How to Calculate Mortar for Brickwork: Formula, Cement & Sand Example

Mortar for brickwork is calculated by subtracting the bricks from the wall:

Wet mortar volume = wall volume − (number of bricks × volume of one brick)
Dry material volume = wet mortar volume × 1.33
Order volume = dry material volume × (1 + waste factor)

Assumptions: the wall is one wythe or one leaf thick, every joint is filled solid, the brick volume uses the brick’s specified size rather than its nominal size, and the joint thickness is the same for bed and head joints.

That is the whole method. The rest of this guide works it through twice, in imperial and in metric, and explains where the 1.33 factor comes from, because using it without knowing what it does is where most estimates go wrong.

Step 1: measure the wall

Take length, height and thickness, and keep every dimension in the same unit family. Mixing inches with feet, or millimetres with metres, is the single most common arithmetic error in masonry estimating.

Subtract openings from the face area before you calculate. A door and two windows in a 20 ft wall can take out 15 percent of the brickwork.

For wall thickness, use the thickness of the masonry itself. A single-wythe brick wall is one brick wide, so the thickness equals the brick’s width. A one-brick metric wall is two leaves plus the collar joint between them.

Step 2: get the brick size right

Bricks have three sets of dimensions, and Brick Industry Association Technical Note 10 is firm about keeping them apart.

• Nominal dimensions are the brick plus its intended joint, so they land on round numbers. A “4 inch” brick is nominal.
• Specified dimensions are the size the brick is manufactured to, and these are what appear on a purchase order.
• Actual dimensions are what turns up, within the tolerance the relevant ASTM standard allows.

Use specified dimensions for the brick volume. Nominal dimensions already contain the joint, so using them for the brick itself counts the joint twice and inflates the mortar figure by roughly 10 percent for a standard brick.

Assumptions: joint thicknesses shown are the values the nominal or coursing dimensions are built on. Non-modular brick such as the US standard size has no nominal dimension by definition.

Step 3: count the bricks

One brick occupies its own volume plus one bed joint and one head joint. In running bond, each brick carries one of each, because the joint it shares with its neighbour is counted once.

Module volume = (brick length + joint) × (brick height + joint) × brick width
Number of bricks = wall volume ÷ module volume

A quicker route for a single-wythe wall is to work per unit of face area:

Bricks per sq ft = 144 ÷ [(brick length + joint) × (brick height + joint)]   (inches)
Bricks per m²    = 1,000,000 ÷ [(brick length + joint) × (brick height + joint)]   (mm)

For a US standard brick with a 3/8 in joint that gives 144 ÷ (8.375 × 2.625) = 6.55 bricks per sq ft, which is the same 655 per 100 sq ft that BIA Technical Note 10 publishes. For a UK brick with a 10 mm joint it gives 1,000,000 ÷ (225 × 75) = 59.3 bricks per m², the familiar “60 per square metre”.

Step 4: subtract the bricks

Wet mortar = wall volume − (brick count × brick length × brick height × brick width)

The result is the volume of mortar sitting in the joints of the finished wall. It is a wet, in-place volume, and that matters for the next step.

Step 5: convert wet mortar to dry material

Here is the factor that most guides quote and few explain.

If you measure out one cubic metre of loose dry sand and the cement to go with it, mix them with water, you do not get more than a cubic metre of mortar. You get less. Loose sand has roughly 30 to 35 percent voids between its grains, and the cement paste settles into those voids instead of adding to the pile.

So the conversion runs the other way when you are buying:

Dry material volume = wet mortar volume × 1.33

Four things are worth knowing about that factor.

It is a range, not a constant. Values between 1.30 and 1.35 are all in common use. The exact figure depends on the sand grading and how loosely it is measured. The Mortar Calculator uses 1.33.

It is not a waste allowance. Waste is a separate multiplier applied afterwards. Guides that treat 1.33 as “wastage” end up either double counting or under-ordering.

It is not sand bulking. Damp sand swells, often by 20 to 30 percent, so a bucket of damp sand holds less actual sand than a bucket of dry sand. If you batch by volume with damp sand, that is a further correction on top.

It does not apply to ready-mixed wet mortar. If you are buying mortar in tubs or from a silo, already mixed, the volume you buy is the wet volume. Skip this step entirely.

Step 6: add waste

BIA guidance is to add 15 to 25 percent to a calculated mortar quantity for waste, and at least 5 percent to the brick count for breakage. Ten percent is defensible for a clean, repetitive job with an experienced crew, and is the default the calculator starts from. Fifteen percent upward suits stone, irregular units, tight sites and first-time work.

Waste on brickwork is genuinely large. Mortar goes on the board, on the scaffold, on the ground and into the mixer’s corners. If you are ordering pre-blended bags rather than raw material, the bag coverage figures manufacturers publish already include a very generous allowance, and you should not add waste on top of them.

Step 7: split the total by the mix ratio

The final volume is divided in the proportions of the mortar type. For a Type N cement-lime mortar at 1 part cement, 1 part hydrated lime and 6 parts sand, the total is 8 parts.

Assumptions: proportions by volume of dry, loose material. These sit inside the proportion specification of ASTM C270, which sets the aggregate at not less than 2¼ and not more than 3 times the sum of the cementitious volumes.

