Concrete Block Calculator
How many concrete blocks do I need for a 20 ft wall 4 ft high?
A 20 × 4 ft wall is 80 sq ft, which takes 90 standard 8 × 8 × 16 in blocks at 1.125 per sq ft, or 95 with 5% extra for breakage. Plan on about 3 bags of 80 lb mortar mix (1 per 30 blocks). Filling every other core takes roughly 11.7 cu ft of grout, about 20 bags of 80 lb.
This concrete block calculator counts the CMU blocks for a wall from its length and height, subtracts openings and adds a breakage allowance. It works for standard 8 × 8 × 16 in blocks and for 6 and 12 in wide blocks with the same 8 × 16 in face.
It also estimates bags of mortar mix from a rule of thumb you can adjust, and the grout needed to fill the cores, either every core or every other core, in cubic feet, cubic yards and 80 lb bags. The defaults show a 20 ft wall 4 ft high.
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How to Use the Concrete Block Calculator
- 1
Enter the wall size
Type the wall length and height in feet. Include any courses below grade that sit on the footing.
- 2
Subtract openings
Enter the total area in square feet of any doors, windows or vents the wall will have. Leave it at 0 for a solid wall.
- 3
Choose the block and allowances
Pick 8, 6 or 12 in wide blocks, set the breakage allowance (5% by default) and the number of blocks one 80 lb bag of mortar mix will lay (30 by default).
- 4
Read the results
The tool shows blocks to buy, blocks before the allowance, courses, bags of mortar mix and core fill grout for every core and every other core.
How the concrete block calculator works
Concrete blocks are sold by nominal size. A standard block is actually 7 5/8 × 7 5/8 × 15 5/8 in; adding a 3/8 in mortar joint brings the face to 8 × 16 in, or 128 sq in. A square foot is 144 sq in, so a wall takes 144 ÷ 128 = 1.125 blocks per square foot regardless of the block width.
The calculator multiplies the net face area by 1.125 and rounds up to get blocks before the allowance, then adds the breakage percentage and rounds up again. Mortar and grout are based on the block count before the allowance, because spare blocks are not laid.
Grout per block depends on the core size. The tool uses about 0.19 cu ft for a 6 in block, 0.26 cu ft for an 8 in block and 0.43 cu ft for a 12 in block with every core filled, and half that for every other core. These are approximate; the real figure varies with the block design and how much grout runs into joints.
Blocks = (length ft × height ft − openings sq ft) × 1.125, rounded up
Blocks to buy = blocks × (1 + breakage %), rounded up
Mortar bags = blocks ÷ blocks per bag (default 30), rounded up
Grout cu ft = blocks × grout per block × 1 (every core) or 0.5 (every other core)
Grout bags = grout cu ft ÷ 0.60 (80 lb bag), rounded up; cu yd = cu ft ÷ 27Worked example: a 20 × 4 ft garden wall
- Face area: 20 × 4 = 80 sq ft, with no openings.
- Blocks: 80 × 1.125 = 90 blocks. Add 5%: 90 × 1.05 = 94.5, so buy 95.
- Courses: 4 ft × 12 ÷ 8 in = 6 courses of 15 blocks each.
- Mortar: 90 ÷ 30 = 3 bags of 80 lb mortar mix.
- Grout, every core: 90 × 0.26 = 23.4 cu ft (0.87 cu yd), or 23.4 ÷ 0.6 = 39 bags of 80 lb.
- Grout, every other core: 11.7 cu ft (0.43 cu yd), or 19.5, so 20 bags.
Blocks, mortar and grout for common walls
Standard 8 × 8 × 16 in blocks, no openings, 5% breakage, 30 blocks per bag of mortar mix. Grout bags are 80 lb at 0.60 cu ft, rounded up.
| Wall | Face sq ft | Blocks | Buy (+5%) | Mortar bags | Grout every core | Grout every other core |
|---|---|---|---|---|---|---|
| 10 × 4 ft | 40 | 45 | 48 | 2 | 0.43 cu yd · 20 bags | 0.22 cu yd · 10 bags |
| 20 × 4 ft | 80 | 90 | 95 | 3 | 0.87 cu yd · 39 bags | 0.43 cu yd · 20 bags |
| 30 × 4 ft | 120 | 135 | 142 | 5 | 1.30 cu yd · 59 bags | 0.65 cu yd · 30 bags |
| 40 × 4 ft | 160 | 180 | 189 | 6 | 1.73 cu yd · 78 bags | 0.87 cu yd · 39 bags |
| 50 × 3 ft | 150 | 169 | 178 | 6 | 1.63 cu yd · 74 bags | 0.81 cu yd · 37 bags |
| 100 × 4 ft | 400 | 450 | 473 | 15 | 4.33 cu yd · 195 bags | 2.17 cu yd · 98 bags |
Block sizes compared (20 × 4 ft wall)
The block count does not change with width, because every size here has the same 8 × 16 in face. Wider blocks weigh more and hold more grout.
