How Many Concrete Blocks Do I Need for a Wall?
A 20×8 ft wall takes 189 standard CMU blocks. Here's the formula, a quick reference table, and what most people forget about mortar.
The magic number is 1.125. That's how many standard 8×8×16 concrete blocks (CMU) you need per square foot of wall. Memorize it and you can estimate any block wall in your head.
20-foot wall, 8 feet tall? That's 160 square feet. Times 1.125 = 180 blocks. Add 5% for waste and breakage: 189 blocks.
The Formula
Here it is in full:
Wall length (ft) × wall height (ft) × 1.125 × 1.05 = blocks needed
The 1.125 comes from the block dimensions. A standard CMU is nominally 8 inches tall by 16 inches wide (including the 3/8-inch mortar joint). That's 0.667 × 1.333 feet, or 0.889 square feet per block. Flip it around: 1 ÷ 0.889 = 1.125 blocks per square foot.
The 1.05 is your 5% waste factor. Blocks chip, crack during delivery, and sometimes you need to cut one and the offcut is too small to use.
Quick Reference Table
| Wall Size | Area (sqft) | Blocks (5% waste) | Mortar Bags (80-lb) | |-----------|-------------|-------------------|-------------------| | 10 × 4 ft | 40 | 48 | 5 | | 20 × 4 ft | 80 | 95 | 9 | | 20 × 8 ft | 160 | 189 | 17 | | 40 × 8 ft | 320 | 378 | 33 | | 50 × 8 ft | 400 | 473 | 41 |
Don't Forget the Mortar
This is what most people miss when budgeting a block wall. Plan for 8.5 bags of 80-lb mortar mix per 100 blocks. That 189-block wall? You need 17 bags of mortar. At $8–$12 per bag, that's $136–$204 just for mortar on top of the block cost.
Use Type S mortar for load-bearing walls and anything below grade. Type N is fine for non-load-bearing partition walls above grade.
Subtracting Openings
Every opening saves you blocks:
- Standard door (3×7 ft): 21 sqft = roughly 24 fewer blocks
- Window (3×4 ft): 12 sqft = roughly 14 fewer blocks
- Garage door (16×7 ft): 112 sqft = roughly 126 fewer blocks
But don't get too aggressive with deductions. You'll still need blocks around the openings for cutting and fitting, and you might add a few extra courses for lintels and bond beams above the openings.
About the Weight
Here's something that catches first-timers off guard: a standard 8×8×16 CMU weighs about 35 pounds. That 189-block wall is over 3.3 tons of block plus another half ton of mortar.
Plan your delivery carefully. Have the pallets set close to the wall location — you do not want to carry 35-pound blocks across a jobsite 189 times. If you're doing a backyard wall, make sure the delivery truck can actually get the pallets where you need them.
Cost
Standard CMU blocks run $1.50–$3.00 each. For our 189-block wall:
- Blocks: $284–$567
- Mortar: $136–$204
- Total materials: $420–$771
That doesn't include rebar, grout for filled cores, or cap blocks — all of which you'll probably need for any wall over 4 feet tall. But it gives you a solid ballpark for the main materials.
Use our block wall calculator to get exact counts including door and window deductions for your specific wall.
Frequently Asked Questions
- How many concrete blocks do I need per square foot of wall?
- Standard 8×8×16-inch concrete blocks cover about 0.89 square feet each (face area minus typical mortar joints). You need roughly 1.125 blocks per square foot of wall face area. For a 100 sq ft wall, that's about 113 blocks — always round up and add 5% for cuts and breakage.
- What are standard concrete block sizes?
- The most common is the 8×8×16-inch CMU (concrete masonry unit), which is the industry standard for most walls. Other sizes include 4×8×16 (half-height), 6×8×16, and 12×8×16 for thicker structural walls. The nominal dimensions include the mortar joint; actual block size is about 3/8 inch smaller.
- How much mortar do I need for a concrete block wall?
- A typical rule of thumb is one 70-lb bag of mortar mix per 30–35 standard blocks. For a 100-block wall, plan on 3–4 bags. Mortar coverage varies with joint thickness and block texture, so buy a little extra — leftover mortar is cheap and running out mid-project is frustrating.
- Does a concrete block wall need a foundation?
- Any block wall more than 2–3 courses (16–24 inches) high should rest on a concrete footing below the frost line. Shorter decorative walls can sit on a compacted gravel base. Check local building codes — most require permits and footings for retaining walls over 4 feet and freestanding walls over 3 feet.
- Can I build a block wall myself?
- Yes, for simple garden walls and short retaining walls. Block work requires careful layout, level courses, and consistent mortar joints. Structural walls, tall retaining walls, and anything requiring engineering or permits should involve a professional. Mistakes in retaining walls can be expensive (and dangerous) to fix.