I once watched a bricklayer refuse to use the mix his labourer had made up. He tapped it with his trowel, tasted it (yes, literally - I am told this was once common practice to feel the lime content), and said it was too strong. The labourer had used 1:3 for a standard garden wall, where 1:5 was correct.
The wall would probably have been fine structurally. But the mortar would have been harder than the facing brick, and any movement - thermal expansion, slight foundation settlement - would have spalled the brick face rather than cracking the joint. You want mortar joints to be the sacrificial element. They are designed to be repointed. Brick faces are not.
Understanding mortar ratios is not complicated. Here is what you actually need to know.
What the ratio means
A mortar mix ratio describes the proportions of cement to sand by volume - not by weight.
A 1:4 mix means: 1 part cement + 4 parts sand. Measured by the bucket, gauge box or shovel - consistency matters more than the measuring method, but you must use the same measure throughout.
The total parts = 5. So cement is 1/5 (20%) of the dry mix and sand is 4/5 (80%).
Water is added separately. The water:cement ratio affects workability and strength but is not captured in the main mix ratio. As a guide, roughly 0.5 litres of water per kg of cement.
Mix ratios and their strengths
| Mix ratio | Strength (approx) | UK designation | Typical use |
|---|---|---|---|
| 1:3 | Very strong | Designation (i) | Engineering bricks, retaining walls, below DPC, manholes |
| 1:4 | Strong | Designation (ii) | Standard brickwork, most domestic applications |
| 1:5 | Medium | Designation (iii) | Internal blockwork, softer bricks, general masonry |
| 1:6 | Weak | Designation (iv) | Repointing, very soft historic brick, lime-modified work |
These are Portland cement and sand ratios. Where lime is added (a traditional practice now often used for breathability on older buildings), the ratio changes - typically 1:1:6 cement:lime:sand for a lime-modified mortar.
When to use each mix
1:3 - strong mix
Reserved for applications where maximum strength and water resistance are needed: below-ground work, engineering brickwork, retaining walls, the first course of blockwork in a garage or outbuilding slab, and manholes. It is too strong for most facing brickwork - it constrains thermal movement and can damage softer bricks.
1:4 - standard mix
The workhorse of domestic brickwork. Suitable for most walls above DPC in normal exposure conditions. Block paving mortar, garden walls in sheltered positions, internal block walls. This is what most builders mean when they say "standard mix."
1:5 - medium mix
For softer or more porous bricks (many reclaimed bricks, handmade bricks, some estate brick), internal walls, and situations where you want slightly more flexibility in the mortar joint. Also used for bedding coping stones and cappings where a little give is useful.
1:6 - lean mix
Primarily used for repointing - especially on older or period properties where you need to match an existing weak mortar rather than applying a modern hard mix. Also used where a lime-type performance is wanted without using lime. Too weak for structural work.
Common mistakes
Using 1:3 on soft bricks. The mortar becomes harder than the brick. Thermal movement then spalls the brick arris rather than cracking the mortar joint. Repointing is easy. Re-facing a brick wall is not.
Using 1:6 below DPC. Below ground, mortar is exposed to constant water pressure and frost. A lean mix will not withstand this. Use 1:3 for any below-DPC work.
Mixing by weight instead of volume. Cement is denser than sand, so a 1:4 ratio by weight gives a very different mix to 1:4 by volume. Volume is the correct way to measure mortar.
Adding too much water. A wet mortar may be easier to use but it is weaker, more prone to shrinkage cracking, and takes much longer to reach full strength. The mix should be stiff enough to hold its shape - it should not slump off the trowel.
Sand type matters too
The mix ratio assumes you are using sharp sand (coarse sand) for structural work. Building sand (soft sand, plastering sand) produces a more workable mortar but weaker - it is suited to pointing and render, not structural beds.
For most domestic brickwork, a blend of 3 parts sharp sand to 1 part building sand gives good workability and adequate strength. Some bricklayers work this way as standard.
How much mortar do I need?
Once you know the right ratio for your job, the Mortar Calculator works out the cement bags and sand you need based on wall area. Enter the area, select your wall type (brickwork or blockwork) and pick your mix ratio - it calculates mortar volume, cement bags and sand in tonnes.
My tips on mortar mixing
Measure consistently. Whether you use a bucket, a gauge box or count shovels, use the same measure every time. Eyeballing proportions is the main cause of variable results across a job.
Mix dry first. Always mix the dry materials together thoroughly before adding water. Unmixed dry cement in a corner of the trough becomes a lump of set cement in the finished wall.
Add water in stages. Add about two thirds of your estimated water first. Mix fully, then add more in small increments until you reach the right consistency. You can always add water, you cannot take it out.
Do not re-temper. Mortar that has started to stiffen should not be wetted up and used again. The cement hydration process has begun. Re-tempered mortar has reduced strength and is more prone to cracking.
Match existing mortar when repointing. Rake out a small test area and compare the colour of your new mix. New mortar often looks considerably lighter when wet than when dry - test a small patch and let it cure before committing to the full job.
Use the Mortar Calculator to get cement and sand quantities for your project before ordering.