40 ft × 8 ft garden wall
378 blocks with 5% waste
- Gross area: 320 sq ft
- 8×8×16 in face: 0.889 sq ft per block
- Base count: 360 blocks
- With 5% waste, rounded up: 378 blocks
Free construction estimator
Cinder Block Calculator
Calculate how many cinder blocks you need for your wall, including openings, waste, mortar, concrete fill, and estimated cost.
- Accurate calculations
- Multiple wall sections
- Door & window deductions
- Material & cost estimates
Calculator
1
Wall dimensions
Enter length and height. Add more sections for corners or extra walls.
ft
ft
No wall sections calculated yet
2
Block size
Nominal face size includes the mortar joint. Width is the wall thickness.
Face dimensions are the wall face (length × height). Width is how deep the block sits in the wall.
3
Doors & windows
Subtract doors and windows from the wall area for a more accurate block estimate.
No openings yet. Solid walls skip this step.
4
Waste allowance
Extra blocks for cuts, breakage, and layout. Actual waste varies by project — this is an estimate, not a requirement.
5
Mortar & core fill
Optional material estimates. Assumptions are listed with the results.
Mortar
in
Default assumes about 33 standard 8×8×16 in blocks per 80 lb bag with a 3/8 in face-shell joint.
Not every wall has grouted cores. This is a volume estimate only — it does not determine reinforcement or engineering requirements.
6
Cost estimate
Optional. Enter your local prices — this tool does not assume market rates.
Enter your wall dimensions to see your estimate.
The diagram updates as you type.
Your estimate will appear here
Add a wall length and height to calculate blocks, mortar, and optional cost.
Live results
Numbers update as you type. No submit button, no waiting.
See the wall
A scaled diagram shows courses, openings, and proportions.
Transparent math
Every estimate shows the formula with your actual values.
How the calculator works
- Step 1
Measure the wall
Enter length and height for each section. Add extra walls for corners or a full enclosure.
- Step 2
Choose a block and openings
Pick a standard CMU size or enter a custom face. Subtract doors and windows so you do not overbuy.
- Step 3
Read the estimate
Get a rounded block count plus optional mortar, grout fill, and cost using prices you provide.
Cinder block calculator formula
Calculations use nominal block face dimensions, which is how modular masonry is typically laid out. Mortar joints are included in the 8×16 in module. Intermediate math is not rounded; only the final purchase count is rounded up.
Wall area
Length × Height
Net wall area
Gross wall area − Opening area
Block quantity
Net wall area ÷ Block face area
Final blocks
Base blocks × (1 + Waste %)
Cinder block calculator examples
These examples are generated from the same calculation engine as the tool above, using 8×8×16 in blocks.
Same wall with a 3 ft × 7 ft door
354 blocks with 5% waste
- Opening area: 21 sq ft
- Net area: 299 sq ft
- Base count: 336.375 blocks
- With 5% waste, rounded up: 354 blocks
20 ft × 8 ft wall
189 blocks with 5% waste
- Gross area: 160 sq ft
- Base count: 180 blocks
- With 5% waste, rounded up: 189 blocks
About 100 square feet
119 blocks with 5% waste
- A 12.5 × 8 ft wall is 100 sq ft
- Base count: 112.5 blocks
- With 5% waste, rounded up: 119 blocks
What is a cinder block calculator?
A cinder block calculator estimates how many masonry units you need to build a wall. It starts from wall area, subtracts openings, divides by the block face, then adds waste. This one also estimates mortar bags, optional core fill, and cost if you enter prices. It is a planning tool, not an engineering design.
How many cinder blocks do I need?
For standard 8×8×16 in blocks, a quick rule is about 1.125 blocks per square foot of wall face, because each block covers 8×16 in including the mortar joint. A 40×8 ft wall is 320 sq ft, which is 360 blocks before waste. Add openings, extra wall sections, and a waste percentage in the calculator for a tighter number.
How to calculate cinder blocks
- Measure each wall length and height in the same units.
- Multiply to get gross area, then subtract door and window area.
- Divide net area by the block’s nominal face area.
- Multiply by (1 + waste). Round up to a whole number of blocks.
Common cinder block sizes
Nominal US sizes are usually 16 in long and 8 in high, with widths of 4, 6, 8, 10, or 12 in. The width is the wall thickness. Metric CMUs are often 20×20×40 cm, which is close to the same module. Actual manufactured size is typically 3/8 in smaller than nominal so a mortar joint completes the module. The Concrete Masonry & Hardscapes Association (opens in a new tab) documents typical CMU shapes, sizes, and that 3/8 in difference between nominal and specified dimensions.
| Nominal size | Typical use | Face coverage |
|---|---|---|
| 4 × 8 × 16 in | Partition / veneer | 0.889 sq ft |
| 6 × 8 × 16 in | Light-duty walls | 0.889 sq ft |
| 8 × 8 × 16 in | Most common CMU | 0.889 sq ft |
| 10 × 8 × 16 in | Heavier walls | 0.889 sq ft |
| 12 × 8 × 16 in | Foundation / retaining | 0.889 sq ft |
How many blocks are in a wall?
Courses run horizontally. A standard 8 in high block makes 12 courses in an 8 ft wall. Along a 40 ft wall, 16 in units need 30 blocks per course. 12 courses × 30 blocks is 360 — the same result as the area method when there are no openings.
How much mortar do I need?
Mortar quantity depends on joint thickness, whether you bed the full face or only the face shells, and how much is wasted on the board. This calculator’s default is about 33 standard 8×8×16 in blocks per 80 lb bag with a 3/8 in joint, scaled for other block faces. You can switch to a custom yield if your supplier quotes a different coverage.
How much concrete is needed to fill cinder blocks?
Hollow cores are only filled when the wall design calls for grout — for example around reinforcement, in bond beams, or in a fully grouted wall. The fill calculator uses an estimating factor of about 0.30 cubic feet per 8 in two-core block when 100% filled, scaled by width and fill percentage. Confirm grout volume with the project drawings; this site does not size rebar or specify grout strength.
Cinder block wall cost
Material cost is blocks + mortar + optional grout, plus delivery and labor if you include them. Prices vary widely by region and finish, so the calculator never invents a local average. Enter what you were quoted. Labor especially depends on height, access, reinforcement, and whether the wall is structural.
FAQ
Short answers to the questions people usually have before they buy blocks.
Divide the net wall area by the face area of one block. For a common 8×8×16 in block, the nominal face is 128 square inches (about 0.889 sq ft), so a 40×8 ft wall needs 360 blocks before waste. This calculator does that math, then subtracts doors and windows and adds your waste allowance.
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