A mortar ratio is two numbers and a decision. The two numbers — cement to sand by volume — set where the mix sits between strong-and-brittle and weak-and-forgiving. The decision is what the wall actually needs, which changes with the units, the exposure and the age of the building.
This guide explains what the ratio means in the bucket, walks the common ratios and their uses, shows how wet mortar volume converts into dry materials and bags of cement, and covers the two subjects that ruin more walls than any bad ratio: plasticiser versus lime, and matching mortar in older brickwork.
Traditional guidance, by volume: 1:3 strong and damp-resistant; 1:4 general external work; 1:5 internal; 1:6 weak, for soft units and internal work. Multiply the wet mortar volume by 1.33 to get dry ingredients (0.3 m³ of 1:5 needs about 96 kg of cement — four 25 kg bags — and 0.33 m³ of sand). On old, soft-fired brickwork, use a lime-based mortar instead; strong cement mortar is what damages those walls. The Mortar Calculator does the volume-to-bags arithmetic for any ratio.
What the Ratio Actually Means
A ratio of 1:5 means one part cement to five parts sand by volume — not by mass, not by shovel-heaped optimism. Volume is how the trade has always gauged mortar, and it survives because it works: a bucket marked at 10 litres measures cement and sand with the same error in both, and the errors cancel.
Gauge with a bucket or a gauge box, not shovels. A "shovel of cement" varies by a third between heap and strike, and on a small mix that swings the ratio by a full grade. Old bricklayers could cut a shovel consistently; the rest of us should use the bucket.
The Common Ratios and Where They Belong
| Ratio (cement:sand) | Character | Typical uses |
|---|---|---|
| 1:3 | Strong, dense, low-flexibility | Engineering brickwork, below-DPC work, dense stone; damp and frost-exposed positions |
| 1:4 | The general external mix | Facing brickwork, garden walls, most exposed site work |
| 1:5 | General purpose, slightly more forgiving | Internal blockwork, partitions, non-exposed lay work |
| 1:6 | Lean, weak, most deformable of the cement-only mixes | Soft, low-strength units; internal work where movement is expected |
The logic is a hierarchy the other way round from what beginners expect: mortar should be weaker than the units it joins. A slightly soft joint absorbs movement and lets the brickwork behave as a mass; a rock-hard joint transfers every stress straight into the bricks, and the bricks lose. Stronger mortar is not better mortar — it is mortar chosen for a specific reason, or a mistake.
One caveat, because practice has moved on: many countries now specify mortar through designated classes and strength categories rather than site ratios, and the recipes behind each class — including whether lime counts as part of the binder — vary by standard. Treat the table above as traditional guidance and a planning tool; if your project is engineered or inspected, use the mix the design and the local standard call for, exactly as written.
The Dry-Volume Factor: Why 1.33
Sand is mostly air. The grains stack with voids between them, and when cement paste and water are worked through, those voids fill — the mixed material occupies less space than the ingredients did loose. The working correction is the dry-volume factor:
dry ingredient volume = wet mortar volume × 1.33. Need 0.3 m³ of mortar in the barrow? Measure out about 0.4 m³ of dry cement-plus-sand first.
Like everything mortar, 1.33 is a typical figure — sand grading and moisture both move it — but it is the standard planning constant, and it beats guessing by the amount that guessing usually loses.
From Volume to Bags
Cement is sold by mass, so the last conversion is volume to kilograms. The bulk density of Portland cement is about 1,440 kg/m³ — a figure worth remembering, because it turns any cement volume into a bag count in one line:
- In the UK and much of Europe, cement comes in 25 kg bags — 1,440 ÷ 25 means a bag is about 17.4 litres of cement.
- Across much of the world — India, much of Africa and Asia, parts of the Americas — the 50 kg bag is standard, worth about 34.7 litres.
- A "tonne bag" or bulk bag of sand is commonly quoted around 0.5 m³ / 800–850 kg, but bag sizes vary by supplier — check before you count them.
Plasticiser vs Lime
A 1:5 or 1:6 mix mixed with just cement, sand and water is harsh — it does not spread or cling. Two traditions fix that. Plasticiser is a liquid admixture that entrains microscopic air bubbles, making the mix fatty and workable; a few capfuls replace what lime used to do, with none of the mixing discipline lime demands. Lime — traditionally gauged into the mix, as in a 1:1:6 cement:lime:sand — adds workability plus two things plasticiser cannot: flexibility under small movements, and a degree of self-healing as slight cracks react with water and re-carbonate.
Modern new-build work usually takes plasticiser. Solid-wall, soft-brick and most repair work usually takes lime. And the old site trick of squiring washing-up liquid into the mixer does neither job properly — it entrains the wrong kind of air, and the salts are no friend of brickwork.
Matching Mortar in Older Brickwork
This is where ratio choices do damage rather than good. Bricks fired before about 1900 — and many since — are soft and porous, and they rely on the mortar to let moisture escape. Those walls were built with lime mortar precisely because it is weaker and more vapour-permeable than the bricks. Repoint or extend them with a strong cement mix and the moisture path through the joints closes; water sits in the bricks instead, and frost eventually pops their faces off in sheets.
