Openings wider than a few feet also need a lintel over them.
The footing
The calculator counts the wall, not what it stands on. A block wall sits on a continuous concrete footing, usually twice as wide as the wall is thick. The concrete calculator works out its cubic yards and bags, and concrete bags per yard converts the result into bags of each size.
Retaining walls
A block retaining wall holds back wet soil, so it needs drainage stone and a drain pipe behind it. Work out the stone with the gravel calculator, using the length of the wall by the width of the drainage zone. Above about 4 ft, many jurisdictions require an engineered design and a permit, and the rebar and grout schedule comes from that design.
Frequently asked questions
How many concrete blocks per square foot?
1.125 standard 8x8x16 blocks per square foot of wall. Each block covers 8 in x 16 in of wall face once the mortar joint is counted, which is 128 square inches, or 0.889 sq ft, and 1 / 0.889 = 1.125.
How many blocks for a 100 square foot wall?
113 blocks laid, so 119 to buy with 5 % extra for breakage and cuts. That matches the 119 units per 100 sq ft, waste included, of the concrete masonry estimating tables for this unit.
How many blocks does a bag of mortar lay?
Up to 13 standard 8x8x16 blocks per 80 lb bag of mortar mix, according to the manufacturer data sheet. Divide the number of blocks by 13, round up, and keep a spare bag or two on site.
How much concrete does it take to fill an 8 inch block?
A fully grouted 8 in wall takes about 36.1 cu ft of grout per 100 sq ft, or 0.361 cu ft per square foot of wall, which is about 0.32 cu ft per block. Grouted every other cell, it takes half as much.
Do all the cells of a block wall need to be filled?
No. Cells are filled where the wall is reinforced, at the spacing your plans give, plus corners, ends and the sides of openings. Retaining walls and basement walls are often filled solid. The structural design decides, not the block count.
Source
Blocks per square foot follow from the nominal 8 in x 16 in face of the unit. Mortar coverage from QUIKRETE, Mortar Mix No. 1102 data sheet. Grout volumes and units per 100 sq ft from NCMA TEK 4-2A, Estimating Concrete Masonry Materials, Table 3. Core-fill bag yield from QUIKRETE, Core-Fill Grout No. 1585-08 data sheet. Actual coverage varies with the joint, the unit and the mason; the bag you buy wins.