1 bag of cement makes volume of Approximately 1.25 + / - 0.05 cubic feet. you can also measure this by calculating density of cement in Kg per cubic feet and dividing by this value to 50Kg or whatever amount you want to check for its volume.
Example - You have a cement which has density of 1440 Kg/Cum
Your 50 Kg Bag will have a volume of = 50 / 1440 = 0.0347 Cum
1 Cum = 35.32 Cft
So 50 Kg bag will have volume in Cft = 0.0347 x 35.32 = 1.226 Cft