How the mortar calculation works
Mortar fills the joints between bricks — the bed joint under each brick and the head (perpend) joint at its end. For each brick, the mortar volume is the brick's "cell" (brick plus one bed and one head joint) minus the brick itself:
wet mortar = bricks × mortar per brick (+ collar joint for double walls) × (1 + mortar waste)
dry mix = wet mortar × 1.33
cement = dry mix ÷ (1 + sand parts) sand = dry mix − cement
L, H and W are the brick length, height and width, and j is the joint thickness. The 1.33 factor allows for the way dry cement and sand shrink in volume once water is added and the voids between sand grains fill.
Mortar needed per 1,000 bricks
| Brick | Joint | Wet mortar per 1,000 bricks | Per ft² / m² of wall |
|---|---|---|---|
| US modular | ⅜ in | 8.1 ft³ | 0.056 ft³ per ft² |
| US standard | ⅜ in | 8.4 ft³ | 0.055 ft³ per ft² |
| US king | ⅜ in | 7.5 ft³ | 0.036 ft³ per ft² |
| UK standard | 10 mm | 0.30 m³ | 0.018 m³ per m² |
| South Africa NFP | 10 mm | 0.32 m³ | 0.017 m³ per m² |
| Australian standard | 10 mm | 0.35 m³ | 0.017 m³ per m² |
These are theoretical volumes for a single-skin wall with full joints and no waste. In practice, masons lose mortar to droppings, the board and partly-used batches, and frogged or cored bricks take extra mortar into the frog or holes — which is why the calculator adds a 15% mortar waste allowance by default. Increase it to 20–25% for frogged bricks laid frog-up or for inexperienced builders.
Choosing a mortar mix ratio
The ratio is parts of cement to parts of sand, by volume. A stronger (richer) mix isn't always better: mortar should be weaker than the bricks so that any cracking happens in the joints, which are easy to repoint.
| Mix | Typical use |
|---|---|
| 1 : 3 | Strong mortar for below ground, retaining walls, copings and very exposed work. |
| 1 : 4 | General-purpose mortar for most external and load-bearing walls. |
| 1 : 5 | Ordinary walls in sheltered positions; often used with a plasticiser. |
| 1 : 6 | Softer mortar for internal walls and soft bricks, usually with lime or plasticiser added. |
In the US, mortar is usually specified by type rather than ratio. Common proportions (cement : lime : sand) are roughly: Type M 1 : ¼ : 3¾ (below grade), Type S 1 : ½ : 4½ (structural and below-grade), Type N 1 : 1 : 6 (general above-grade walls and veneer), and Type O 1 : 2 : 9 (interior, non-load-bearing). Many US masons use pre-blended masonry cement (70 lb bags) mixed with about 3 parts sand — pick "70 lb masonry cement" and "1 : 3" in the calculator for that.
Cement bags and sand
The calculator converts the cement volume to weight (about 1,440 kg/m³ for cement, or 70 lb per ft³ for masonry cement) and divides by your bag size: 94 lb or 92 lb Portland, 70 lb masonry cement, or 50, 40 or 25 kg bags. Sand is shown by volume and by weight (about 1,600 kg/m³ or 1.35 tons per cubic yard), because suppliers sell it both ways. Damp sand bulks up, so a little extra sand is rarely wasted.
Using pre-mixed mortar? An 80 lb bag of pre-mixed mortar typically makes about 0.6–0.75 ft³ of wet mortar — check the bag. Divide the calculator's wet mortar volume by your bag's yield to get the number of bags.
Frequently asked questions
How much mortar do I need for 100 bricks?
About 0.8 ft³ of wet mortar for 100 US modular bricks with ⅜ in joints, or about 0.03 m³ (30 liters) for 100 UK bricks with 10 mm joints, before waste.
How many bags of cement do I need for 1,000 bricks?
At a 1 : 4 mix, 1,000 UK bricks need about 130 kg of cement including 15% mortar waste — 3 × 50 kg or 6 × 25 kg bags. 1,000 US modular bricks need about 4 × 70 lb bags of masonry cement at 1 : 3. Use the calculator for your exact brick and mix.
What is the best mortar mix for bricks?
1 : 4 cement to sand is a good general mix for external walls. Use 1 : 3 below ground or for retaining walls and 1 : 5 or 1 : 6 (with lime or plasticiser) for sheltered and internal walls. In the US, Type N is standard for above-grade walls and Type S for structural or below-grade work.
Why is the dry volume bigger than the wet mortar volume?
When water is added, cement particles fill the gaps between sand grains, so the mixed mortar takes up less space than the dry ingredients. A factor of about 1.33 is the standard allowance.
How thick should a mortar joint be?
⅜ in (about 10 mm) is standard for most brickwork. Thicker joints use more mortar and fewer bricks; the calculator lets you change the joint size.