CMU BLOCK · REVIEWED MAY 2026 · BY BRENT

CMU BLOCK COUNT

blocks = ft² ÷ 0.889

Wall length

Wall height

Openings (doors/windows)

ft²

Block size (W×H×L)?

Include rebar??

RESULT

—

FILL IN ABOVE

1.125 blocks/ft² (8×8×16 face). 5% waste. Mortar = 3 × 80-lb bags per 100 blocks. Estimate only — verify structural designs with engineer.

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About this calculator

This CMU calculator estimates the number of concrete masonry units (blocks) for a wall, plus the mortar bags and optional rebar runs. The math assumes standard nominal sizes — face dimension 16" × 8" with a 3/8-inch mortar joint, giving 1.125 blocks per square foot of wall (about 0.889 ft² per block face). Mortar adds three 80-lb bags of Type S per 100 blocks. Rebar follows the IRC and IBC pattern of #4 vertical at 4 ft on center plus #4 horizontal in every fourth course bond beam — typical for non-engineered residential and light commercial. ESTIMATE ONLY — for engineered or load-bearing CMU walls verify with a structural engineer.

How to use this calculator

Enter wall length and height in feet. Subtract any door or window opening area. Pick the block depth — all standard CMU has the same 16×8 face dimension; only the depth changes. 4-inch is for partitions; 6-inch and 8-inch are typical for structural walls; 12-inch is for heavy loads or basement walls.

Indicate whether you want rebar in the calculation. Reinforced CMU adds #4 vertical bars at 4 ft on center plus #4 horizontal bars in every fourth course as a bond beam — the typical non-engineered residential pattern. The result includes block count, mortar bags, and rebar linear feet.

Worked example

For a 30 × 8 ft wall with no openings, 8×8×16 standard block, with rebar:

Wall area: 240 ft². Blocks: ⌈(240 ÷ 0.889) × 1.05⌉ = 284 blocks. Mortar: ⌈(284 ÷ 100) × 3⌉ = 9 bags of 80-lb Type S.

Rebar: vertical #4 at 4 ft o.c. = ⌈30 ÷ 4⌉ + 1 = 9 bars × 8 ft = 72 lin ft. Horizontal #4 in every 4th course = ⌈12 courses ÷ 4⌉ = 3 rows × 30 ft = 90 lin ft. Total rebar: 162 lin ft (9 sticks of 20-ft #4).

At $2-$3 per 8×8×16 block + $7-$10 per mortar bag + $10-$15 per stick of #4: materials run $720-$1,065 (blocks) + $63-$90 (mortar) + $90-$135 (rebar) = $873-$1,290 total.

With 12-inch block (basement walls): same 240 ft² needs 284 blocks at $4-$6 each = $1,136-$1,704 in blocks alone. The 12-inch is double the cost.

Common mistakes & waste factors

Forgetting to subtract openings. A wall with a 7-ft × 3-ft garage door (21 ft²) needs 24 fewer blocks than calculated without the subtraction — easy to overorder.

Mixing block depths. All CMU has the same 16×8 face; only depth changes. The block COUNT is the same regardless of depth — what changes is bond beam steel, mortar consumption, and weight.

Underordering rebar. The default pattern (vertical at 4 ft o.c., horizontal every 4th course) is for NON-engineered residential. Engineered or load-bearing walls usually need vertical at 2 ft o.c. and horizontal every 2nd course — twice the steel.

Forgetting grouting. Reinforced CMU walls need cells with rebar grouted solid (filled with high-slump concrete or grout). Adds about 1 cubic yard of grout per 100 ft² of wall at 4 ft o.c. vertical spacing.

Rules of thumb

Standard CMU face: 16×8 in. 1.125 blocks per ft² of wall (0.889 ft² per block face) with 3/8" mortar joint.

Mortar: 3 × 80-lb bags of Type S per 100 blocks.

5% waste for cut blocks at corners and openings.

Rebar (non-engineered): #4 vertical at 4 ft o.c., #4 horizontal in every 4th course bond beam.

Grout: ~1 yd³ per 100 ft² of wall when reinforced cells are grouted at 4 ft o.c.

Block weight: 4-inch ~25 lb, 8-inch ~35 lb, 12-inch ~60 lb. Pace: 100-200 blocks per day for DIY, 400-600 for a pro mason.

Common questions

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How many concrete blocks per square foot?

1.125 blocks per square foot of wall using standard 8×8×16 face dimensions with a 3/8" mortar joint — that's 0.889 ft² per block face. A 100 ft² wall needs 113 blocks; round up 5% for breakage and you order 119. Smaller faces (4×16 half-high block, for example) double the block count because the face is half the height.

How many bags of mortar for 100 concrete blocks?

Three 80-lb bags of Type S or N mortar per 100 blocks — about 0.5 ft³ of mortar per 100 blocks at a 3/8 in joint. Type S is the standard for load-bearing CMU walls (1,800 psi compressive strength). For below-grade foundations or seismic regions, the engineer may spec Type M (2,500 psi). Add 10% waste for spillage and partial bags. A block trowel is wider than a brick trowel and sized specifically for CMU coursing.

When does a CMU wall need rebar?

Per IRC R606 and IBC Chapter 21, every CMU wall in seismic zones D1/D2 needs vertical reinforcement. Most jurisdictions also require rebar for any retaining wall, basement wall, or wall over 8 ft tall. Typical pattern is #4 vertical at 4 ft on center grouted into the cells plus #4 horizontal in every fourth course bond beam. For engineered designs (tall walls, heavy loads, seismic), a structural engineer specifies bar size and spacing — the calculator's rebar option assumes the standard non-engineered residential pattern.