Expert: Stephen Major (Owner--Major Design Group) - 3/30/2007

Question
QUESTION: im going to poor a 10'long 18"widex12'high footing
how many so called "yards" is this
and do you know how to figure out the math to do such a thing so that i can tell the concrete yard exactly what i need for the job
ANSWER: A cubic yard is 3' x 3' x 3', or 27 cubic feet.  I'm not sure if you meant 12 feet or 12 inches for the footing height.  For a 12' high footing multiply 10' x 1.5' x 12' = 180 cubic feet.  Divide this by 27 cu. ft./yard for a total of 6.67 yds.  Round up to 7 and you should be OK.  

If you meant 12", then the calculation is 10' x 1.5' x 1' = 15 cu. ft., or about 0.6 yards.  Many suppliers require a 3-yard minimum.  

If your sub-grade is  uneven you may need more concrete.  Same goes for weak formwork.  If your forms bow during the pour you will need more concrete.  It's always better to order a little extra than to be a little short.

Also, most concrete suppliers will tell you the yardage if you give them the pour dimensions.

---------- FOLLOW-UP ----------

QUESTION: thanks for the info very cool
do you also know the formula for getting square
say 18'x 22'what would be the diagonal measurement
thankyou
simon

Answer
Sure,

Take the two adjacent sides (a and b) and connect them with a diagonal line (c).  This forms a right triangle (assuming all is square).  The formula to find the hypotenuse (diagonal) of a right triangle is (a) squared + (b) squared = (c) squared.  In your case (a) squared + (b) squared is 324 + 484 = 808.  The diagonal is the square root of 808, or 28.425.