How Much Mortar Do I Need for Cinder Blocks? Bags Per Block Calculated
A standard 8x8x16 cinder block requires approximately 0.05 to 0.07 cubic feet of mortar, which works out to roughly one 60-pound bag of mortar mix for every 12 to 15 blocks. For a 100-block wall, you need 7 to 8 bags. The exact amount depends on joint thickness, block type, and whether you are laying a single or double-wythe wall.
Mortar is one of those materials that is easy to underestimate. Order too little and you are making an emergency hardware store run mid-project while your first batch goes off. Order too much and you are paying for material that ends up in the dumpster. Neither is a good outcome on a project where timing matters.
This guide gives you the exact calculation for how much mortar you need for cinder blocks, the bag count by wall size, the mix type that matches your application, and the three most common mistakes that waste mortar on a CMU job.
Enter your wall dimensions. The mortar calculator gives you the bag count and volume for any block size and joint thickness.
Open the Mortar Calculator →The Mortar Calculation for Cinder Blocks
To figure out how much mortar for cinder blocks, you need two numbers: the volume of a single mortar joint and the number of joints in your wall.
A standard 8x8x16 CMU block laid in running bond has two types of joints: the bed joint (the horizontal mortar between courses) and the head joint (the vertical mortar between block ends). A standard joint is 3/8 inch thick. Here is the calculation per block:
- Bed joint per block: 16” x 8” x 0.375” = 48 cubic inches = 0.028 cubic feet
- Head joint per block: 8” x 8” x 0.375” = 24 cubic inches = 0.014 cubic feet
- Total mortar per block: approximately 0.042 cubic feet (before waste)
- With 25% waste factor: approximately 0.053 cubic feet per block
A 60-pound bag of pre-mixed mortar yields about 0.45 cubic feet when mixed. Dividing 0.45 by 0.053 gives you approximately 8.5 blocks per 60-pound bag. In practice, most masons plan on one 60-pound bag of mortar for every 8 to 10 blocks, accounting for mix variation, spills, and bed joint thickness that runs slightly thick in practice.
Bags Per Block Reference Table
| Number of blocks | 60-lb bags needed | 80-lb bags needed | Cubic feet mortar |
|---|---|---|---|
| 25 blocks | 3 bags | 2 bags | 1.3 |
| 50 blocks | 6 bags | 5 bags | 2.7 |
| 100 blocks | 12 bags | 9 bags | 5.3 |
| 150 blocks | 18 bags | 14 bags | 8.0 |
| 200 blocks | 24 bags | 18 bags | 10.6 |
| 300 blocks | 36 bags | 27 bags | 15.9 |
| 500 blocks | 60 bags | 45 bags | 26.5 |
These figures are for standard running bond CMU walls. Stack bond (blocks aligned vertically) uses slightly more head-joint mortar. Double-wythe walls essentially double the mortar requirement. Use the mortar calculator for non-standard configurations.
Which Mortar Type to Use for Cinder Blocks
Not all mortar is the same, and using the wrong type is one of the fastest ways to get a wall that cracks at the joints. The ASTM C270 standard defines four main mortar types for masonry construction in the US.
| Mortar type | Compressive strength | Best for | Not suitable for |
|---|---|---|---|
| Type M | 2,500 PSI | Below-grade, retaining walls, heavy loads | Interior, decorative |
| Type S | 1,800 PSI | At-grade exterior, most residential CMU walls | Interior non-structural |
| Type N | 750 PSI | Interior, above-grade non-structural, chimneys above roofline | Below grade, heavy loads |
| Type O | 350 PSI | Interior non-load-bearing, repointing soft historic brick | Any exterior or load-bearing |
For most residential cinder block walls, including garden walls, retaining walls, and foundation walls, Type S mortar is the correct choice. It has the right balance of strength and flexibility. Type M is used for retaining walls below grade or walls bearing very heavy loads. Type N is fine for an interior garden wall or a decorative block feature, but it is too weak for anything structural or below grade.
Pre-mixed mortar bags (just add water) are convenient and consistent. Site-mixed mortar from Portland cement, lime, and sand is less expensive at large scale but requires careful proportioning. For projects under 300 blocks, pre-mixed bags are almost always the better choice. For larger projects, a contractor will typically site-mix to control cost. The ASTM C270 standard governs both approaches.
Mortar Bags by Common Wall Size
| Wall size | Wall area | Block count (approx) | 60-lb bags |
|---|---|---|---|
| 4’ long x 4’ tall (6 courses) | 16 sq ft | 18 blocks | 2 to 3 bags |
| 8’ long x 4’ tall | 32 sq ft | 36 blocks | 4 to 5 bags |
| 10’ long x 6’ tall | 60 sq ft | 68 blocks | 8 to 9 bags |
| 20’ long x 4’ tall | 80 sq ft | 90 blocks | 10 to 11 bags |
| 20’ long x 8’ tall | 160 sq ft | 180 blocks | 21 to 23 bags |
| 40’ long x 8’ tall | 320 sq ft | 360 blocks | 42 to 46 bags |
Three Real Cinder Block Projects, Mortar Calculated
Example 1: Backyard garden wall in Austin, Texas
A homeowner wants a 12-foot long x 3-foot tall garden wall (3 courses of 8x8x16 blocks, roughly 27 blocks in a single-wythe wall). Type N mortar is sufficient for this non-structural, above-grade application. Calculation: 27 blocks needs about 3 to 4 bags of 60-pound pre-mixed Type N mortar. Ordered 5 bags to account for waste and mixing variation. Total mortar cost: about $35. The wall went up in an afternoon.
