1st. determine cu. yds of whetted grout needed by multiplying L x W x H (where the least common denominator is ft. If we use the L x W x H measurements to get the volume, the equation would look like this: 140 lf x 3/12 ft x 5/12 ft = 14.6 cu ft ( of whetted grout )
2nd. add the volume of water needed to wet a 50 lb bag of General construction, mineral-aggregate non-shrink grout to the industry "flow-able mix" standard . According to a popular grout industry website, approx 1.07 gal water mixed with a 50 lb bag of grout yields 0.45 cu ft of whetted grout.
3rd. divide the volume of whetted grout needed, 14.6 cu ft by the volume of whetted grout a 50 lb bag will produce, 0.45 cu ft, to get the # of bags of dry grout needed for the job: 14.6 cu ft / .45 cu ft = 17.7777 ( 18 bags dry grout )