How Mortar Volume is Determined
Mortar serves to bond masonry units and account for dimensional variations. The required volume is heavily dependent on the ratio of the unit's surface area to the area of the joints between them.
Geometric Mortar Volume Ratio:
$$\text{V}_{\text{ratio}} = \frac{\text{Joint Thickness}}{\text{Unit H} + \text{Joint Thickness}} \times \left[ 1 + \frac{\text{Unit H}}{\text{Unit L}} \right]$$ Where: $\text{V}_{\text{ratio}}$ is the Cubic Feet of Mortar per Square Foot of Wall, $\text{H}$ is Height, and $\text{L}$ is Length (all in feet). This formula accounts for both the horizontal bed joints and the vertical head joints to provide an accurate volumetric estimate.
Key Factors Affecting Mortar Use
Joint Thickness (0.375" is Standard)
- **Thicker Joints** (e.g., $1/2$"): Require significantly **more** mortar and are generally used for irregularly shaped, hand-made, or reclaimed units.
- **Thinner Joints** (e.g., $1/4$"): Require less mortar but demand highly uniform bricks/blocks and meticulous labor.
- **Standard Joint:** The industry default of $\mathbf{3/8}$ inch ($\mathbf{0.375}$") offers the best balance for most modern units.
Mortar Yield & Waste
Calculating Bags
- **Yield Varies:** Always check the bag! Our estimates are $\mathbf{0.65}$ cu ft for $\mathbf{80}$ lb mortar bags, but this changes based on the mix type (Type N, S, M) and water content.
- **Waste Allowance:** A professional mason typically accounts for a $\mathbf{5\%}$ to $\mathbf{10\%}$ waste factor to cover droppings, overfilling, and cleanup. Round up your bag count after adding this buffer.
Note on Unit Size:
For **Concrete Blocks (CMUs)**, use dimensions of $\mathbf{15.625}$" long and $\mathbf{7.625}$" high (to account for the $\mathbf{3/8}$" joint on a nominal $\mathbf{16}$" x $\mathbf{8}$" block). The calculation remains the same, but the mortar volume per square foot will be lower due to the larger unit size.