Calculating rebar for a foundation project is as important as calculating concrete for the footings. If you guess at the amounts, you may end up with too much rebar and push your project over budget. Or you may end up with too little, which will render your project useless and underdeveloped. The average doityourselfer can successfully calculate rebar amounts for foundations by following a few triedandtrue methods.
Things You'll Need
 Calculator
 Tape measure
 Writing pad

Calculate the total number of linear feet of your foundation. For example, if your foundation is 30 feet wide and 40 feet long, then add all the sides together to get the total footage. The total for this foundation would be 30+30+40+40=140 linear feet.

Determine the amount of rebar needed for the footings. Generally, one stick of rebar per 8 inches of footing width will suffice. If your footings are 16 inches wide, you will need to add two sticks of rebar along the width of the footing; however, if your footings are 24 inches wide, you will need three sticks. For example, if your footings are 140 total feet long and your footings are 24 inches wide, then multiply 140 feet times three. You will get 420 feet. This is how much vertical rebar you will need for your 24inch footers. Because rebar comes in 20foot pieces, divide 420 by 20 and you get 21 sticks of rebar.

Determine how much rebar for your foundation walls. Your foundation walls will require that every four feet be filled with concrete and rebar. If you have 140 feet of wall, just divide 140 feet by 4 and you get 35. This will be the amount of block cells that will be filled with concrete and rebar; however, you will need to determine the lengths of the rebar. If your foundation is 32 inches, then multiply 32 inches times 35 slots and you get the total number of inches of rebar you need, which is 1,120 inches. You will then need to convert that to feet, which is 1,120 divided by 12 equals 93.3 feet. You will need five sticks of 20 foot rebar for the vertical runs, as well.

Add stepup rebar to your total figures for your foundation. Stepup rebar, or S bars, allow you to attach together at the step ups of your footings. You will most certainly need these to keep your footings continuous.
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References
 "Construction management jumpstart"; Barbara J. Jackson; 2004
 "Ortho's Basic home building: An Illustrated Guide;" Ron Hildebrand, Ortho Books; 1991
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