Calculate mortar bags, cement, lime and sand for brick, block and stone walls. Uses ASTM C270 mix ratios, accounts for joint width and waste, and works in pre-mixed bags or site-mixed bulk material.
- Brick & Block
- CMU Block
- Stone Veneer
- ASTM C270 Types
- Pre-Mixed & Bulk
- Cost Estimate
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Material breakdown
What Is Mortar and Why the Mix Matters
Mortar is the bonding agent between masonry units. It is a mixture of cement, lime, sand and water, and the proportions of the first three are set by ASTM C270, the standard that governs masonry mortar in North America[reference:0]. Unlike concrete, mortar is not designed to be as strong as possible. It is designed to be slightly weaker than the masonry units it bonds, so that movement and stress cracking happens in the mortar joint rather than in the brick or block[reference:1].
Type M — 2500 psi
Highest cement content. Used for foundations, retaining walls and structures carrying heavy compressive load. Mix: 1 cement : 1/4 lime : 3 sand.
Type S — 1800 psi
The structural workhorse. Used for exterior walls, below-grade masonry and any wall exposed to high wind or seismic load. Mix: 1 cement : 1/2 lime : 4.5 sand.
Type N — 750 psi
The general-purpose above-grade mortar. Used for brick veneer, chimneys, and most residential brickwork where load is light. Mix: 1 cement : 1 lime : 6 sand.
Type O — 350 psi
Lowest strength. Used for interior non-load-bearing walls and historic restoration. Mix: 1 cement : 2 lime : 9 sand.
Both the National Concrete Masonry Association and the Brick Industry Association recommend using the weakest mortar that will meet structural requirements. Stronger mortar is not better for masonry — it causes stress cracks, poor water drainage and premature failure. Type N is best for most masonry veneer[reference:2].
ASTM C270 Mix Proportions
The proportion specification in ASTM C270 defines each mortar type by the volume ratio of cement to lime to sand. These are the ratios used when mixing from bulk materials on site[reference:3].
| Type | Cement : Lime : Sand | 28-Day Strength | Cement per ft³ mortar | Lime per ft³ mortar | Best use |
|---|---|---|---|---|---|
| M | 1 : 1/4 : 3 | 2500 psi | 0.33 ft³ | 0.08 ft³ | Foundations, heavy load |
| S | 1 : 1/2 : 4.5 | 1800 psi | 0.22 ft³ | 0.11 ft³ | Exterior, below grade |
| N | 1 : 1 : 6 | 750 psi | 0.17 ft³ | 0.17 ft³ | Above-grade veneer |
| O | 1 : 2 : 9 | 350 psi | 0.11 ft³ | 0.22 ft³ | Interior, non-load-bearing |
Proportions by volume, not weight. A common site mistake is mixing by weight instead of volume. One bag of cement (94 lb) occupies about 1 cubic foot. One bag of hydrated lime (50 lb) occupies about 1.25 cubic feet. Sand is measured damp and loose. Mixing by weight changes the ratio and weakens the mortar.
Mortar Coverage Rates
The number of masonry units one bag of pre-mixed mortar will lay depends on the unit type and the joint width. These are the accepted coverage rates for standard 80 lb bags at 3/8-inch joints.
| Masonry unit | Size | Units per 80 lb bag | Units at 1/2 in joint | Mortar per 1,000 units |
|---|---|---|---|---|
| Concrete masonry units | ||||
| 6 in CMU | 6 × 8 × 16 in | ~13 | ~9 | ~28 ft³ |
| 8 in CMU | 8 × 8 × 16 in | ~12 | ~8–9 | ~25 ft³ |
| 12 in CMU | 12 × 8 × 16 in | ~11 | ~8 | ~22 ft³ |
| Brick | ||||
| Modular brick | 3 5/8 × 2 1/4 × 7 5/8 in | ~41 | ~24–28 | 19 ft³ |
| Queen size brick | 3 1/8 × 2 3/4 × 9 5/8 in | ~34 | ~22 | ~22 ft³ |
| King size brick | 3 5/8 × 2 5/8 × 9 5/8 in | ~30 | ~20 | 22 ft³ |
| Utility brick | 3 5/8 × 3 5/8 × 11 5/8 in | ~25 | ~18 | 32 ft³ |
| Stone | ||||
| Manufactured stone veneer | Varies by profile | ~17 sq ft | ~12 sq ft | ~59 sq ft per 80 lb |
Joint width matters more than most people think. Moving from a 3/8-inch joint to a 1/2-inch joint increases mortar volume by roughly 30%, which means 30% more bags for the same wall[reference:4]. A 1/2-inch joint is not just a cosmetic choice — it is a material quantity decision.
How the Mortar Calculator Works
Six steps from wall dimensions to the order list. Every conversion is printed with the result so you can check the arithmetic against the supplier's yield sheet.
