Mortar Calculator

What Is the Mortar Calculator?

The Mortar Calculator estimates how many bags of premixed mortar are needed to build a brick or concrete block wall. You enter the wall dimensions, choose the type of masonry unit and the bag size, and it returns the number of bags along with the approximate number of units in the wall.

Mortar is the material that bonds masonry units together and seals the joints against weather. It is easy to underestimate. A wall that looks like it is mostly brick is in fact around 15 to 20 percent mortar by face area. Running out partway through a course means stopping work, and a new batch mixed differently can show as a colour change in the finished joints.

How the Calculator Works

The calculator first finds the wall area by multiplying its length and height. It then converts that area into a number of masonry units. For modular brick with 3/8 inch joints, there are about 6.86 bricks per square foot. For standard 8 by 16 inch concrete block, there are 1.125 blocks per square foot.

It then applies a yield figure. An 80 pound bag of premixed mortar lays approximately 36 modular bricks or 12 standard blocks with 3/8 inch joints. A 60 pound bag yields three quarters of that: about 27 bricks or 9 blocks.

The number of units is increased by the waste percentage and divided by the units per bag. The result is rounded up to whole bags.

The Mortar Formula

Units = Wall area × Units per square foot

Bags = Units × (1 + Waste % ÷ 100) ÷ Units per bag, rounded up

Yield figures used

• 80 lb bag: about 36 bricks or 12 blocks
• 60 lb bag: about 27 bricks or 9 blocks

These figures come from typical manufacturer data for full bed and head joints at 3/8 inch. Wider joints, deep frogs in the bricks or very absorbent units use more.

Step-by-Step Explanation

1. Measure the wall length and height and multiply for the area.
2. Multiply the area by 6.86 for modular brick or 1.125 for standard block.
3. Add 10 percent for waste, dropped mortar and joint variation.
4. Divide by 36 for brick or 12 for block if using 80 pound bags.
5. Round up to the next whole bag.

Worked Example

A brick garden wall is 20 feet long and 6 feet high.

• Area: 20 × 6 = 120 square feet.
• Bricks: 120 × 6.86 = 823.
• With 10 percent waste: 905.
• Bags: 905 ÷ 36 = 25.1, so 26 bags of 80 pounds.

The same wall built in concrete block:

• Blocks: 120 × 1.125 = 135.
• With 10 percent waste: 148.5.
• Bags: 148.5 ÷ 12 = 12.4, so 13 bags of 80 pounds.

Block needs about half the mortar of brick for the same wall area, because there are far fewer joints.

How to Use the Calculator

1. Enter the wall length and height.
2. Choose brick or concrete block.
3. Choose the bag size you plan to buy.
4. Set the waste allowance.
5. Press Calculate.
6. Read the number of bags and the approximate unit count.

For an exact count of units with your own brick or block dimensions, use the Brick Calculator or the Concrete Block Calculator. The footing under the wall is poured concrete, estimated with the Concrete Calculator. For tiled surfaces, the Tile Calculator counts the tiles, and base layers for paved areas are covered in the Paver Calculator.

Practical Tips

• Buy all the mortar from the same product line. Colour and working properties differ between brands.
• Measure water consistently. The amount of water changes the colour of cured mortar.
• Mix to the consistency of thick peanut butter. It should hold its shape on the trowel and still spread.
• Let the mix rest for a few minutes after the first mixing, then remix briefly. This is called slaking and improves workability.
• Use mortar within about 90 minutes of mixing. Do not add water to revive mortar that has started to set.
• In hot or windy weather, mix smaller batches and shade the mixing area.
• Protect new work from rain, frost and direct sun for at least 24 to 48 hours.
• Keep unopened bags dry and off the ground. Damp bags form lumps and lose strength.

Common Mistakes

Confusing mortar with concrete. Mortar has no coarse aggregate and is designed to bond, not to bear load on its own. Concrete mix must not be used to lay bricks.

Using the wrong mortar type. A mix that is too strong for soft old brick can damage the brick.

Estimating by wall area alone. Mortar use depends on the number of joints, so the unit size matters as much as the area.

Not allowing for waste. Mortar falls off the trowel, stiffens on the board and is trimmed from joints.

Retempering old mortar. Adding water after the initial set weakens the bond.

Inconsistent joint thickness. Joints that drift thicker use more mortar and throw off the course heights.

Mortar Types and Where to Use Them

Type N is a medium-strength mix suitable for most above-ground walls, chimneys and veneers. Type S has higher bond and compressive strength and is used below ground, in retaining walls and where walls face wind or seismic loads. Type M is the strongest and is used for foundations and driveways. Type O is a low-strength mix for interior non-load-bearing work and for repointing historic masonry.

Mixing From Scratch

Mortar can also be mixed on site from Portland cement, hydrated lime and sand. A common Type N recipe is 1 part cement, 1 part lime and 6 parts sand by volume. Type S is 1 part cement, half a part lime and 4.5 parts sand. As a rough guide, laying 1,000 modular bricks takes about 7 to 8 bags of masonry cement and around one cubic yard of sand. Mixing on site costs less on large jobs, while premix is more convenient and consistent for small ones.

Repointing

Repointing replaces the outer part of the joints in an existing wall. Old mortar is raked out to a depth of about three quarters of an inch and new mortar is pressed in. It uses far less material than new work. As a rough figure, one 80 pound bag repoints 40 to 50 square feet of brickwork, depending on joint width and depth.

Joint Profiles

The way a joint is finished affects how well it sheds water. A concave joint, formed with a rounded jointing tool, compresses the mortar against the brick and is the most weather-resistant. A V joint performs similarly. A flush joint is cut level with the face and suits walls that will be painted or rendered. Raked joints are recessed for a shadow line and are best kept to sheltered or interior walls, because the ledge holds water. Struck and weathered joints slope to shed water and are common in traditional work.

Weather and Curing

Mortar cures by hydration, as concrete does, and it needs moisture and moderate temperatures. Avoid laying masonry when the temperature is below 40 degrees Fahrenheit unless you can protect the work, and never use frozen sand or lay on frosted units. In hot weather, lightly mist the finished wall for a day or two so the joints do not dry before they gain strength. Mortar reaches most of its strength within 28 days.

Storing and Handling Bags

Stack bags on a pallet under cover and use the oldest first. A bag that has gone hard at the edges has taken up moisture and should not be used for structural work. An 80 pound bag is a heavy lift, so many people prefer 60 pound bags and accept the extra count.

Colour and Additives

Standard mortar cures to a mid grey. White cement and coloured pigments are available where the joint colour matters, as it does with most face brick. If you use pigment, measure it carefully for every batch, because small differences show clearly once the wall dries.