Hi Joe,

You can think of the extension in 3 pieces. First extend the length 6 inches with a slab that is 15 ft by 6 inches by 4 inches deep. Now extend one side with a slab that is 15ft 6 inches long by 6 inches wide by 4 inches deep. Finally extend the other side with an identical sized slab. Thus you have three pieces

• 1 piece 15 feet by 6 inches by 4 inches
  
  
• 2 pieces 15 feet 6 inches (186 inches) by 6 inches by 4 inches.

Use our volume calculator to find the volume of each piece and add to find the total volume. To determine the number of bags of cement required either look on the bag or ask the dealer who is going to sell you the cement. The volume of concrete you can make from a bag of cement depends on the cement you are using, the aggregate you put with the cement and the ratio of aggregate to cement in the mixture.

Good luck in this,
Harley