Determine unit count
Either enter the wall area and the calculator converts to unit count using the unit's nominal face area, or enter the count directly.
Apply joint factor
A 3/8-inch joint is the baseline. A 1/2-inch joint multiplies mortar volume by 1.3. A 1/4-inch joint multiplies by 0.85.
Compute mortar volume
Unit count ÷ units per bag for that unit type and joint width, then convert to total cubic feet of mortar.
Add waste
A separate waste buffer (default 10%) covers spillage, tooling loss and over-mixing.
Convert to bags or bulk
Divide total volume by bag yield (0.6 ft³ for 80 lb, 0.45 ft³ for 60 lb) or by the ASTM C270 mix proportions for site-mixed.
Cost estimate
Bag count or bulk material quantities × unit price gives the total material cost.
Worked Example — 20 ft × 10 ft CMU wall, Type S, 3/8 in joint
| Step | Calculation | Result |
|---|---|---|
| Wall area | 20 ft × 10 ft | 200 sq ft |
| 8 in CMU count | 200 sq ft × 1.125 blocks/sq ft | 225 blocks |
| Mortar bags (3/8 in) | 225 ÷ 12 blocks/bag | 18.75 bags |
| With 10% waste | 18.75 × 1.10 | 20.6 → 21 bags |
| Mortar volume | 21 × 0.6 ft³ | 12.6 ft³ |
| Material cost at $11.50/bag | 21 × 11.50 | $241.50 |
Pre-Mixed Bags vs Site-Mixed
Pre-mixed mortar bags are the right choice for most residential jobs. Site-mixing from bulk cement, lime and sand becomes cost-effective on larger projects but requires reliable bulk supply and consistent mixing.
| Factor | Pre-mixed bags | Site-mixed bulk |
|---|---|---|
| Convenience | Rip, dump, add water, mix | Measure cement, lime, sand separately |
| Consistency | Reliable, factory-blended | Depends on mason skill and measuring |
| Cost per ft³ | $11.50–$15.00 per bag (0.6 ft³) | $4.68–$6.00 per ft³ (Type M) |
| Best for | Jobs under 500 blocks | Jobs over 500 blocks |
| Water needed | 5–6 qt per 80 lb bag | Varies, add until workable |
| Storage | Keep dry, 1 year shelf life | Bulk sand and cement, moisture-sensitive |
The break-even point is around 500 blocks or 2,000 bricks. Below that, the convenience of pre-mixed wins. Above that, the per-cubic-foot cost of site-mixing becomes significant — typically $400–$600 saved on a 2,000-block commercial job[reference:5].
Why Joint Width Changes Everything
Joint width is the single most underestimated factor in mortar estimating. Most calculators use a fixed 3/8-inch joint and never ask. But a 1/2-inch joint adds 30% more mortar volume — enough to run a full crew short on a large wall.
1/4 in joint
Used in some brickwork for a tighter, more uniform appearance. Uses about 15% less mortar than the 3/8-inch standard.
3/8 in joint
The industry standard for brick and block. All coverage rates on this page are based on this width unless noted.
1/2 in joint
Used for rougher units or textured block. Adds about 30% to mortar volume. If your spec calls for 1/2-inch joints, order 30% more mortar.
Rake vs flush
A raked joint (recessed) exposes more mortar surface to weather and uses slightly more material than a flush joint. Tooling type matters at the margins.
Mortar is specified by ASTM C270 (Standard Specification for Mortar for Unit Masonry), which defines four types (M, S, N, O) by both proportion and property specification[reference:6]. Compressive strength requirements are set by the property specification: Type M at 2500 psi, Type S at 1800 psi, Type N at 750 psi, Type O at 350 psi[reference:7]. Bag yields follow manufacturer published data for pre-mixed products, typically 0.6–0.8 ft³ per 80 lb bag[reference:8].
One 80 lb bag of pre-mixed mortar lays about 12 standard 8×8×16 CMU blocks or 35–41 bricks at 3/8-inch joints[reference:9]. A 1/2-inch joint drops that to 8–9 blocks or 25 bricks. Roughly 8 bags cover 100 sq ft of brick wall. For site-mixing, one cubic foot of Type N mortar needs about 0.17 ft³ cement, 0.17 ft³ lime and 1 ft³ sand. Always add 10% waste on pre-mixed and 15% on site-mixed to cover spillage and over-mixing.
People Also Ask
How much mortar do I need for 100 blocks?
For standard 8×8×16 CMU blocks at 3/8-inch joints, plan on about 8–9 bags of 80 lb pre-mixed mortar per 100 blocks[reference:10]. With 10% waste, round up to 9–10 bags. If you are using 1/2-inch joints, the count rises to about 11–12 bags per 100 blocks.
How many bricks will one bag of mortar lay?