To turn volumes into bags or weights, common loose bulk densities are about 90 lb per cu ft (1,440 kg/m³) for portland cement, 40 lb per cu ft (640 kg/m³) for hydrated lime and 100 lb per cu ft (1,600 kg/m³) for sand. US practice rounds cement to a slightly denser packing and treats a 94 lb bag as 1 cu ft, and a 50 lb bag of hydrated lime as 1.25 cu ft, which is what the calculator uses.

Worked example 1: imperial

A single-wythe garden wall, 20 ft long by 8 ft high, in US standard brick (3⅝ × 2¼ × 8 in) with 3/8 in joints, Type N mortar, 10 percent waste.

1. Face area = 20 × 8 = 160 sq ft
2. Wall thickness = brick width = 3⅝ in = 0.302 ft. Wall volume = 160 × 0.302 = 48.33 cu ft
3. Bricks per sq ft = 144 ÷ (8.375 × 2.625) = 6.55. Bricks = 160 × 6.55 = 1,048 bricks
4. Solid volume of one brick = (8 × 2.25 × 3.625) ÷ 1,728 = 0.0378 cu ft
5. Total brick volume = 1,048 × 0.0378 = 39.57 cu ft
6. Wet mortar = 48.33 − 39.57 = 8.76 cu ft (248 L)
7. Dry material = 8.76 × 1.33 = 11.65 cu ft
8. Plus 10 percent waste = 12.82 cu ft (0.36 m³)
9. Cement = 12.82 ÷ 8 = 1.60 cu ft, so 2 bags of 94 lb
10. Hydrated lime = 1.60 cu ft ÷ 1.25 = 1.28, so 2 bags of 50 lb
11. Sand = 12.82 × 6/8 = 9.6 cu ft (0.36 cu yd)
12. Or as pre-blended bags at 0.66 cu ft each, 12.82 ÷ 0.66 = 20 bags of 60 lb

Order 1,100 bricks to cover the 5 percent breakage allowance.

A note on wall thickness. This example used the brick’s specified width of 3⅝ in. The calculator fills in the nominal 4 in wythe by default, because that is the dimension drawings work to, and for this wall that returns about 22 bags rather than 20. The difference is the 3/8 in between the nominal module and the brick. Both are reasonable ordering bases, and the nominal figure is the more conservative of the two. Type your own thickness into the calculator if you would rather work from the specified size.

Worked example 2: metric

A half-brick wall, 5 m long by 3 m high by 0.09 m thick, in 190 × 90 × 90 mm brick with 10 mm joints, mixed at 1 part cement to 6 parts sand, 10 percent waste.

1. Wall volume = 5 × 3 × 0.09 = 1.35 m³
2. Brick face plus joints = 0.2 × 0.1 = 0.02 m², so bricks per m² = 50
3. Bricks = 15 m² × 50 = 750 bricks
4. Solid volume of one brick = 0.19 × 0.09 × 0.09 = 0.001539 m³
5. Total brick volume = 750 × 0.001539 = 1.154 m³
6. Wet mortar = 1.35 − 1.154 = 0.196 m³, which is 14.5 percent of the wall volume
7. Dry material = 0.196 × 1.33 = 0.260 m³
8. Plus 10 percent waste = 0.286 m³
9. Cement = 0.286 × 1/7 = 0.041 m³, about 59 kg, so 2 bags of 50 kg
10. Sand = 0.286 × 6/7 = 0.245 m³, about 393 kg

The 14.5 percent in step 6 is worth pausing on, because many published tables put mortar at 23 percent of brickwork volume for this brick. That higher figure comes from treating each brick as filling a 200 × 100 × 100 mm box, which adds a mortar joint through the wall thickness that a 90 mm single-leaf wall does not have. A 190 mm two-leaf wall does have a collar joint, and its mortar share works out at 19 percent. The bricks per cubic metre guide takes that difference apart in full.

Checking your own calculation

Four checks catch nearly every mistake.

Is the mortar a plausible share of the wall? For single-wythe brickwork with normal joints it should be between about 15 and 25 percent of the wall volume. Anything above 30 percent means the brick volume is too small or the joint is too thick.

Is sand the largest quantity? In every standard mortar type sand is 6 of 8 parts or 9 of 12. If cement comes out larger than sand, the ratio has been applied upside down.

Does the brick count match the face area? Multiply your brick count by the face area of one brick plus its joints and see whether it returns the wall area you started with.

Are the units consistent? Cubic feet from feet, cubic metres from metres. A thickness in inches inside a formula that expects feet is a factor of twelve error, and it will look almost plausible.

For a quick cross-check on a real wall, put your dimensions into the Mortar Calculator. It runs the same seven steps, keeps the units straight, and gives both the pre-blended bag count and the cement, lime and sand split. If your hand calculation and the tool disagree by more than about 10 percent, the difference is nearly always the wall thickness or the brick size, not the arithmetic.

Where the estimate goes next

Once you have a volume you can approach it from the other direction as a check. The mortar per m² figures let you multiply the wall area by a single number and see whether the long calculation lands in the same place. If your joints are anything other than the standard, settle that first with the joint thickness guide, because it moves the total more than any other input.

And when it comes time to actually mix what you have ordered, the step-by-step mixing guide covers proportioning by bucket, mixing order and the water you add at the end.