The rule: soft old bricks need soft, lime-rich mortar. For repair and repointing on traditional solid walls, a natural hydraulic lime mortar — commonly around 1:3 lime to well-graded sharp sand, with the lime grade chosen to the exposure — is the typical answer. Where you must match an existing mortar, chip a piece out and have it assessed; strength and permeability, not just colour, are what you are matching. The strongest mortar on the site should still be weaker than the masonry it joins.
Water: Gauge by Feel, Not by Litres
No mortar recipe specifies water, because sand moisture changes daily. The water demand of a mix is read by feel: add water gradually until the mortar holds a firm ridge when the trowel is drawn through, clings to the underside of the trowel held flat, and does not slump or weep. That feel — "workable but stiff" — is the specification. If a mix needs reviving an hour in, a splash of water and a hard remix is fine; repeatedly chasing water in to keep a dying mix alive will weaken the joint.
Worked Example: 0.3 m³ at 1:5
0.3 m³ is roughly the mortar in 11–12 m² of single-skin brickwork (0.3 ÷ 0.026 ≈ 11.5 m²) — a decent garden-wall session. At 1:5 by volume:
- Dry volume: 0.3 × 1.33 = 0.399 ≈ 0.4 m³ of dry ingredients.
- Parts: 1 + 5 = 6.
- Cement: 0.4 ÷ 6 ≈ 0.067 m³. As mass: 0.067 × 1,440 ≈ 96 kg — four 25 kg bags (or two 50 kg bags), with a little margin in the fourth.
- Sand: 0.4 × 5/6 ≈ 0.33 m³ — about two-thirds of a bulk bag, or roughly half a tonne at typical sand densities.
- Water: gradually, to the ridge-and-cling feel — likely 20–25 litres across the batch, but let the sand decide.
That is the whole calculation the Mortar Calculator automates — enter the wall or the mortar volume, pick the ratio, and it returns cement by the bag and sand by the m³.
Common Mistakes
- Gauging by the shovel. Heaps vary by a third; the bucket does not.
- Skipping the 1.33 factor. A wet-volume order of materials comes up a third short by the end of the job.
- Reaching for 1:3 "for strength". On soft or old bricks the mortar wins the argument, and the wall loses.
- Strong cement repointing on a lime-built wall. The classic way to spall brick faces within a few winters.
- Washing-up liquid as plasticiser. It foams the wrong way and adds salts; the real admixture is measured in capfuls.
- Chasing water into a stiffening mix. Revive with a splash and a remix; do not keep watering a mix past its working time.
- Mixing a full day's mortar at once. Gauge batches you can use within the working time printed on the cement bag — typically an hour or two in temperate conditions.
Plan It With the Mortar Calculator
Know your wall area? The Mortar Calculator applies the per-m² allowances, expands by the 1.33 dry-volume factor, and splits the result into cement bags and sand volume for any ratio from 1:3 to 1:6 — or take the brick count from the Brick & Block Calculator and let it work from there.
Open the Mortar CalculatorFrequently Asked Questions
What is the best mortar ratio for bricklaying?
There is no single best — follow the units and the exposure. As traditional guidance: 1:4 general external, 1:5 to 1:6 internal, 1:3 only where a strong dense mix is genuinely required. Old soft-fired bricks want a lime-based mortar instead; strong cement mortar is what damages them.
How much mortar does one 25 kg bag of cement make?
At 1:5: one bag is 25 ÷ 1,440 ≈ 0.017 m³ of cement; with its five parts of sand and the 1.33 expansion folded back out, that is about 0.08 m³ of wet mortar — roughly 3 m² of single-skin brickwork at the typical 0.026 m³ per m².
Can I use plasticiser instead of lime?
For modern work, yes — it entrains a little air and makes lean mixes workable. Lime still earns its place where flexibility and vapour permeability matter: solid walls, soft bricks, and most repairs on older buildings.
Why is my mortar cracking and crumbling?
Usually a mix mismatched to the units (too strong cracks, too weak crumbles), over-watering, fast drying in sun and wind, or frost in the first days. Strong mortar on soft bricks fails by cracking the bricks; weak mortar on dense engineering brick fails at the joints.
What ratio should I use to repoint old brickwork?
Match the existing, and keep the mortar weaker than the masonry. On pre-1900 solid walls that usually means a lime mortar — typically around 1:3 natural hydraulic lime to well-graded sharp sand. Cement repointing traps moisture and spalls the brick faces.
Related MyHouseLogic Guides
Mortar is one of three quantities on a brick job.
- How many bricks do I need? — the per-m² method that produces the mortar volume in the first place.
- Concrete mix ratios — the same ratio logic for the footing under the wall, and why water dominates strength there too.
- Concrete curing guide — the drying-and-protection discipline that mortar shares after the joints are struck.
- All Concrete & Building guides — slabs, sub-bases, brickwork and quantities.
The bottom line: the ratio is by volume, gauged by bucket; 1:4 external, 1:5 internal, 1:3 only on demand; wet volume × 1.33 gives the dry materials, and cement runs 1,440 kg per m³. Keep the mortar weaker than the units — and on old brickwork, that means lime, not cement.