| Block | Blocks | Grout per block (approx.) | Every core | Every other core |
|---|---|---|---|---|
| 6 × 8 × 16 in | 90 | 0.19 cu ft | 17.1 cu ft · 29 bags | 8.6 cu ft · 15 bags |
| 8 × 8 × 16 in | 90 | 0.26 cu ft | 23.4 cu ft · 39 bags | 11.7 cu ft · 20 bags |
| 12 × 8 × 16 in | 90 | 0.43 cu ft | 38.7 cu ft · 65 bags | 19.4 cu ft · 33 bags |
Edge cases the simple formula misses
- Corners and ends: walls that turn corners or end at a pier often use corner or half blocks. Order them separately; the area method counts them as standard blocks.
- Cap blocks: solid caps on top of a garden wall are not included. Count them as wall length ÷ cap length.
- Bond beams and lintels over openings use special blocks and are usually grouted solid with rebar.
- Mortar use varies a lot with the mason, joint tooling and waste. The 30 blocks per bag figure is a common rule of thumb; change it if your supplier or bag label suggests a different yield.
- For large grout volumes, ready-mix grout or a grout pump is often easier than bagged material. Ask your supplier.
Footings and reinforcement
Every block wall needs a continuous concrete footing below the frost line, typically wider than the block. Work out its volume with the concrete calculator using the Footing tab, and the gravel base with the gravel calculator.
For a short landscape wall, interlocking segmental blocks that need no mortar may be simpler; compare with the retaining wall calculator.
When to call a professional
A low freestanding garden wall is a reasonable DIY project. Foundation walls, retaining walls, load-bearing walls and anything over about 4 ft need vertical and horizontal rebar, grouted cores and a footing sized for the soil and load. Those details are set by your local building code or an engineer, and usually require a permit and inspection. Use this calculator to budget materials, not to design a structural wall.
Features
- Blocks from wall face area at 1.125 nominal 8 × 16 in blocks per sq ft
- 6, 8 and 12 in wide blocks
- Subtracts door, window and vent openings
- Breakage allowance (default 5%)
- Mortar mix bags at an adjustable blocks-per-bag rule (default 30)
- Approximate core fill grout for every core or every other core, in cu ft, cu yd and 80 lb bags
Frequently Asked Questions
How many concrete blocks are in a square foot?
A standard block has a nominal face of 8 × 16 in including the mortar joint, which is 128 sq in. A square foot is 144 sq in, so a wall takes 1.125 blocks per square foot, or 112.5 blocks per 100 sq ft. The same applies to 6 and 12 in blocks with an 8 × 16 in face.
How many blocks do I need for a 20 ft wall 4 ft high?
The wall is 80 sq ft, so 80 × 1.125 = 90 standard 8 × 8 × 16 in blocks. Add about 5% for breakage and cuts, which brings it to 95 blocks. The wall is 6 courses high with 15 blocks per course.
How many bags of mortar do I need per block?
A common rule of thumb is one 80 lb bag of mortar mix for about 30 blocks, so 90 blocks need about 3 bags. Actual use depends on joint size, tooling and waste, so buy a spare bag and adjust the blocks per bag setting if your bag label gives a different figure.
How much grout does it take to fill concrete block cores?
An 8 in block takes roughly 0.26 cu ft of grout with every core filled. For 90 blocks that is about 23.4 cu ft, or 0.87 cu yd, which is 39 bags of 80 lb at 0.60 cu ft per bag. Filling every other core takes about half: 11.7 cu ft, or 20 bags. Treat these as approximate.
Do I need to fill every core in a block wall?
Not always. Many walls only grout the cores that hold vertical rebar, at a spacing set by code or the engineer, while foundation and retaining walls may be grouted more fully. A short decorative wall may need no grout at all. Follow your local code or the design you are working from.
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