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

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.