How the mortar calculator works
Repointing replaces only the outer part of each joint, so the amount of mortar depends on how many metres of joint the wall contains, not on the number of bricks. For each square metre the calculator counts the horizontal bed joints (one per course) and the short vertical perpends between bricks, then multiplies that length by the joint width and the depth you rake out.
Mortar = area × joint length per m² × width × rake depth × 1.15The 15% covers mortar dropped from the hawk, pressed into hollows behind the old pointing and left to harden in the bucket. To split the mix into ingredients, the wet volume is multiplied by 1.3, because dry cement and sand shrink by roughly a quarter once water fills the gaps between grains. Cement is taken at 1,440 kg/m³ and damp building sand at 1,600 kg/m³.
Worked example
A garden wall of 10 m² in 215 × 65 mm bricks with 10 mm joints has 13.3 m of bed joint and 3.9 m of perpends per square metre, 171.9 m in all. Raked 20 mm deep, the gap to fill is 171.9 × 0.010 × 0.020 = 0.0344 m³, or 39.5 L with the allowance. Dry volume is 51.4 L; at 1:4 one part in five is cement, so 10.3 L or 14.8 kg of cement and 41.1 L or about 66 kg of sand. Buying premix instead, 39.5 L at 1.9 kg of dry mix per litre is 75 kg, just over three 25 kg bags, so four are needed.
Joint length and mortar per square metre
| Unit (10 mm joints) | Joint length per m² | Litres per m² at 20 mm rake |
|---|---|---|
| Metric brick 215 × 65 mm | 17.2 m | 3.4 L |
| US modular brick 7⅝ × 2¼ in | 19.1 m | 3.8 L |
| German NF brick 240 × 71 mm | 15.9 m | 3.2 L |
| Concrete block 390 × 190 mm | 7.4 m | 1.5 L |
Figures exclude the 15% allowance. Small bricks mean far more joint: the same area of US modular brick takes two and a half times the pointing mortar of block.
Choosing the mix
| UK designation | Cement : lime : sand | Nearest ASTM type | Typical use |
|---|---|---|---|
| (i) | 1 : ¼ : 3 | M | Below ground, heavy loads |
| (ii) | 1 : ½ : 4–4½ | S | Exposed walls, copings, chimneys |
| (iii) | 1 : 1 : 5–6 | N | General walls above ground |
| (iv) | 1 : 2 : 8–9 | O | Soft brick in sheltered spots |
The golden rule of repointing is that new mortar must be weaker and more porous than the brick, so moisture leaves through the joints rather than through the brick face. Houses built before about 1920 were usually laid in lime mortar; repointing them with a hard cement mix traps water and makes the brick faces spall. Use a lime-based mortar there and take advice for listed or historic buildings.
Repointing tips
- Rake square, not V-shaped. A joint cleaned to a flat back holds new mortar far better.
- Protect your lungs. Grinding out joints releases silica dust; wear a proper respirator and damp down the work area.
- Dampen before filling. Wet the raked joints so dry brick does not suck water out of the fresh mortar.
- Mix in small batches. Cement mortar is workable for roughly one to two hours; do not re-temper mortar that has started to stiffen.
- Match the finish. Copy the existing joint profile and sand colour on a test panel first.
Frequently asked questions
How much mortar do I need to repoint 1 m² of brickwork?
With standard 215 × 65 mm bricks, 10 mm joints and a 20 mm rake, about 3.4 L per square metre, or 4 L with waste. A 25 kg bag of premix repoints roughly 3 m².
How many bags of mortar to repoint a 100 sq ft wall?
US modular brick with 3/8 in joints raked 3/4 in needs about 1.3 ft³ of mortar, which is two 80 lb bags of premixed mortar.
What is the best mortar mix for repointing?
For modern bricks, 1:1:5–6 cement, lime and sand is common. For old, soft brick use a lime mortar, and never a mix stronger than the original.
How deep should I rake out mortar joints?
Two to two and a half times the joint width, so 20–25 mm for a 10 mm joint, and at least 15 mm (5/8 in), always back to sound mortar.
How much cement and sand for 1 cubic metre of mortar?
At 1:4, 1 m³ of wet mortar needs about 1.3 m³ of dry materials: roughly 375 kg of cement (15 bags of 25 kg) and 1.04 m³ of sand, about 1,660 kg.
Last reviewed: 2026-09-27 · How we calculate