Concrete Block Calculator
Planning a block wall shouldn’t feel like doing taxes. With a few measurements you can estimate how many concrete blocks you’ll need, what it might cost, and roughly how many bags of mortar to buy. This guide walks you through the logic our calculator uses, shows the formulas behind it, and answers practical questions about block sizes, mortar, and unit conversions. You’ll see clear examples and shortcuts you can use on site or at your desk.
How the Concrete Block Calculator Works
The calculation follows a simple chain: measure the wall, determine the block’s face area, divide to get quantity, then add cost and mortar. That’s it. You can enter wall height and width or type the area directly if you already know it. For blocks, pick a common preset size or enter a custom face size when you’re using specialty CMUs.
- Wall area = height × width (use feet and inches or metric; the calculator converts under the hood).
- Block face area uses the nominal size (includes the mortar joint) because that’s how walls are laid out in the field. See the size table below.
- Blocks needed = ceiling(wall area ÷ block face area). We round up because you can’t buy half a block and you’ll trim a few.
- Total cost = blocks × price per block in your selected currency.
- Mortar bags use a widely cited rule of thumb: ~3 standard bags per 100 standard 16×8 blocks, rounded up to full bags for purchase. This is a planning estimate, not a specification.
Formulas & Units
You can do this math by hand in two steps. First, compute areas. Second, divide and round up.
1) Wall area
Wall area (ft²) = Height (ft) × Width (ft)
If you measured in inches or metric, convert to feet first. The calculator takes care of conversions automatically.
2) Block face area
For a 16″ × 8″ nominal CMU, the face area is (16 × 8) in² = 128 in². Convert to square feet:
128 ÷ 144 = 0.8889 ft².
Why nominal? Construction follows the module that includes the typical 3/8″ mortar joint, which is the basis of the 4″ modular grid in U.S. practice. Using nominal sizes simplifies layout and yields accurate takeoffs for most walls
3) Block quantity
Blocks needed = ⌈ Wall area ÷ Block face area ⌉
4) Total cost
Total cost = Blocks × Price per block
5) Mortar estimate (rule of thumb)
A common estimating rule is 3 bags per 100 blocks. Convert that proportion to your count, then round up to whole bags:
Mortar bags = ⌈ Blocks × (3 ÷ 100) ⌉
Multiple regional guides and suppliers publish the same factor for quick estimating . Manufacturer data sheets also publish coverage per bag; use those when you need tighter accuracy for bidding or ordering.
Nominal vs. Actual CMU Sizes (with a quick table)
You’ll often hear “8×8×16 block.” That’s a nominal size, which already includes the typical mortar joint. The actual size of a standard CMU is 7⅝″ × 7⅝″ × 15⅝″; the extra ⅜″ joint brings it back to the nominal 8″ module. This convention lines blockwork up with the common 4″ design grid in North America and appears in industry references and ASTM practice.
| Nominal size (in) | Typical actual size (in) | Nominal face area (ft²) | Notes |
|---|---|---|---|
| 16 × 8 | 15⅝ × 7⅝ | 0.8889 | Most common “standard” block; used for examples. |
| 12 × 8 | 11⅝ × 7⅝ | 0.6667 | Often used for openings and layout adjustments. |
| 8 × 8 | 7⅝ × 7⅝ | 0.4444 | Half-length units, still on the 8″ module. |
| 16 × 4 | 15⅝ × 3⅝ | 0.4444 | Half-height units for coursing. |
| 12 × 4 | 11⅝ × 3⅝ | 0.3333 | Half-height, shorter length. |
| 8 × 4 | 7⅝ × 3⅝ | 0.2222 | Quarter block of the 16 × 8 module. |
Step-by-Step: Calculate Blocks, Cost, and Mortar
1) Measure or enter the wall area
Measure the wall’s height and width. Multiply the two numbers to get area. If you already know the area from a plan set or a takeoff, type it in directly.
2) Choose a block size
Pick the preset that matches your block. If you’re using a non-standard face size, enter a custom width and height and the calculator will convert it to square feet automatically.
3) Get the block quantity
Divide the wall area by the block’s face area then round up. That gives you the count of blocks to lay out the face area of the wall. You’ll cut or trim in the field as needed.
4) Estimate total cost
Multiply by the price per block. If you’re buying pallets, ask your supplier about volume pricing. Many yards discount per-unit cost at the pallet level.
5) Estimate mortar bags
Use the simple rule of thumb: 3 bags per 100 blocks. Round up to whole bags since you can’t buy a fraction. For tighter estimates, check the coverage table for the exact mortar you’ll use example manufacturer data.
