Mortar Mix Ratio & Quantity Calculator

Mortar Mix Guide

What's the key thing to understand about Mortar Mix Ratios by Application?

Mortar strength must match the application. Too strong a mortar damages softer masonry by trapping moisture. Bricklaying (internal): 1 cement: 5 sand. Bricklaying (external): 1 cement: 4 sand. Repointing (hard brick): 1 cement: 3 sand. Repointing (soft brick or stone): 1 lime: 2.5 sand (softer, sacrificial — protects the masonry). External rendering: 1 cement: 4 sand (scratch coat), 1 cement: 5 sand (finish coat). Paving bedding mortar: 1 cement: 4 sharp sand. Floor screed: 1 cement to 4 sharp sand, laid at a consistent thickness to provide a smooth, level base for the final floor covering.

What do I need to know about Mixing Technique?

Always mix dry materials first before adding water. Measure by volume using a bucket, not by weight on site. Mix to an even colour before adding water — incomplete dry mixing produces weak spots. Water addition: add approximately 80% of the expected water and mix thoroughly, then add remainder to achieve the right consistency. Correct consistency: mortar should hold its shape when squeezed in the hand but not stick to clean surfaces. Too wet: runs off the trowel, gives poor adhesion, and leads to shrinkage cracking as the excess water evaporates out of the cured mortar.

What's the difference between Ready-Mixed and Site-Mixed Mortar?

Ready-mixed mortar (tub, dry bags, or pre-batched wet): convenient, consistent, no mixing equipment needed. Dry bag mortar (just add water): good for small jobs. Pre-batched wet mortar in tubs: includes a retarder that keeps mortar workable for up to 36 hours — excellent for larger jobs. Site mixing: economical for large quantities (1 tonne of bricklaying mortar costs approximately £35 mixed vs £80-120 pre-mixed). For repointing: always use a pre-mixed repointing mortar or mix carefully to match the colour and texture of the surrounding original mortar.

What's the difference between Lime Mortar and Cement Mortar?

Traditional lime mortar (NHL 3.5 or NHL 5 natural hydraulic lime + sand) should be used for repointing all pre-1920 brick and stone buildings and any soft brick regardless of age. Cement mortar is too rigid and too impermeable for soft or historic masonry — it traps moisture and causes spalling (surface crumbling). Lime mortar is sacrificial — it slowly wears away and can be repointed rather than damaging the surrounding masonry. If you see crumbling, spalling, or blown brickwork on an older property, suspect incorrectly specified hard cement mortar rather than the traditional lime it should have been repointed with.

What should I know about Mortar Designation Classes (M1-M12)?

UK mortar is classified by designation (i to iv, or increasingly by compressive strength class M2 to M12) reflecting cement content and resulting strength/flexibility balance. Designation iii (roughly 1:1:6 cement:lime:sand, or 1:5-6 cement:sand with plasticiser) is the standard general-purpose mix for most above-ground masonry. Designation ii (1:½:4-4½) is stronger, used for more exposed brickwork or retaining structures. A common and costly mistake is using too strong a mortar (too much cement) for repointing older solid-wall buildings — older brick and stone is typically softer than the mortar joint is meant to be, and modern cement-rich mortar that's stronger than the masonry itself can cause the brick or stone face to spall and crack as it's forced to take stresses the original lime mortar was designed to absorb instead.

What do I need to know about Mortar Workability and Retardation?

Mortar workability (how easily it can be spread and shaped before it stiffens) depends on water content, sand grading, and whether a plasticiser or lime is added. In hot weather, mortar can stiffen (go 'off') within 1-2 hours, while in cold weather it may remain workable for 3-4 hours but also cures far more slowly, risking frost damage to the unset mortar if temperatures drop below about 2-5°C overnight. Retarding admixtures extend working time for large pours or hot-weather work, while accelerators speed up setting for cold-weather work — though frost precautions (covering fresh brickwork, avoiding laying in freezing conditions altogether) are generally preferred over chemical accelerators for ordinary DIY projects, since accelerators can affect long-term durability and appearance if not dosed correctly.

