Block Mortar Calculator
Calculate how much mortar you need for concrete block (CMU) walls
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Quick Answer
One 80-pound bag of mortar mix lays approximately 12-13 standard 8x8x16 concrete blocks with 3/8" joints. For 100 blocks, you'll need about 8-9 bags plus 10% for waste. Type S mortar is recommended for most block wall applications due to its high strength and versatility.
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Mortar Coverage by Block Size
| Block Size | Blocks per 80lb Bag | Bags per 100 Blocks |
|---|---|---|
| 8" x 8" x 16" (Standard) | 12.5 | 8 |
| 8" x 12" x 16" | 10 | 10 |
| 4" x 8" x 16" (Half) | 20 | 5 |
| 6" x 8" x 16" | 15 | 7 |
| 10" x 8" x 16" | 10 | 10 |
| 12" x 8" x 16" | 8 | 13 |
* Based on 3/8" mortar joints. Thicker joints require more mortar.
Types of Mortar Explained
Type N
750 PSI
General purpose mortar for above-grade applications.
- • Non-load-bearing walls
- • Interior partitions
- • Soft stone/brick veneer
- • Most flexible option
Type S
1800 PSI
High-strength mortar for structural applications.
- • Most common choice
- • Load-bearing walls
- • Below grade OK
- • Retaining walls
Type M
2500 PSI
Maximum strength for demanding conditions.
- • Foundations
- • Heavy load-bearing
- • Below grade/frost
- • Earthquake zones
How to Mix Mortar
Using Pre-Mixed Bags (Recommended)
- Add about 5 quarts of clean water per 80lb bag to mixing container
- Slowly add mortar mix while mixing
- Mix for 3-5 minutes until smooth, workable consistency
- Let rest 5 minutes, then remix briefly
- Add small amounts of water if needed
From Scratch Ratio (Type S)
- 1 part Portland cement
- ½ part hydrated lime
- 4½ parts mason sand
- Water as needed for workability
Mix dry ingredients first, then add water gradually.
Pro Tip: Mortar Consistency
Properly mixed mortar should hold its shape when squeezed but not crumble. It should be "buttery" and stick to the trowel when held upside down briefly. If it's too wet, it will slump; too dry and it won't bond properly.
Block Laying Tips
- 1.Dampen blocks before laying to prevent them from absorbing moisture from mortar too quickly.
- 2.Apply mortar to block ends (called "buttering") before placing for strong vertical joints.
- 3.Check level frequently - every 3-4 blocks horizontally and every course vertically.
- 4.Strike joints when mortar is "thumbprint hard" (holds shape but still slightly soft).
- 5.Work in small batches - mortar becomes unworkable after 90 minutes. Never re-temper.
- 6.Protect from weather - cover fresh work if rain threatens; don't lay blocks in freezing temps.
Frequently Asked Questions
How many blocks will one bag of mortar lay?
One 80-pound bag of mortar mix will typically lay 12-13 standard 8x8x16 concrete blocks with 3/8" joints. This varies based on block size, joint thickness, and worker efficiency. Larger blocks or thicker joints will reduce coverage.
What type of mortar is best for concrete blocks?
Type S mortar is most commonly used for concrete block walls. It provides high strength (1800 PSI) and is suitable for both above and below grade applications. Type N is adequate for above-grade, non-load-bearing walls. Type M is used for foundations and heavy structural applications.
How thick should mortar joints be for concrete blocks?
Standard mortar joints for concrete blocks are 3/8" (10mm) thick. This provides good structural integrity while minimizing mortar usage. Thicker joints (1/2") may be used for irregularly sized blocks or when extra adjustment is needed.
How much waste should I add for mortar?
Add 10% waste factor for experienced masons and up to 15-20% for DIY projects. Waste occurs from dropped mortar, cleaning, and mortar that sets before use. For first-time block layers, 20% extra is recommended.
Can I mix my own mortar for blocks?
Yes, you can mix mortar using Portland cement, hydrated lime, and mason sand. A common Type S mix ratio is 1 part cement, 1/2 part lime, and 4-1/2 parts sand. Pre-mixed bags are more convenient and ensure consistent quality.
How long does mortar stay workable?
Mortar remains workable for about 90 minutes after mixing in moderate conditions. Hot, dry, or windy weather shortens this time significantly. Never add water to mortar that has begun to set (re-tempering) as it weakens the final bond.
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Dr. Snezana Lawrence
Mathematical Historian | PhD from Yale
Dr. Lawrence is a published mathematical historian with a PhD from Yale University. She ensures mathematical precision and accuracy in all our calculations, conversions, and academic score calculators. Her expertise spans computational mathematics and educational assessment.
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