How to Calculate the Size and Amount of Rebar

Even an inexperienced builder can calculate re-bar size and amount. All you need is a tape measure, writing pad and calculator, and you can accurately determine the proper size and amount of rebar for your own project. You can estimate your rebar quantities in one to two hours, depending on the size and scope of the project.

Rebar size and amount depends on the size of the structure to which it will be inserted.


Step 1

Determine the overall length of rebar necessary for the installation by measuring the combined footer perimeter. For example, if you have a footer that is 10 feet long and 10 feet wide, then the total length will be 40 feet (10x4=40 feet).

Step 2

Determine the amount of sticks of re-bar wide that will be installed into the footer. One stick of rebar per 8 inches of width is the general rule when considering how many sticks wide to place in the footer. If you have a footer that is 24 inches wide, then you will need three sticks of re-bar across the width of the footer.

Step 3

Combine the rebar amounts to determine total rebar needed. If you have 40 feet of footer that is 24 inches wide, then multiply 40 feet times 3 (amount of rebar sticks wide) to get 120 total feet of rebar. Since rebar comes in 20 foot lengths, divide 120 feet by 20 feet to get six. You will need six pieces of rebar for the footer.

Step 4

Determine the size of rebar needed by taking into consideration the wall height and width that will rest on the footer. Use 1/2-inch rebar for walls less than 8 feet high and 8 inches wide. If the wall to be placed on the footer is greater than 8 feet and 8 inches wide, use 5/8-inch rebar.


Step 5

Determine the amount of rebar in a slab by dividing the slab width by 2 feet and multiplying that by 3 (amount of rebar in 24 inches). For example, if you have a slab that is 20 feet long and 20 feet wide, then divide 20 feet by 2 feet to get 10.

Step 6

Multiply ten times 3 to get the total number of 20 foot rebar that will be installed into the slab. You will need 30 20-foot rebar sticks for the slab. Double the total since the slab rebar will be tied together in a 24-by-24 inch square grid.

Step 7

Determine the size of the rebar based on the desired use of the slab. If large objects such as tractors and trucks will be placed on the slab, use 5/8-inch rebar. Otherwise, use 1/2-inch rebar.