Calculating quantities of materials for mortar by weight is convenient, as that is how the materials are sold, and it also eliminates the need to take account of volume shrinkage on mixing! When working out quantities of bagged materials, the number must always be rounded up to the next full bag. Rounding down will lead to insufficient quantities. Caution should also be exercised when buying large quantities of sand in bags – it is likely to be much more economic to buy it loose by the tonne.
Project: a 100mm-thick block wall 12m long and 2.6m high (measured from the top of its foundation), constructed using a mortar mix of 1:2:4 (cement:lime:sand).
Number of blocks = 12m ^ 2.6 x 10 per square metre = 312 blocks
Add an amount for waste (typically 10 per cent): 312 + 10 per cent = 343 blocks
Quantity of mortar = 343 x 2.25kg/block = 772kg in a ratio of 1:2:4 (total of 7 component parts)
Cement: (772kg + 7 parts) x 1 = 110kg + 25kg/bag = 4.40 = 5 bags
Lime: (772kg + 7 parts) x 2 = 221kg + 25kg/bag = 8.84 = 9 bags
Sand: (772kg + 7 parts) x 4 = 441kg + 25kg/bag = 17.64 = 18 bags