Concrete Block Calculator (CMU)
Count concrete masonry units (CMU), mortar bags, and wall area for block walls — basement, retaining, garden, or structural — at typical 8×8×16 in nominal block size.
Concrete Block Calculator
Enter project dimensions below — results update instantly. Switch units freely.
Estimates assume typical industry density and waste factors. Always verify with your supplier and local building code before purchasing material.
CMU vs. Poured Concrete: Where Each Wins
Concrete Masonry Unit (CMU) walls are built from hollow concrete blocks bonded with mortar. They compete with poured concrete walls for basement foundations, retaining walls, and commercial buildings.
CMU wins when:
- No ready-mix access — remote sites or limited truck access
- DIY or small crew — no form set-up; two workers and a trowel
- Phased construction — build 4 courses today, 4 more next weekend
- Simple rectangular walls — no complex geometry
Poured concrete wins when:
- Speed — one pour vs. days of laying blocks
- Waterproofing — poured walls have no joints to seal
- Straight walls in one pour — less labor per linear foot above a certain project size
- Heavy engineered loads — easier to reinforce and vibrate for density
Cost: CMU typically runs $12-18 per ft² of wall installed; poured concrete $15-22 per ft². CMU slightly cheaper for DIY; poured cheaper for professional crews.
Block Coverage and Mortar Math
Block Area (8×16 in) = 128 in² = 0.89 ft²
Coverage rule for standard 8×8×16 in blocks:
- 1 block covers 0.89 ft² of wall (with 3/8-in mortar joints)
- ~1.125 blocks per ft² of wall area
- A 4-ft course contains 3 blocks of length + 1/8 block for the corner
Mortar requirements:
- One 80-lb bag of Type N mortar mix yields ~1.0 ft³ of mortar
- Standard 8×8×16 blocks use ~1/30 ft³ of mortar per block (joints + bed)
- Therefore: 1 bag lays ~30 blocks
- Round up for spillage: order 1 bag per 25 blocks for safety
| Nominal Size | Actual Size | Coverage per Block | Blocks per ft² |
|---|---|---|---|
| 4×8×16 (half) | 3⅝×7⅝×15⅝ | 0.89 ft² | 1.125 per ft² |
| 6×8×16 | 5⅝×7⅝×15⅝ | 0.89 ft² | 1.125 per ft² |
| 8×8×16 (standard) | 7⅝×7⅝×15⅝ | 0.89 ft² | 1.125 per ft² |
| 10×8×16 | 9⅝×7⅝×15⅝ | 0.89 ft² | 1.125 per ft² |
| 12×8×16 | 11⅝×7⅝×15⅝ | 0.89 ft² | 1.125 per ft² |
| 8×8×8 (half block) | 7⅝×7⅝×7⅝ | 0.45 ft² | 2.25 per ft² |
| 8×4×16 (cap) | 7⅝×3⅝×15⅝ | 0.45 ft² | 2.25 per ft² |
Block face area is same for all 8-in heights; differ only in wall depth. Block names use nominal (including 3/8-in mortar); actual size is 3/8-in smaller.
| Wall Type | Block Size | Rebar | Grout |
|---|---|---|---|
| Garden / ornamental (< 3 ft) | 6×8×16 | None required | Optional |
| Retaining wall (3-4 ft) | 8×8×16 | #4 @ 48 in vertical | Grout fill at rebar cells |
| Retaining wall (4-6 ft) | 8×8×16 or 10×8×16 | #4 @ 32 in vertical | Grout all cells |
| Basement wall (7-8 ft) | 8×8×16 or 10×8×16 | #4 @ 24 in vertical | Grout alternating cells |
| Structural wall (load-bearing) | 8×8×16 or 10×8×16 | Engineered | Engineered |
Grout = flowable concrete used to fill hollow CMU cells with rebar. Typically 2,500 psi pumpable mix.
Real-World Example Calculations
Garden Wall 20 × 3 ft, standard CMU
Decorative garden wall along property line.
- Wall length
- 20 ft
- Wall height
- 3 ft
- Block
- 8×8×16 in
- Waste
- 5%
Takeaway: Weekend DIY project. 4 blocks high × 15 blocks long. Cost ~$200 blocks + $50 mortar.
Basement Walls 30 × 8 ft, 8×8×16
Full basement wall, 8 ft tall, alternating-cell grouted.
- Wall length
- 140 ft perimeter
- Wall height
- 8 ft
- Block
- 8×8×16 in
- Waste
- 5%
Takeaway: 2-3 masons × 2-3 days. Include grout pump for filling rebar cells.
Retaining Wall 40 × 5 ft, 10×8×16
Full-height retaining wall along driveway grade change.
- Wall length
- 40 ft
- Wall height
- 5 ft
- Block
- 10×8×16 in
- Waste
- 5%
Takeaway: Add grout fill (~12 yd³) for all cells; retaining walls require full reinforcement and drainage.
Frequently Asked Questions
How many concrete blocks do I need?
For standard 8×8×16 in CMU: 1.125 blocks per square foot of wall area. A 20 × 8 ft wall = 160 ft² = ~180 blocks. Always add 5% for breakage and cutting waste.
How much does a concrete block cost?
In 2026: $1.75-3.00 per standard 8×8×16 block at big-box stores. Specialty blocks (corner, half, end-cap) cost more. Delivery adds $100-200 for 100+ block orders. Buying by the pallet (~90 blocks) is cheapest.
How much mortar per concrete block?
For standard 8×8×16 blocks: ~1 cubic foot of mortar per 30 blocks — or roughly 1 bag of 80-lb Type N mortar mix per 25-30 blocks. Order 25% extra for spillage.
What mortar do I use for concrete blocks?
Type N for general above-grade applications (garden walls, partitions). Type S for below-grade and heavy-load applications (retaining walls, basement walls). Type M for the heaviest structural walls. Premixed bagged mortar labeled with these types is available at most home centers.
How long does it take to build a concrete block wall?
Experienced mason: 200-300 blocks per day. DIY: 100-150 per day. For a 20 × 8 ft basement section (180 blocks), allow 1 day professional or 2 weekend days DIY. Slow rate for first-time masons; improve dramatically by day 3.
Do I need to fill concrete blocks with concrete?
Only where rebar is placed. Standard practice: fill cells containing vertical rebar with pumpable grout; leave other cells hollow. Fill every cell (solid-filled CMU) only for heavy structural loading or fire ratings. Full-filled walls require grout ≈ 1 yd³ per 150 ft² of wall.
How high can a concrete block wall be built?
Unreinforced: up to 4 ft. Reinforced with vertical rebar and grouted cells: up to 10 ft residential, higher with engineering. Retaining walls above 4 ft always require engineering sign-off. Don't exceed these heights without engineer verification.