The ratio of RCC 1:2:4 means one part of cement by volume, two parts of fine aggregate/sand by volume and four parts of coarse aggregate/bajri by volume. To achieve this ratio accordingly in field we make measuring box having size 1 feet x 1 feet x 1¼ feet that is the volume of box is 1¼ cft and is equal to the volume of one bag of cement. Now for making cement concrete mix, we take one bag of cement and two boxes of fine aggregate and four boxes of coarse aggregate for the said ratio 1: 2: 4. 
 
However in design mix process the ratio is mixed by weight and is given by laboratory after testing different materials and mixes.

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