Mortar Calculator

What Is the Mortar Calculator?

The Mortar Calculator estimates the volume and weight of mortar mix needed for brickwork, blockwork, or stone masonry walls based on wall area, masonry unit size, and joint thickness. Masons, bricklayers, and builders use it to work out accurate material orders, avoiding the productivity loss of running out of mortar mid-lift or wasting expensive pre-mixed product on over-ordering. According to the American Concrete Institute Masonry Guide, consistent mortar joint width and fill quality directly affects the compressive strength and weathertightness of the finished wall, making correct mortar volume just as important as the masonry unit order itself.

Mortar volume is a direct function of the joint density in the wall, which increases as masonry unit size decreases. A wall of standard bricks has roughly three to four times more joint surface area per square metre than a wall of standard CMU blocks, even at the same joint width. Given this, the mortar order for a brick wall and a block wall of the same face area can differ by a factor of three or more, making a calculator more reliable than a rule-of-thumb estimate for any multi-trade project.

How Mortar Volume Is Calculated

The mortar volume in a masonry wall is the sum of all bed joint (horizontal) and perp joint (vertical) volumes in the completed wall. For standard brickwork with 10mm joints, bed joints account for approximately 13 percent of wall volume and perp joints add another 5 to 7 percent, giving a total joint fraction of approximately 17 to 20 percent of wall volume. Multiplying the wall volume by 0.20 gives the theoretical mortar void volume, which is then adjusted upward by 15 percent for shrinkage, waste during mixing, and mortar lost from the joints during the initial bedding and squeezing process.

On top of that, mortar yields vary between products. A 25 kg bag of pre-mixed mortar typically yields 12 to 15 litres depending on the product density and mix design. Site-batched mortar from bulk cement and sand allows larger volumes at lower cost, but requires accurate measurement of constituent materials to maintain a consistent specification. That said, pre-mixed bagged mortar is more practical for residential projects because it eliminates batching errors and ensures consistent colour across the project.

Mortar Requirements by Masonry Unit Type

The table below shows approximate mortar requirements per square metre of single-leaf wall for common masonry unit types, based on standard joint widths and the ASTM C270 standard specification for mortar for unit masonry.

Mortar Designation and Mix Selection

Mortar designation determines the cement-to-lime-to-sand ratio and the resulting compressive strength. Using too strong a mortar relative to the masonry unit causes units to crack rather than joints when movement occurs, making repair work harder. Using too weak a mortar in exposed or below-grade locations allows water ingress and freeze-thaw deterioration. M4 mortar (1:1:5.5 cement:lime:sand) is the standard for above-grade internal and sheltered external brickwork. M10 (1:0:3 cement:sand) is used below the damp proof course and in highly exposed locations. Lime in the mix improves workability, reduces early stiffening, and gives the mortar flexibility to accommodate minor structural movement without cracking.

For heritage and listed buildings, matching the original mortar specification is critical. Applying a hard modern Portland cement mortar to a pre-1920 building constructed with lime-sand mortar causes significant damage as the soft masonry units crack under the rigidity of the new mortar. A heritage or conservation mortar specialist should be consulted for any repair work on historic masonry.

Worked Example: Mortar for a Brick Garden Wall

A bricklayer is building a 6-ft-high × 30-ft-long single-leaf brick wall using standard modular bricks (3⅝ × 2¼ × 7⅝ in) with ⅜-in mortar joints.

Step 1 : Wall area: 6 ft × 30 ft = 180 ft²

Step 2 : Brick count: Using standard estimating factor of 6.75 bricks per ft² (with ⅜-in joints): 180 × 6.75 = 1,215 bricks. Add 5% waste = 1,276 bricks → order 1,280.

Step 3 : Mortar volume: Rule of thumb for standard brick with ⅜-in joints: approximately 0.5 ft³ of mortar per 100 bricks. 1,276 bricks / 100 × 0.5 = 6.38 ft³

Step 4 : Bags of pre-mixed mortar: One 70 lb bag of Type S mortar mix yields ≈ 0.47 ft³. 6.38 / 0.47 = 13.6 → 14–15 bags

Step 5 : Add 15% waste: Mortar waste from spills and re-mixing is consistently higher than concrete. On top of that, the NRMCA Concrete in Practice guide notes that fresh mortar left in a bucket for more than 2 hours should be discarded rather than re-tempered with water. 17 bags recommended.

Mortar Coverage by Masonry Unit and Joint Width

Choosing the Right Mortar Type: ASTM C270 Guide

ASTM C270 is the governing standard for mortar used in unit masonry construction. Mortar designation under this standard affects both workability and long-term durability. Over-specifying strength (e.g., using Type S on a flexible garden wall) can cause the mortar to crack the brick rather than absorb movement through joint failure. With that in mind, this is one of the most upvoted issues in masonry Quora threads: homeowners choose the strongest mortar assuming it is the best, when in practice a weaker, more flexible mortar is the right call for most above-grade brickwork rather than absorb movement through joint failure, which is much cheaper to repair.

Accuracy and Limitations

The calculator provides estimates based on average joint proportions for each masonry type. Actual mortar consumption varies with the skill level of the mason, the surface condition of the masonry units, and the amount of waste from dropped and trimmed mortar. Field waste is typically 10 to 15 percent for experienced masons and up to 20 percent for less experienced workers. Always add the appropriate waste allowance to the calculated volume before ordering.

The calculator does not include mortar for pointing or repointing, which is a separate operation from bedding and jointing. Pointing mortar covers only the surface joint and requires a significantly smaller volume than full-bed mortar for new construction. The American Concrete Institute (ACI) sets the professional standard for mix design tolerances and site testing protocols that apply when estimates will be used for structural or load-bearing applications. For masonry walls with hollow core blocks that require fill concrete, our concrete block fill calculator handles that core fill estimate separately from the mortar bed.

The Most Common Mortar Calculation Mistake

The most consistent mortar estimating mistake is using the wall face area alone and applying a single coverage figure without adjusting for the specific masonry unit and joint width. Applying the brick mortar coverage figure to a block wall underestimates the mortar order by approximately 65 percent, because blocks have far fewer joints per square metre than bricks. On a 50 square metre block wall, this error produces an order roughly 400 litres short, requiring a second delivery that delays the project by half a day. With that in mind, always specify the exact masonry unit size and joint width in the calculation rather than using a generic coverage figure. This error turns up most often in multi-trade estimates prepared by someone other than the mason before anyone looks into why the mortar ran out halfway up the second lift. The National Ready Mixed Concrete Association (NRMCA) identifies material under-estimation as the leading cause of mid-project concrete shortages, recommending a 5-10% waste factor buffer on all project orders. If the project also requires grout for tile or masonry joint finishing, our grout calculator gives you the jointing material volume alongside the mortar figure.