How to calculate total water quantity in a concrete mix

This construction video is very informative for quantity surveyor as it briefly explains the process for computing the quantity of water required in concrete mix. Here concrete is mixed in the proportion of 1:2:4. The measuring unit for water is litre. The water quantity is also calculated in gallons.

Generally water required for 1 bag of cement is 11.667 litres or 3 gallons that is 1/3 of cement bag and 5% for aggregates.

It is assumed that volume of 1 bag of cement = 1.25 cft or 0.035 Cu.m
Volume of sand (also called as fine aggregate) = 0.035 x 2 = 0.07 m3 (as mix ratio is 1:2:4)
Volume of bajri (also called as coarse aggregate) = 0.035 x 4 = 0.14 m3 (as mix ratio is 1:2:4)
So, total volume of aggregates is obtained by adding both fine and coarse aggregates i.e.
0.07 + 0.14 = 0.21 m3

Therefore, volume of water for 1 bag of cement = 11.667 litres

Volume of water for aggregates = 5/100 x 0.21 = 0.0105 m3

To conver it to litre, just multiply it with 1000 = 10.50 litre (we know 1 Cu.m = 1000 litres)

In order to get total quantity of water, add both water for cement and water for aggregates
= 11.667 + 10.50 = 22.16 litres

Video Source: SL Khan