Example 2: Retaining wall in Seattle, Washington
A 16-foot long x 4-foot tall retaining wall (6 courses) holding back a sloped backyard. This is below-grade on the soil side and structural, so Type S mortar is required. Block count: approximately 72 blocks. Mortar needed: 8 to 10 bags of 60-pound Type S mortar. The contractor also core-filled every other block with grout for additional strength, which required a separate concrete core-fill calculation. Mortar cost was a minor portion of the total project.
Example 3: Basement foundation repair in Cleveland, Ohio
Repointing 20 linear feet of an older CMU basement wall where mortar joints had deteriorated. The repointing work required grinding out old mortar to 3/4-inch depth and filling fresh. Approximately 350 linear feet of joints needed filling. At about 0.01 cubic feet per linear foot of joint, that is 3.5 cubic feet of mortar, or roughly 8 bags of 60-pound Type S mortar. The project also required a structural assessment of the wall before repointing began to confirm no blocks needed replacement. For the core lessons about avoiding material failures in masonry, the same principles that apply to slab cracks apply to joint failures in block walls.
Waste Factor and Ordering Tips
Always order 20% to 25% more mortar than your calculated volume. Mortar is wasted through: excess that squeezes out of joints and is trimmed off, evaporation and stiffening in the mixing tub before use, spillage during mixing and application, and the end of each batch that cannot be fully used before setting. A 25% waste factor is standard for experienced masons working in controlled conditions. First-time block layers should use 30% to 35% because mixing consistency and joint thickness control take practice.
Pre-mixed mortar begins to stiffen within 90 minutes of mixing. Mix in batches sized to what your crew can lay in that window. Remixing or adding water to stiffened mortar weakens the final joint. Plan your batch size by how many blocks your crew can lay per hour, then work backwards from 90 minutes.
Tell the cinder block calculator your wall length, height, and block size. It shows you how much mortar for cinder blocks in your specific project, with the waste factor already included.
Open the Block Calculator →Frequently Asked Questions
How much mortar do I need per cinder block?
A standard 8x8x16 cinder block requires approximately 0.042 cubic feet of mortar at 3/8-inch joint thickness, or about 0.053 cubic feet with a 25% waste factor included. This works out to roughly one 60-pound bag of pre-mixed mortar for every 8 to 10 blocks.
How many bags of mortar for 100 cinder blocks?
For 100 standard 8x8x16 cinder blocks at standard 3/8-inch joints, you need approximately 11 to 13 bags of 60-pound pre-mixed mortar with a 25% waste factor included. Buy 12 bags as a safe starting point for a project this size.
What type of mortar should I use for cinder blocks?
Type S mortar is the correct choice for most exterior, at-grade, and structural cinder block applications. It has 1,800 PSI compressive strength and sufficient flexibility for standard residential CMU walls. Type M is for below-grade retaining walls with heavy loads. Type N is only appropriate for interior non-structural walls or decorative applications above grade.
How thick should mortar joints be for cinder blocks?
The standard mortar joint thickness for CMU construction is 3/8 inch. Joints that are thicker than 1/2 inch are weaker and prone to cracking as they cure. Joints thinner than 1/4 inch do not have enough material to bond properly. Most pre-mixed mortar bags are formulated for 3/8-inch joints and their coverage estimates assume this thickness.
How many cinder blocks does a 60-lb bag of mortar lay?
A 60-pound bag of pre-mixed mortar yields approximately 0.45 cubic feet of mixed mortar. At 0.053 cubic feet per block (with 25% waste), one 60-pound bag lays approximately 8 to 9 standard 8x8x16 blocks. This is a useful planning number: for every 10 blocks, budget 1.2 bags and round up to the nearest whole bag when ordering.
Can I mix mortar and concrete together?
No. Mortar and concrete are different materials designed for different jobs. Mortar is a sand-cement mixture with lime, designed to create flexible, workable joints between masonry units. Concrete contains coarse aggregate and is designed for structural pours. Using concrete as mortar produces a too-stiff mixture that does not bond properly and cracks at the joints.
How long does mortar take to cure?
Mortar reaches initial set in 24 to 48 hours and full cure in 28 days, similar to concrete. You can typically continue building on mortar after 24 hours in normal temperatures. In cold weather below 40 degrees Fahrenheit, mortar curing slows significantly and can fail entirely if it freezes before reaching initial set. Protect fresh mortar from freezing with insulating blankets or delay masonry work in cold conditions.
How do I estimate mortar for a retaining wall?
Calculate the total number of blocks in the wall, then use the standard 0.053 cubic feet per block figure with the 25% waste factor. For a retaining wall that requires core filling (grout in the hollow cores for structural strength), calculate the core fill separately: a standard 8x8x16 block core volume is about 0.17 cubic feet per cell, with typically 2 cells per block. Add that volume to your mortar order as a separate material.
Related Calculators and Guides
For mortar type specifications and testing standards, the ASTM C270 Standard Specification for Mortar for Unit Masonry is the governing US reference for all residential and commercial masonry applications.