Mortar Calculator UK
Calculate exactly how much cement and sand you need for bricklaying or repointing. Enter your wall area, brick size and mix ratio, and get an exact bag count for both.
Last reviewed: 8 September 2026
Job Type
Mix & Wall Area
Options
How to Calculate Mortar for Brickwork
Here's the method behind this calculator:
- Work out the joint area: the gaps between bricks that need filling. This depends on brick size and joint thickness. For a standard UK brick (215 × 102.5 × 65mm) with 10mm joints, the joints make up about 17% of the wall face.
- Multiply by depth: for new brickwork, mortar fills the full depth of the brick (102.5mm for a single skin). For repointing, only the raked-out depth matters, typically 15-20mm.
- Add an allowance: real mortar use runs higher than pure geometry suggests because bricks have frogs (indents) that need filling and some material is always lost in mixing and application. We build this in automatically.
- Split by mix ratio: a 1:5 mix (cement:sand) is 1 part cement to 5 parts sand by volume, so 1/6 of the mortar is cement and 5/6 is sand.
- Convert to bags: a 25kg bag of cement is about 0.0174 m³. Sand is sold by weight, typically in 850kg bulk bags or 25kg mini bags.
Our calculator does all of this automatically based on your brick size, joint thickness, mix ratio and wall area.
Choosing a Mortar Mix Ratio
| Mix Ratio | Strength | Best For |
|---|---|---|
| 1 : 3 | Strong | Engineering brick, below damp-proof course, exposed coastal locations |
| 1 : 4 | Medium-strong | General brickwork without plasticiser, load-bearing walls |
| 1 : 5 | Medium | The most common general-purpose mix for garden walls, extensions and general bricklaying, usually with a plasticiser |
| 1 : 6 | Weak/flexible | Soft old handmade bricks, rendering backup coats (a stronger mix can crack softer, older brick) |
As a rule, mortar should always be slightly weaker than the brick or block it's holding together. Using too strong a mix on soft old brickwork causes the brick to crack and spall before the joint does, which is much harder to repair.
New Brickwork vs Repointing: What's Different
New brickwork uses mortar to fill both the bed joints (horizontal, between courses) and perpend joints (vertical, between bricks) across the full depth of the brick. This is the calculation used when building a wall from scratch.
Repointing only replaces the outer layer of an existing joint. Old, crumbling mortar is raked out to a depth of 15-20mm and refilled, leaving the sound mortar behind it in place. This uses far less material per m² than new brickwork, which is why our calculator treats it as a separate job type with its own depth setting.
Never repoint with a mortar mix stronger than the original. Modern cement-rich mixes trap moisture in older, softer brickwork and accelerate frost damage. For pre-1919 solid brick walls, use a lime mortar instead of cement, which stays permeable and flexes with the building.
Mixing Mortar: Ratios by Volume, Not Weight
Mortar mix ratios (like 1:5) are always by volume, not weight, traditionally measured with a bucket or gauge box, not scales. Cement and sand have different densities, so mixing by weight in the same ratio would give a different, weaker mix.
Dry materials also "bulk up" when loose: a given weight of dry sand and cement takes up more space than the same materials once mixed and compacted into wet mortar. Our calculator accounts for this automatically when converting mortar volume into bags to buy.
Plasticiser: most bricklayers add a liquid plasticiser (or washing-up liquid as an old trade trick) to a lime-free mix, which entrains tiny air bubbles that make the mortar more workable and frost-resistant. Follow the dosage on the bottle; typically a capful per mix.
Frequently Asked Questions
How much mortar do I need per 1000 bricks?
For a standard UK brick with 10mm joints in a single-skin wall, 1000 bricks typically need approximately 0.4-0.5 m³ of wet mortar, which works out to roughly 6-7 bags of cement (25kg) and one bulk bag of sand (850kg) at a 1:5 mix. Exact figures vary with brick size, joint width and mix ratio.
How much sand and cement do I need for a m² of brickwork?
A single-skin (half-brick) wall with standard UK bricks and 10mm joints needs approximately 0.025 m³ of wet mortar per m². At a 1:5 mix, that's roughly 0.4 bags of cement and 43kg of sand per m² once you account for dry sand bulking up compared to mixed wet mortar. It adds up fast: for a 15 m² wall that's already over 640kg of sand.
What mortar mix should I use for a garden wall?
A 1:5 or 1:6 cement:sand mix with a plasticiser is standard for garden walls built with modern facing bricks. Go stronger (1:3 or 1:4) only where the wall is retaining earth, below ground, or in an exposed coastal location.
How much mortar do I need to repoint a wall?
Repointing uses much less material than new brickwork since only the outer 15-20mm of each joint is replaced. A typical house elevation of 30-40 m² needs roughly 3-5 bags of cement and a similar volume of sand at a standard rake-out depth. Our calculator gives you an exact figure for your wall.
Can I use ready-mixed bagged mortar instead of mixing my own?
Yes, pre-blended bagged mortar (just add water) is widely available and convenient for smaller jobs like repointing or garden walls. It costs more per m³ than buying cement and sand separately, but saves you gauging the ratio yourself. Our cement and sand figures let you compare cost against a pre-mixed alternative.
Should I use lime mortar on an old house?
For solid brick or stone walls built before around 1919 (before cavity walls and modern cement became standard), yes: lime mortar stays breathable and flexible, letting moisture escape through the joints rather than the brick face. Cement mortar on old solid walls traps damp and accelerates frost damage to the brick itself.
Figures include a typical allowance for frog-fill and mixing losses, and are based on standard UK brick sizes and mortar mix conventions. Actual usage varies with brickwork condition, mixing technique and wastage on site.