Concrete Block Calculator
Enter your wall dimensions to estimate standard 16×8 blocks, mortar, and rebar.
Runs entirely in your browser — your numbers never leave this page.
How it works
Standard CMU (16×8×8 in) covers 0.89 sq ft of wall face. Mortar runs about one 80-lb bag per 30–40 blocks:
Blocks = (Length × Height) ÷ 0.89 × (1 + Waste)
- Footing first: 24 inches wide × 12 inches deep minimum, below frost line — block walls crack when footings move.
- Running bond: stagger vertical joints by half a block every course. Stack bond (aligned joints) is weak and usually not code-allowed.
- Rebar: #4 verticals every 4 ft in filled cores for structural/retaining walls, tied to the footing dowels.
- Mortar joints: 3/8 inch — thicker joints waste mortar and weaken the wall. Butter the ends, not just the beds.
- Control joints: every 20–25 ft on long walls to manage shrinkage cracking.
Frequently asked questions
How many concrete blocks do I need?
Wall face area ÷ 0.89, plus 10%. A 24×4 ft wall is 96 sq ft → ~108 blocks + waste ≈ 119 blocks.
How much mortar per block?
Roughly one 80-lb bag per 30–40 blocks with 3/8-inch joints. A 119-block wall needs ~4 bags. Pre-mixed Type S is the standard choice.
Do I need rebar in a block wall?
For structural, retaining, or tall walls: yes — #4 verticals every 4 ft in concrete-filled cores, tied into the footing. Garden walls under 2 ft can skip it.
How deep should the footing be?
Below the frost line, minimum 12 inches deep × 24 inches wide for an 8-inch wall. The footing carries everything — skimp it and the wall cracks.
What is running bond?
Staggering each course half a block so vertical joints do not align. It is the standard pattern — aligned "stack bond" is weak and often against code.
Can I lay block in cold weather?
Above 40°F and rising is the rule. Mortar will not cure properly near freezing — tent and heat the work area or wait for spring.
How do I cut concrete block?
Masonry saw with a diamond blade (wet is cleaner), or score-and-split with a brick hammer for rough cuts. Always wear a respirator — silica dust.
Should cores be filled?
Fill cores with concrete wherever rebar runs, and on all retaining/structural walls. Unfilled cores are fine for freestanding garden walls.
What is the difference between Type N and Type S mortar?
Type S is stronger and the default for structural and below-grade work. Type N suits above-grade non-structural walls. Do not use Type M (too rigid) for most block work.
How long does a block wall last?
Decades to a century with a proper footing and drainage. The failure points are always the same: moving footings, water pressure, and missing control joints.