One 80 lb bag of pre-mixed mortar lays approximately 35 bricks at 3/8-inch joints, or 25 bricks at 1/2-inch joints for standard modular brick. Coverage varies by brick size — king size bricks run about 30 per bag, utility bricks about 25 per bag[reference:11].
What is the difference between Type M, S, N and O mortar?
The difference is cement content and compressive strength. Type M (2500 psi) has the most cement and is used for foundations. Type S (1800 psi) is the structural standard for exterior and below-grade walls. Type N (750 psi) is the general-purpose above-grade mortar for brick veneer and residential walls. Type O (350 psi) is for interior non-load-bearing work[reference:12].
How many bags of mortar are in a cubic foot?
An 80 lb bag of pre-mixed mortar yields about 0.6 to 0.75 cubic feet of mixed mortar, depending on the manufacturer and mortar type[reference:13]. That means approximately 1.3 to 1.7 bags per cubic foot. A 60 lb bag yields about 0.45 cubic feet. Always check the specific product data sheet — yields vary slightly between manufacturers.
How much water do I add to mortar mix?
For an 80 lb bag of pre-mixed mortar, start with about 5 to 6 quarts of clean water and add gradually until the mortar holds a ridge without slumping or cracking[reference:14]. A 60 lb bag takes about 4 quarts. Add water slowly — too much water weakens the mortar. The correct consistency is like thick peanut butter, not runny pancake batter.
Can I mix my own mortar instead of buying pre-mixed?
Yes. Site-mixing uses separate Portland cement, hydrated lime and sand in the ASTM C270 proportions. For Type N mortar: 1 part cement, 1 part lime, 6 parts sand by volume. It is cheaper per cubic foot but requires careful measuring and consistent mixing. Site-mixing makes sense on jobs over 500 blocks or 2,000 bricks — below that, the labour and consistency advantages of pre-mixed usually win[reference:15].
Does joint width really change how much mortar I need?
Yes, significantly. Moving from a 3/8-inch joint to a 1/2-inch joint increases mortar volume by roughly 30%[reference:16]. On a 200 sq ft block wall, that is the difference between 21 bags and 27 bags. Always confirm the specified joint width before ordering — it is the single most underestimated factor in mortar estimating.
How much mortar do I need for a stone veneer wall?
Manufactured stone veneer uses about 17 square feet per 80 lb bag of mortar at standard joint width. That is much lower coverage than brick or block because stone veneer units are larger and the mortar bed is thicker. For a 200 sq ft stone wall, plan on about 12 bags of mortar plus 10% waste.
What happens if I use the wrong mortar type?
Using mortar that is too strong causes stress cracking in the masonry units — the mortar does not yield, so the brick or block cracks instead. Using mortar that is too weak causes joint failure and water penetration. Match the mortar type to the application: Type M for foundations, Type S for exterior and below-grade, Type N for general above-grade, Type O for interior non-load-bearing[reference:17].
How long does mortar take to cure?
Mortar sets in 24 hours and reaches about 70% of its design strength in 7 days. Full 28-day strength is what ASTM C270 specifies for compliance testing. Keep the wall moist for the first 3 days after laying — rapid drying weakens the mortar. In hot weather, mist the wall twice daily. In cold weather below 40°F, use heated water and protect the wall from freezing.
Can I use mortar for a concrete slab?
No. Mortar is designed as a thin bonding layer between masonry units, not as a structural concrete. A concrete slab needs a proper concrete mix with coarse aggregate (gravel or crushed stone). Mortar has no coarse aggregate and will not achieve the compressive strength or durability of concrete. Use the concrete calculator for slab and footing volumes.
How long can I store leftover mortar bags?
Unopened bags stored dry and off the ground last about 12 months. Check for lumps before use — if the powder is hard or clumpy, the cement has hydrated and the bag is no good. Opened bags should be used within a few weeks. Never mix old and new bags in the same batch unless you test the old ones first.
Does mortar need to match the brick colour?
Colour match is a design choice, not a structural requirement. White mortar makes brick stand out; grey mortar blends with the brick colour. Pigments can be added to pre-mixed mortar or to site-mixed batches. One caution: pigment must be measured consistently across every batch, or the wall will show colour variation. Buy pre-pigmented bags if colour accuracy matters.
Related Calculators
References
- ASTM C270 — Standard Specification for Mortar for Unit Masonry. ASTM International, current edition.
- National Concrete Masonry Association (NCMA). TEK 9-1A: Mortar Properties and Specifications.
- Brick Industry Association (BIA). Technical Notes 8: Mortars for Brickwork.
- Portland Cement Association (PCA). Portland Cement Plaster (Stucco) Manual — mixing and proportioning guidance.
- American Concrete Institute (ACI). ACI 530: Building Code Requirements for Masonry Structures.
Confirm mortar type, joint width and unit dimensions against the project specification and local code before ordering. Bag yields and coverage rates vary by manufacturer and unit size.