Mortar Calculator

What is a mortar calculator?

A mortar calculator tells you how many bags of mortar mix to buy for a wall. Mortar is sold by the bag, but a wall is measured by its area, so the two never line up without a bit of arithmetic. You give the calculator the wall’s length and height, say whether you are laying bricks or concrete blocks, and add a wastage allowance — and it returns the wall area, how many masonry units the wall takes, and how many bags of mortar it takes to lay them.

It is the natural companion to a brick or block count: the units tell you what to stack, the mortar tells you what holds them together. Because a bag of mortar lays a predictable number of units, the bag count follows directly from the unit count.

How does the calculator work?

Enter the wall length and wall height, choose the masonry unit (brick or concrete block), and set a wastage percentage for breakage and offcuts. The calculator multiplies length by height to get the wall area, multiplies that by the number of units the chosen masonry unit needs per square metre, pads the result by the wastage percentage, and rounds up to whole units. It then divides the unit count by the number of units one bag of mortar lays and rounds up again, because you can only buy whole bags.

Every measurement has its own unit selector, so you can enter the wall in metres, centimetres, feet, inches, or yards — the calculator converts everything internally before doing the math. The wall area is shown in whichever unit suits you: square metres, square feet, square centimetres, or square yards.

Formulas

The wall area is length multiplied by height. With length $L$ and height $H$:

$$A = L \times H$$

The number of masonry units is the area multiplied by the units per square metre $u$, padded by the wastage percentage $w$ and rounded up, because you cannot buy a fraction of a brick:

$$N = \left\lceil A \times u \times \left(1 + \frac{w}{100}\right) \right\rceil$$

The number of mortar bags is the unit count divided by the number of units one bag lays, $y$, again rounded up:

$$B = \left\lceil \frac{N}{y} \right\rceil$$

The coverage figures this calculator uses

The calculator is built on the standard published rules of thumb for a single-leaf (single-wythe) wall:

The 60 bricks per m² figure comes from a 215 × 65 mm brick face with 10 mm joints, which covers 0.016875 m² per brick. The bag yields — about 30 bricks or about 12 standard blocks per 80 lb (36.3 kg) bag — are the coverage figures published on mortar-mix bags by the major manufacturers.

Treat the result as an estimate. Real yield depends on the joint thickness you actually strike, whether the blocks are hollow or solid, how thick the wall is, and how much mortar ends up back on the board. A thicker joint or a double-leaf wall will eat noticeably more mortar than these figures suggest.

Worked examples

Example 1: a brick wall

A wall is 10 m long and 2.5 m high, laid in brick, with a 10% wastage allowance:

$$A = 10 \times 2.5 = 25 \, \text{m}^2$$ $$N = \left\lceil 25 \times 60 \times 1.1 \right\rceil = \lceil 1650 \rceil = 1650$$ $$B = \left\lceil \frac{1650}{30} \right\rceil = 55$$

So a 25 m² brick wall takes about 1,650 bricks and 55 bags of mortar.

Example 2: the same wall in concrete block

The same 10 m × 2.5 m wall, this time laid in 8 × 8 × 16 in concrete block, still with a 10% wastage allowance:

$$N = \left\lceil 25 \times 12.5 \times 1.1 \right\rceil = \lceil 343.75 \rceil = 344$$ $$B = \left\lceil \frac{344}{12} \right\rceil = \lceil 28.67 \rceil = 29$$

The same wall takes only 344 blocks — blocks are much bigger than bricks — and 29 bags of mortar. Blocks need fewer bags because there are far fewer joints to fill per square metre of wall.

Estimating mortar for a wall

• Measure the wall you are actually building. Subtract large openings such as doors and windows from the area before you enter it, or you will buy mortar for a wall that is not there.
• Set the wastage deliberately. About 10% is a sensible default: it covers broken units, mortar dropped off the board, and the extra you mix at the end of the day. Tight, simple rectangles can run lower; walls with many cuts and returns run higher.
• Buy whole bags, and buy one spare. Mortar is mixed in batches and the last batch is rarely a full bag, so rounding up is not optional — and running out mid-course means a cold joint.
• Check your joint thickness. These figures assume a standard 10 mm (⅜ in) joint. A 15 mm joint uses roughly half as much mortar again, so bump the wastage up or buy extra if you are laying thick beds.