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How to Calculate Cement and Sand Ratios for Mortar
Mortar mix ratios are critical for the right application. Using the wrong strength can crack walls or cause tiles to fail. Here is the correct mix for pointing, plastering, bricklaying, and rendering.
Mortar is not one mix -- there are at least six different strengths (designation classes) specified in BS EN 998-2, each suited to different applications. Using structural mortar for floor tiles or weak mix for foundation brickwork are common and costly mistakes.
Mortar Mix Designations (UK)
DesignationCement:SandStrengthUse
M12 (i)1:3Very highBelow DPC brickwork, engineering brick
M6 (ii)1:4 to 1:4.5HighExternal walls, copings, sills
M4 (iii)1:5 to 1:6MediumInternal blockwork, general brickwork
M2 (iv)1:7 to 1:8LowInternal partitions, non-structural
Pointing mix1:4Medium-highRepointing external brickwork
Floor tile adhesive1:3HighWet areas, heavy use
Calculating Quantities
For 1:4 mix (1 part cement, 4 parts sand) by volume:
Total parts = 1 + 4 = 5
Cement fraction = 1/5 = 20%
Sand fraction = 4/5 = 80%
Volume required: 100 litres of mortar
Cement: 100 x 0.20 = 20 litres
Sand: 100 x 0.80 = 80 litres
Converting to bags:
25kg cement bag ≈ 16.5 litres
Cement bags needed: 20 / 16.5 = 1.21 bags -- buy 2 bags
Builders sand (850kg/m3 bulk density):
80 litres = 0.08 m3 x 850 = 68kg sand
Volume Yield (Mortar Shrinks When Mixed)
Dry materials mixed with water yield less volume than the inputs.
Rule of thumb: 1.3:1 bulking factor
(1.3 volumes of dry materials = 1 volume of mortar)
To get 0.1 m3 (100 litres) of fresh mortar:
Dry material needed: 100 x 1.3 = 130 litres
For 1:4 mix: cement = 130/5 = 26 litres = ~2 bags
Sand: 104 litres = ~88kg
For tiling:
Coverage: approximately 5-8 litres per m2 at 10mm bed depth
10 m2 tiled area: 50-80 litres mortar
Dry materials (x1.3): 65-104 litres
1:3 mix: cement = 65-104/4 = 16-26 litres (1-2 bags)
Sand: 49-78 litres (50-66kg)