Concrete Block Calculator
Find how many concrete blocks (cinder blocks) you need for a wall, plus bags of mortar, with door and window openings and waste.
Results
| Item | Value |
|---|
Quick Answer
Standard 8 × 8 × 16 in blocks cover 8 × 16 in of wall with the mortar joint, so you need 1.125 blocks per square foot. A wall 30 ft long and 4 ft high is 120 sq ft, or 135 blocks. With 5% extra for waste, buy 142 blocks and about 11 bags of 80 lb mortar mix.
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Table of Contents
- Concrete Block Calculator
- Quick Answer
- Related Calculators
- How This Concrete Block Calculator Works
- How to Use the Concrete Block Calculator
- Core Formulas
- Key Terms
- Worked Example
- Real-Life Examples
- Concrete Block vs Cinder Block
- Blocks for Common Wall Sizes
- Planning Tips
- Common Mistakes to Avoid
- Frequently Asked Questions
- References & Sources
How This Concrete Block Calculator Works
Enter the length and height of the wall, choose the block face size, and subtract any doors or windows. The calculator works out the wall area, divides it by the area one block covers, and adds extra for broken blocks and cuts. It also estimates the bags of mortar and the number of courses.
Quick explanation: block count depends on the face of the block you see in the wall, not its depth.
- Standard blocks: 1.125 per sq ft, whatever their width (4, 6, 8, 10 or 12 in).
- Nominal size: real size plus a 3/8 in joint.
- Waste: 5% to 10% is common.
How to Use the Concrete Block Calculator
- Enter the wall length and height in feet.
- Choose the block face size. Most walls use standard 8 × 8 × 16 in blocks.
- Enter the area of any doors or windows to subtract.
- Set the waste allowance, usually 5% to 10%.
- Press Calculate and read the blocks, courses and mortar bags.
Core Formulas
- Wall area: length × height − openings
- Blocks: wall area (sq ft) × 144 ÷ block face area (sq in)
- Standard block: 144 ÷ (8 × 16) = 1.125 blocks per sq ft
- With waste: blocks × (1 + waste %), rounded up
- Mortar: standard blocks ÷ 13 per 80 lb bag
Key Terms
- CMU: concrete masonry unit, the trade name for a concrete block.
- Nominal size: the block size plus a 3/8 in mortar joint, such as 8 × 8 × 16 in.
- Course: one horizontal row of blocks.
- Mortar joint: the layer of mortar between blocks, usually about 3/8 in thick.
- Footing: the concrete base the wall sits on.
Worked Example
A garage wall is 24 ft long and 8 ft high, with a 3 × 7 ft door.
- Wall area = 24 × 8 − 21 = 171 sq ft
- Blocks = 171 × 1.125 = 192.4
- With 5% waste: 202 blocks and about 16 bags of mortar
Real-Life Examples
- A garden wall: 20 ft long and 3 ft high needs 71 standard blocks with 5% waste.
- A shed foundation wall: 4 walls of 12 ft, 2 ft high, is 96 sq ft, or 114 blocks.
- A retaining wall: 40 ft long and 4 ft high needs 198 blocks with 10% waste. Check local rules for walls over about 4 ft.
- A barbecue surround: 16 ft of wall, 3 ft high, needs 57 blocks and about 5 bags of mortar.
Concrete Block vs Cinder Block
- Concrete block: made with cement, sand and gravel. Today almost all blocks sold in the US are concrete.
- Cinder block: an older name for blocks made with coal ash (cinders). Many people still use the name.
- Same count: the size and number of blocks are the same either way.
Blocks for Common Wall Sizes
| Wall (ft) | Area (sq ft) | Blocks | Mortar (80 lb bags) |
|---|---|---|---|
| 10 × 3 | 30 | 36 | 3 |
| 10 × 4 | 40 | 48 | 4 |
| 20 × 4 | 80 | 95 | 8 |
| 20 × 6 | 120 | 142 | 11 |
| 30 × 4 | 120 | 142 | 11 |
| 40 × 4 | 160 | 189 | 15 |
| 24 × 8 | 192 | 227 | 18 |
| 50 × 6 | 300 | 355 | 28 |
Planning Tips
- Plan in 8 in steps: walls in multiples of 8 in need fewer cut blocks.
- Corners: corner and end blocks have a smooth end. Count them separately if needed.
- Footing: a block wall needs a solid concrete footing below the frost line.
- Reinforcement: taller and retaining walls usually need steel rebar and grout.
Common Mistakes to Avoid
- Using the real block size. Use the nominal size, which includes the mortar joint.
- Forgetting openings. Subtract doors and windows, but add lintels if needed.
- No extra blocks. Blocks chip and need cutting, so order 5% to 10% more.
- Skipping the footing. A wall without a proper footing can crack and lean.
- Short on mortar. The first course uses a full, thick bed, so keep a spare bag.
Frequently Asked Questions
1.125 standard 8 × 8 × 16 in blocks per square foot of wall.
113 standard blocks before waste, or 119 with 5% extra.
Nominal 8 × 8 × 16 in. The real size is 7⅝ × 7⅝ × 15⅝ in, leaving room for 3/8 in joints.
QUIKRETE says one 80 lb bag of Mason Mix lays up to 13 standard blocks. This calculator uses that figure.
No. A 4, 6, 8 or 12 in wide block has the same 8 × 16 in face, so the count is the same.
12 courses of standard 8 in high blocks.
5% for simple walls, and up to 10% for walls with many corners, openings or cuts.
They are the same shape and size. True cinder blocks use coal ash, but most blocks sold today are concrete.
Enter your local price per block to see the block cost. Add mortar, footing, steel and labor for the full cost.
Often yes, for walls that carry loads, retain soil or are tall. Check your local building code.
It varies by supplier and block size. For standard 8 × 8 × 16 in blocks, ask your supplier for their pallet count.
A standard hollow 8 × 8 × 16 in block often weighs about 35 to 40 lb, depending on the mix.
References & Sources
| Source | Used For | Link |
|---|---|---|
| QUIKRETE, Mason Mix (No. 1136) | Mortar for block walls, with a link to the yield data sheet. | https://www.quikrete.com/productlines/masonmix.asp |
| Wikipedia, Concrete block | Standard block sizes and uses. | https://en.wikipedia.org/wiki/Concrete_block |