Mortar Mix Ratio Calculator (Sand:Cement)

Mortar Mix Guide (UK)

What's the difference between Mortar and Concrete?

Mortar bonds masonry units (bricks, blocks, stone). Concrete forms structural elements (foundations, slabs). Key differences: mortar uses fine aggregate (sand) only — no coarse aggregate. Concrete uses both sand and gravel. Mortar must be workable (spread cleanly with a trowel, support a brick without slumping) but firm enough to support load above. Cement-only mortars (1:3 cement:sand without lime) are too brittle — crack as walls move slightly. Always include lime or plasticiser for workability and flexibility in mortar — a rigid, cement-only mix will crack as the wall naturally moves slightly over time.

What's the difference between Lime and Plasticiser?

Traditional mortars use hydrated lime as the third component. Lime adds: workability (mortar spreads cleanly). Plasticity (forgiving during laying, slow setting). Crack tolerance (some flex through seasonal movement). Bond improvement to absorbent units. Modern mortars often substitute plasticiser (Feb-Mix, Sika-Air) for lime: cheaper. Faster to mix (no slaking time). Same workability benefits. Substitute ratio: typically 50-100ml plasticiser per 25kg cement. Lime mortar specifics: hydrated lime, typically CL90 grade, is the standard choice for most traditional masonry repair and repointing work.

What do I need to know about Workability and Mixing?

Good mortar should: hold a sharp edge when trowelled. Spread cleanly without dragging. Support a brick without slumping. Stiffen progressively as it sets (workable 1-2 hours from mixing). Common DIY mistakes: too much water (drops strength dramatically). Mixing too large a batch (sets before fully used). Using wrong sand (soft/plastering sand for brickwork makes weak mortar — use sharp/building sand). Sand quality: building sand (sharp sand): for brickwork mortar. Sharp particles bond strongly with cement, giving a stronger, more durable mortar than soft or rounded sand grains would produce.

What's the key thing to understand about Coverage and Quantity?

Approximate mortar usage per m² of wall: standard 215×65mm brick wall, 10mm joints: 0.025 m³ mortar per m² (half-brick wall). Or about 60 bricks/m². Solid 215mm wall: 0.045 m³ per m². 100mm block wall: 0.015 m³ per m². 140mm or 215mm block wall: 0.020-0.025 m³ per m². Working ratios: 1m³ mortar contains approximately: 1.6 tonnes sharp sand. 250kg cement (1:6 mix with plasticiser) OR 200kg cement + 50kg hydrated lime (1:0.5:4.5 with lime). ~150 litres water. Practical batching: a typical 1m³ wheelbarrow batch will produce enough mortar for a substantial day's bricklaying, though exact coverage varies with joint thickness and brick size.

What do I need to know about Mortar Strength Classes (i, ii, iii, iv)?

UK mortar specifications commonly use designation classes, often written as mortar class i, ii, iii and iv, which describe mortar strength and the typical cement-to-lime-to-sand ratio used to achieve it, running from the strongest and least flexible, class i, down to the weakest and most flexible, class iv. Class i mortar, typically a stronger cement-rich mix, is generally reserved for situations needing high strength, such as retaining walls, below-ground work, or exposed, severe-weather locations, but its relative rigidity means it doesn't accommodate minor building movement as well as weaker mixes, which can in some cases cause cracking to show in the mortar joint, or worse, in the bricks or blocks themselves if the mortar is harder than the masonry it's binding. Class iii mortar, a more moderate, general-purpose cement-lime-sand ratio, is commonly used for standard above-ground brickwork and blockwork in typical domestic construction, offering a reasonable balance of adequate strength with enough flexibility to accommodate normal minor building movement without cracking. The general principle worth understanding, beyond memorising exact ratios, is that mortar should generally be weaker than, or at most similar in strength to, the masonry units it's binding together, since mortar is intended to be the sacrificial, more easily repaired element in a wall, allowing minor movement and weathering to show in the joints, which can be repointed, rather than in the brick or stone itself, which is far harder and more expensive to replace.

What should I know about Mortar for Repointing vs New Build?

Repointing an existing older wall, removing and renewing failed or eroded mortar in the joints between existing bricks or stonework, generally needs a different mortar specification to a new-build wall, and using a modern, strong cement-based mortar to repoint an older property is a common and sometimes costly mistake. Many older UK buildings, particularly those built before around the 1920s, were originally built with a lime-based mortar, which is softer and more breathable than modern cement-based mixes, allowing moisture within the wall to evaporate out through the mortar joints rather than becoming trapped. Repointing such a wall with a hard, modern cement-rich mortar can trap moisture inside the wall, since the mortar is now less permeable than the surrounding softer brick or stone, which can force moisture to escape through the masonry units themselves instead, accelerating frost damage, spalling and erosion of the brick or stone faces over time, a problem that often takes years to become visible and can be expensive to properly correct once it has. For this reason, matching the original mortar type and roughly its historic strength when repointing an older property is generally considered better practice than defaulting to a standard modern mortar mix, and for a genuinely old or characterful building, seeking advice on an appropriate lime-based specification before repointing is worth the extra consideration.

Worked Example: Mortar for 10m² of Blockwork

A standard 440×215mm concrete block laid with 10mm joints needs roughly 10 blocks per m² of wall. For a 1:5 cement:sand mortar mix (a typical general-purpose brickwork/blockwork ratio), coverage guides commonly allow around 0.025-0.03m³ of wet mortar per m² of blockwork with standard block sizes and joint thickness — so for 10m², budget roughly 0.25-0.3m³ of mixed mortar. Converting to materials at a 1:5 ratio by volume: total parts = 6, so cement volume ≈ 0.3 ÷ 6 ≈ 0.05m³ and sand volume ≈ 0.25m³. A 25kg bag of cement is roughly 0.018m³ loose, so 0.05m³ needs about 3 bags (rounding up, and allowing for the usual 10-15% mixing and handling waste pushes this to 3-4 bags). This is exactly why merchants quote mortar mix calculators in bags-per-m² rather than leaving tradespeople to convert ratios from scratch on site — the arithmetic above is what sits behind those simplified tables.

What Mix Ratio Should I Use for Different Mortar Applications?

The right mortar mix depends heavily on what the mortar is for, not just what wall it's going into — a mix that's too strong for an older, softer brick can do more damage than one that's too weak.

What's the Difference Between Cement:Lime:Sand and Plain Cement:Sand Mixes?

A plain cement:sand mix (e.g. 1:5) is cheaper and simpler to batch but sets stiffer, cures faster, and is noticeably less workable under the trowel — it tends to 'go off' before a less experienced bricklayer can finish adjusting joints. Adding lime (making it a cement:lime:sand mix, e.g. 1:1:6) improves workability significantly: the mortar stays plastic and easy to tool for longer, has slightly better water retention (reducing the risk of the brick sucking moisture out of the mortar too fast and weakening the bond), and produces a mortar that is somewhat more flexible once cured, better able to accommodate the tiny day-to-day movements of a wall without cracking. This is why traditional and conservation-grade mortars almost always specify lime, while modern mass-production blockwork more often uses a plain cement:sand mix with a liquid plasticiser instead of lime, since plasticiser gives a similar workability benefit more cheaply and consistently, even though it works by a different mechanism (entraining tiny air bubbles rather than lime's chemistry).