What do I need to know about Mortar Colour Matching for Repairs?

When repointing or patching an existing wall, matching the new mortar's colour to the old is as important as getting the mix ratio right, since mismatched pointing is one of the most visually obvious DIY mistakes on a period property. Colour comes mainly from the sand used, so take a sample of the existing mortar (or photograph a freshly broken joint) and try to match the sand's colour and grain size before buying in bulk — builders' merchants often stock several sand colours (yellow, grey, red/buff) for this reason. Cement type matters too: ordinary grey cement will dull a mix compared to white cement, which is often used for lighter, more traditional-looking pointing. Lime content also affects colour, giving a softer, more matte finish than a hard cement-rich mix. Always mix a test batch, let it dry fully (colour changes significantly between wet and cured mortar, lightening by several shades), and compare it against the original in daylight before repointing an entire wall, since large-scale mismatches are expensive and difficult to correct afterwards.

Worked Example: Mortar for 1000 Bricks

Bricklaying mortar consumption is normally estimated per 1,000 bricks rather than per square metre, since brick size and joint thickness vary.

Laying 1,000 standard bricks with 10mm joints at a typical 1:5 cement:sand ratio consumes roughly 0.28 cubic metres of mixed mortar, built from about six 25kg bags of cement and roughly a bulk bag of building sand. This figure assumes consistent 10mm joints throughout — thicker joints (common on reclaimed or handmade bricks with irregular dimensions) increase mortar consumption noticeably, sometimes by 30-50% more, since joint volume rises faster than brick count as the gap widens. It's also why experienced bricklayers always order a buffer of 10-15% above the calculated figure: wastage from dropped mortar, mixing inconsistency, and the unavoidable extra mortar used at wall ends and around openings all add up across a full job.

Pointing Mortar vs Bedding Mortar

Mortar used to bed bricks or blocks together and mortar used to point (finish) the visible joint face often need different mix characteristics, even on the same wall. Bedding mortar needs good workability and a slightly wetter consistency so it beds the brick evenly and squeezes out excess as the brick is tapped into place, with strength matched to the masonry unit being used — too strong a mix on soft historic brick can actually cause the brick itself to fail before the mortar does, since the mortar is meant to be the slightly weaker, more flexible component that can be repointed later without damaging the brick. Pointing mortar, used either to finish a joint at the time of laying or to repair an existing joint later (repointing), is typically mixed slightly drier and stiffer so it holds a crisp profile (such as a bucket-handle, weather-struck, or flush finish) without slumping before it sets. On repair and restoration work, matching the pointing mortar's mix, colour, and finish to the original is particularly important both for appearance and for genuine technical reasons: a hard, impermeable modern cement pointing mortar on an old lime-mortared wall can trap moisture inside the wall that the original lime mortar was designed to let breathe out, accelerating damage to the brick or stone itself rather than protecting it — which is why conservation-grade repointing work on period properties almost always specifies a lime-based mortar rather than standard cement mix regardless of what the rest of the wall was built with.

Sulfate-Resisting Cement for Below-Ground Work

Standard Portland cement mortar is unsuitable for certain below-ground or persistently damp applications because naturally occurring sulfates in some soils and groundwater react with standard cement over time, causing the mortar to expand, crack, and lose strength — a slow-acting but serious failure mode sometimes called sulfate attack. Footings, retaining walls, and brickwork below damp-proof course level in areas with known sulfate-bearing clay or groundwater typically call for sulfate-resisting cement (often labelled SRC) instead of standard cement in the mix, which is formulated with a different chemical composition specifically to resist this reaction. Building control and structural engineers will usually specify SRC explicitly on drawings for foundation and below-ground work in affected areas, identified through a soil survey, rather than leaving it to the bricklayer's judgement, since the consequence of getting it wrong only becomes visible years later once the damage is already extensive. For straightforward above-ground DIY brickwork and garden walls on ordinary, well-drained sites, standard Portland cement mortar remains entirely appropriate and sulfate-resisting cement is an unnecessary cost.