Worked Examples
Example A: 20 ft × 6 ft garden wall using 16″ × 8″ CMUs
- Wall area = 20 × 6 = 120 ft²
- Block face area (16 × 8) = 128 in² = 0.8889 ft²
- Blocks = ⌈120 ÷ 0.8889⌉ = ⌈135.0⌉ = 135 blocks
- Price per block = $3.75 → Total cost ≈ $506.25
- Mortar = ⌈135 × 3/100⌉ = ⌈4.05⌉ = 5 bags
Example B: 15 m × 2.4 m site wall using 400×200 mm nominal blocks
- Convert to ft for consistency if needed or work in m².
- Wall area = 15 × 2.4 = 36 m² = 387.50 ft².
- Block face area (400×200 mm) = 0.08 m² = 0.8611 ft².
- Blocks = ⌈387.50 ÷ 0.8611⌉ = 451 blocks.
- Mortar = ⌈451 × 3/100⌉ = 14 bags.
Example C: Custom unit 12″ × 4″ nominal
- Face area = (12×4) in² = 48 in² = 0.3333 ft²
- For a 200 ft² wall → Blocks = ⌈200 ÷ 0.3333⌉ = 600 blocks
- Mortar = ⌈600 × 3/100⌉ = 18 bags
These examples focus on the face area of the wall. Reinforcing, bond beams, lintels, control joints, or special shapes will change the mix of units you purchase. Use this estimate to start a conversation with your supplier.
How Many Bags of Mortar Do You Need?
If you want a quick answer, use the 3-per-100 rule and round up. It’s a practical average published by multiple masonry guides . It assumes standard 8×8×16 CMUs, typical ⅜″ joints, and average waste.
Need to tighten the estimate for a purchase order? Look up the coverage table for the exact mortar you’ll use. For example, QUIKRETE lists coverage per 80-lb bag for standard 8×8×16 blocks. Some calculators use “blocks per bag” yields like 12–13 blocks per 80-lb bag which is another way to do the same math. Your yield will vary with joint thickness, block texture, and your crew’s technique.
| Blocks | Rule-of-thumb bags | Rounded up |
|---|---|---|
| 100 | 3.0 | 3 |
| 250 | 7.5 | 8 |
| 400 | 12.0 | 12 |
| 600 | 18.0 | 18 |
Pro Tips for Cleaner Estimates
- Use nominal sizes for layout. It aligns with real-world coursing and simplifies takeoffs.
- Round up generously on small projects. A few extra blocks save a second trip to the yard.
- Check pallet quantities. Suppliers often bundle CMUs by pallet which can lower unit cost.
- Account for openings. Subtract doors and windows from wall area to avoid over-ordering.
- Clarify special shapes. Bond beams, lintel blocks, and corner units change counts.
- Verify local standards. ASTM C90 governs CMU dimensions and tolerances in the U.S. Many references summarize its practical effects on sizing and nominal dimensions.
Concrete Block Calculator FAQs
What’s the difference between nominal and actual CMU size?
Nominal size includes the typical mortar joint so layout follows a 4″ module. Actual size is the physical block you measure with a tape. For a standard “8×8×16” block the actual face is about 7⅝″ × 15⅝″; the ⅜″ joint makes up the difference.
How do I calculate blocks for an opening?
Compute the opening area and subtract it from the wall area. Then divide by the block face area and round up. For a door 3′ × 7′ that’s 21 ft². If your wall is 200 ft² the net is 179 ft².
Which block size should I choose?
16″ × 8″ nominal is the workhorse for many walls, though projects often mix in 12″ and half-height units for coursing and layout. When in doubt ask your supplier to confirm what’s in stock and what ships fast.
Does the mortar rule change for split-face or textured units?
It can. Rough textures and deep joints consume more mortar. Use the manufacturer’s coverage chart for accurate buys example.
What about reinforcing, grout, and cores?
This calculator focuses on face units for estimating. Reinforcement, core fill, lintels, and bond beams are project-specific. Your engineer or local code will dictate spacing and grout volume. Some producers publish grout coverage per bag to help with those calculations.
Can I estimate blocks using only wall area?
Yes. Divide wall area by block face area and round up. You can see the same approach in many industry calculators and guides.
How accurate is the mortar estimate?
It’s a planning figure. Crew technique, joint thickness, block texture, and weather all shift mortar usage. When you need precision for procurement, lean on the coverage table from your selected mortar and include a small waste factor.
